t3701: we don't need no Perl for `add -i` anymore

[git.git] / run-command.c

#include "cache.h"
#include "run-command.h"
#include "exec-cmd.h"
#include "sigchain.h"
#include "strvec.h"
#include "thread-utils.h"
#include "strbuf.h"
#include "string-list.h"
#include "quote.h"
#include "config.h"
#include "packfile.h"
#include "hook.h"
#include "compat/nonblock.h"

void child_process_init(struct child_process *child)
{
        struct child_process blank = CHILD_PROCESS_INIT;
        memcpy(child, &blank, sizeof(*child));
}

void child_process_clear(struct child_process *child)
{
        strvec_clear(&child->args);
        strvec_clear(&child->env);
}

struct child_to_clean {
        pid_t pid;
        struct child_process *process;
        struct child_to_clean *next;
};
static struct child_to_clean *children_to_clean;
static int installed_child_cleanup_handler;

static void cleanup_children(int sig, int in_signal)
{
        struct child_to_clean *children_to_wait_for = NULL;

        while (children_to_clean) {
                struct child_to_clean *p = children_to_clean;
                children_to_clean = p->next;

                if (p->process && !in_signal) {
                        struct child_process *process = p->process;
                        if (process->clean_on_exit_handler) {
                                trace_printf(
                                        "trace: run_command: running exit handler for pid %"
                                        PRIuMAX, (uintmax_t)p->pid
                                );
                                process->clean_on_exit_handler(process);
                        }
                }

                kill(p->pid, sig);

                if (p->process && p->process->wait_after_clean) {
                        p->next = children_to_wait_for;
                        children_to_wait_for = p;
                } else {
                        if (!in_signal)
                                free(p);
                }
        }

        while (children_to_wait_for) {
                struct child_to_clean *p = children_to_wait_for;
                children_to_wait_for = p->next;

                while (waitpid(p->pid, NULL, 0) < 0 && errno == EINTR)
                        ; /* spin waiting for process exit or error */

                if (!in_signal)
                        free(p);
        }
}

static void cleanup_children_on_signal(int sig)
{
        cleanup_children(sig, 1);
        sigchain_pop(sig);
        raise(sig);
}

static void cleanup_children_on_exit(void)
{
        cleanup_children(SIGTERM, 0);
}

static void mark_child_for_cleanup(pid_t pid, struct child_process *process)
{
        struct child_to_clean *p = xmalloc(sizeof(*p));
        p->pid = pid;
        p->process = process;
        p->next = children_to_clean;
        children_to_clean = p;

        if (!installed_child_cleanup_handler) {
                atexit(cleanup_children_on_exit);
                sigchain_push_common(cleanup_children_on_signal);
                installed_child_cleanup_handler = 1;
        }
}

static void clear_child_for_cleanup(pid_t pid)
{
        struct child_to_clean **pp;

        for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
                struct child_to_clean *clean_me = *pp;

                if (clean_me->pid == pid) {
                        *pp = clean_me->next;
                        free(clean_me);
                        return;
                }
        }
}

static inline void close_pair(int fd[2])
{
        close(fd[0]);
        close(fd[1]);
}

int is_executable(const char *name)
{
        struct stat st;

        if (stat(name, &st) || /* stat, not lstat */
            !S_ISREG(st.st_mode))
                return 0;

#if defined(GIT_WINDOWS_NATIVE)
        /*
         * On Windows there is no executable bit. The file extension
         * indicates whether it can be run as an executable, and Git
         * has special-handling to detect scripts and launch them
         * through the indicated script interpreter. We test for the
         * file extension first because virus scanners may make
         * it quite expensive to open many files.
         */
        if (ends_with(name, ".exe"))
                return S_IXUSR;

{
        /*
         * Now that we know it does not have an executable extension,
         * peek into the file instead.
         */
        char buf[3] = { 0 };
        int n;
        int fd = open(name, O_RDONLY);
        st.st_mode &= ~S_IXUSR;
        if (fd >= 0) {
                n = read(fd, buf, 2);
                if (n == 2)
                        /* look for a she-bang */
                        if (!strcmp(buf, "#!"))
                                st.st_mode |= S_IXUSR;
                close(fd);
        }
}
#endif
        return st.st_mode & S_IXUSR;
}

/*
 * Search $PATH for a command.  This emulates the path search that
 * execvp would perform, without actually executing the command so it
 * can be used before fork() to prepare to run a command using
 * execve() or after execvp() to diagnose why it failed.
 *
 * The caller should ensure that file contains no directory
 * separators.
 *
 * Returns the path to the command, as found in $PATH or NULL if the
 * command could not be found.  The caller inherits ownership of the memory
 * used to store the resultant path.
 *
 * This should not be used on Windows, where the $PATH search rules
 * are more complicated (e.g., a search for "foo" should find
 * "foo.exe").
 */
static char *locate_in_PATH(const char *file)
{
        const char *p = getenv("PATH");
        struct strbuf buf = STRBUF_INIT;

        if (!p || !*p)
                return NULL;

        while (1) {
                const char *end = strchrnul(p, ':');

                strbuf_reset(&buf);

                /* POSIX specifies an empty entry as the current directory. */
                if (end != p) {
                        strbuf_add(&buf, p, end - p);
                        strbuf_addch(&buf, '/');
                }
                strbuf_addstr(&buf, file);

                if (is_executable(buf.buf))
                        return strbuf_detach(&buf, NULL);

                if (!*end)
                        break;
                p = end + 1;
        }

        strbuf_release(&buf);
        return NULL;
}

int exists_in_PATH(const char *command)
{
        char *r = locate_in_PATH(command);
        int found = r != NULL;
        free(r);
        return found;
}

int sane_execvp(const char *file, char * const argv[])
{
#ifndef GIT_WINDOWS_NATIVE
        /*
         * execvp() doesn't return, so we all we can do is tell trace2
         * what we are about to do and let it leave a hint in the log
         * (unless of course the execvp() fails).
         *
         * we skip this for Windows because the compat layer already
         * has to emulate the execvp() call anyway.
         */
        int exec_id = trace2_exec(file, (const char **)argv);
#endif

        if (!execvp(file, argv))
                return 0; /* cannot happen ;-) */

#ifndef GIT_WINDOWS_NATIVE
        {
                int ec = errno;
                trace2_exec_result(exec_id, ec);
                errno = ec;
        }
#endif

        /*
         * When a command can't be found because one of the directories
         * listed in $PATH is unsearchable, execvp reports EACCES, but
         * careful usability testing (read: analysis of occasional bug
         * reports) reveals that "No such file or directory" is more
         * intuitive.
         *
         * We avoid commands with "/", because execvp will not do $PATH
         * lookups in that case.
         *
         * The reassignment of EACCES to errno looks like a no-op below,
         * but we need to protect against exists_in_PATH overwriting errno.
         */
        if (errno == EACCES && !strchr(file, '/'))
                errno = exists_in_PATH(file) ? EACCES : ENOENT;
        else if (errno == ENOTDIR && !strchr(file, '/'))
                errno = ENOENT;
        return -1;
}

static const char **prepare_shell_cmd(struct strvec *out, const char **argv)
{
        if (!argv[0])
                BUG("shell command is empty");

        if (strcspn(argv[0], "|&;<>()$`\\\"' \t\n*?[#~=%") != strlen(argv[0])) {
#ifndef GIT_WINDOWS_NATIVE
                strvec_push(out, SHELL_PATH);
#else
                strvec_push(out, "sh");
#endif
                strvec_push(out, "-c");

                /*
                 * If we have no extra arguments, we do not even need to
                 * bother with the "$@" magic.
                 */
                if (!argv[1])
                        strvec_push(out, argv[0]);
                else
                        strvec_pushf(out, "%s \"$@\"", argv[0]);
        }

        strvec_pushv(out, argv);
        return out->v;
}

#ifndef GIT_WINDOWS_NATIVE
static int child_notifier = -1;

enum child_errcode {
        CHILD_ERR_CHDIR,
        CHILD_ERR_DUP2,
        CHILD_ERR_CLOSE,
        CHILD_ERR_SIGPROCMASK,
        CHILD_ERR_ENOENT,
        CHILD_ERR_SILENT,
        CHILD_ERR_ERRNO
};

struct child_err {
        enum child_errcode err;
        int syserr; /* errno */
};

static void child_die(enum child_errcode err)
{
        struct child_err buf;

        buf.err = err;
        buf.syserr = errno;

        /* write(2) on buf smaller than PIPE_BUF (min 512) is atomic: */
        xwrite(child_notifier, &buf, sizeof(buf));
        _exit(1);
}

static void child_dup2(int fd, int to)
{
        if (dup2(fd, to) < 0)
                child_die(CHILD_ERR_DUP2);
}

static void child_close(int fd)
{
        if (close(fd))
                child_die(CHILD_ERR_CLOSE);
}

static void child_close_pair(int fd[2])
{
        child_close(fd[0]);
        child_close(fd[1]);
}

static void child_error_fn(const char *err, va_list params)
{
        const char msg[] = "error() should not be called in child\n";
        xwrite(2, msg, sizeof(msg) - 1);
}

static void child_warn_fn(const char *err, va_list params)
{
        const char msg[] = "warn() should not be called in child\n";
        xwrite(2, msg, sizeof(msg) - 1);
}

static void NORETURN child_die_fn(const char *err, va_list params)
{
        const char msg[] = "die() should not be called in child\n";
        xwrite(2, msg, sizeof(msg) - 1);
        _exit(2);
}

/* this runs in the parent process */
static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
{
        static void (*old_errfn)(const char *err, va_list params);
        report_fn die_message_routine = get_die_message_routine();

        old_errfn = get_error_routine();
        set_error_routine(die_message_routine);
        errno = cerr->syserr;

        switch (cerr->err) {
        case CHILD_ERR_CHDIR:
                error_errno("exec '%s': cd to '%s' failed",
                            cmd->args.v[0], cmd->dir);
                break;
        case CHILD_ERR_DUP2:
                error_errno("dup2() in child failed");
                break;
        case CHILD_ERR_CLOSE:
                error_errno("close() in child failed");
                break;
        case CHILD_ERR_SIGPROCMASK:
                error_errno("sigprocmask failed restoring signals");
                break;
        case CHILD_ERR_ENOENT:
                error_errno("cannot run %s", cmd->args.v[0]);
                break;
        case CHILD_ERR_SILENT:
                break;
        case CHILD_ERR_ERRNO:
                error_errno("cannot exec '%s'", cmd->args.v[0]);
                break;
        }
        set_error_routine(old_errfn);
}

static int prepare_cmd(struct strvec *out, const struct child_process *cmd)
{
        if (!cmd->args.v[0])
                BUG("command is empty");

        /*
         * Add SHELL_PATH so in the event exec fails with ENOEXEC we can
         * attempt to interpret the command with 'sh'.
         */
        strvec_push(out, SHELL_PATH);

        if (cmd->git_cmd) {
                prepare_git_cmd(out, cmd->args.v);
        } else if (cmd->use_shell) {
                prepare_shell_cmd(out, cmd->args.v);
        } else {
                strvec_pushv(out, cmd->args.v);
        }

        /*
         * If there are no dir separator characters in the command then perform
         * a path lookup and use the resolved path as the command to exec. If
         * there are dir separator characters, we have exec attempt to invoke
         * the command directly.
         */
        if (!has_dir_sep(out->v[1])) {
                char *program = locate_in_PATH(out->v[1]);
                if (program) {
                        free((char *)out->v[1]);
                        out->v[1] = program;
                } else {
                        strvec_clear(out);
                        errno = ENOENT;
                        return -1;
                }
        }

        return 0;
}

static char **prep_childenv(const char *const *deltaenv)
{
        extern char **environ;
        char **childenv;
        struct string_list env = STRING_LIST_INIT_DUP;
        struct strbuf key = STRBUF_INIT;
        const char *const *p;
        int i;

        /* Construct a sorted string list consisting of the current environ */
        for (p = (const char *const *) environ; p && *p; p++) {
                const char *equals = strchr(*p, '=');

                if (equals) {
                        strbuf_reset(&key);
                        strbuf_add(&key, *p, equals - *p);
                        string_list_append(&env, key.buf)->util = (void *) *p;
                } else {
                        string_list_append(&env, *p)->util = (void *) *p;
                }
        }
        string_list_sort(&env);

        /* Merge in 'deltaenv' with the current environ */
        for (p = deltaenv; p && *p; p++) {
                const char *equals = strchr(*p, '=');

                if (equals) {
                        /* ('key=value'), insert or replace entry */
                        strbuf_reset(&key);
                        strbuf_add(&key, *p, equals - *p);
                        string_list_insert(&env, key.buf)->util = (void *) *p;
                } else {
                        /* otherwise ('key') remove existing entry */
                        string_list_remove(&env, *p, 0);
                }
        }

        /* Create an array of 'char *' to be used as the childenv */
        ALLOC_ARRAY(childenv, env.nr + 1);
        for (i = 0; i < env.nr; i++)
                childenv[i] = env.items[i].util;
        childenv[env.nr] = NULL;

        string_list_clear(&env, 0);
        strbuf_release(&key);
        return childenv;
}

struct atfork_state {
#ifndef NO_PTHREADS
        int cs;
#endif
        sigset_t old;
};

#define CHECK_BUG(err, msg) \
        do { \
                int e = (err); \
                if (e) \
                        BUG("%s: %s", msg, strerror(e)); \
        } while(0)

static void atfork_prepare(struct atfork_state *as)
{
        sigset_t all;

        if (sigfillset(&all))
                die_errno("sigfillset");
#ifdef NO_PTHREADS
        if (sigprocmask(SIG_SETMASK, &all, &as->old))
                die_errno("sigprocmask");
#else
        CHECK_BUG(pthread_sigmask(SIG_SETMASK, &all, &as->old),
                "blocking all signals");
        CHECK_BUG(pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &as->cs),
                "disabling cancellation");
#endif
}

static void atfork_parent(struct atfork_state *as)
{
#ifdef NO_PTHREADS
        if (sigprocmask(SIG_SETMASK, &as->old, NULL))
                die_errno("sigprocmask");
#else
        CHECK_BUG(pthread_setcancelstate(as->cs, NULL),
                "re-enabling cancellation");
        CHECK_BUG(pthread_sigmask(SIG_SETMASK, &as->old, NULL),
                "restoring signal mask");
#endif
}
#endif /* GIT_WINDOWS_NATIVE */

static inline void set_cloexec(int fd)
{
        int flags = fcntl(fd, F_GETFD);
        if (flags >= 0)
                fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
}

static int wait_or_whine(pid_t pid, const char *argv0, int in_signal)
{
        int status, code = -1;
        pid_t waiting;
        int failed_errno = 0;

        while ((waiting = waitpid(pid, &status, 0)) < 0 && errno == EINTR)
                ;       /* nothing */

        if (waiting < 0) {
                failed_errno = errno;
                if (!in_signal)
                        error_errno("waitpid for %s failed", argv0);
        } else if (waiting != pid) {
                if (!in_signal)
                        error("waitpid is confused (%s)", argv0);
        } else if (WIFSIGNALED(status)) {
                code = WTERMSIG(status);
                if (!in_signal && code != SIGINT && code != SIGQUIT && code != SIGPIPE)
                        error("%s died of signal %d", argv0, code);
                /*
                 * This return value is chosen so that code & 0xff
                 * mimics the exit code that a POSIX shell would report for
                 * a program that died from this signal.
                 */
                code += 128;
        } else if (WIFEXITED(status)) {
                code = WEXITSTATUS(status);
        } else {
                if (!in_signal)
                        error("waitpid is confused (%s)", argv0);
        }

        if (!in_signal)
                clear_child_for_cleanup(pid);

        errno = failed_errno;
        return code;
}

static void trace_add_env(struct strbuf *dst, const char *const *deltaenv)
{
        struct string_list envs = STRING_LIST_INIT_DUP;
        const char *const *e;
        int i;
        int printed_unset = 0;

        /* Last one wins, see run-command.c:prep_childenv() for context */
        for (e = deltaenv; e && *e; e++) {
                struct strbuf key = STRBUF_INIT;
                char *equals = strchr(*e, '=');

                if (equals) {
                        strbuf_add(&key, *e, equals - *e);
                        string_list_insert(&envs, key.buf)->util = equals + 1;
                } else {
                        string_list_insert(&envs, *e)->util = NULL;
                }
                strbuf_release(&key);
        }

        /* "unset X Y...;" */
        for (i = 0; i < envs.nr; i++) {
                const char *var = envs.items[i].string;
                const char *val = envs.items[i].util;

                if (val || !getenv(var))
                        continue;

                if (!printed_unset) {
                        strbuf_addstr(dst, " unset");
                        printed_unset = 1;
                }
                strbuf_addf(dst, " %s", var);
        }
        if (printed_unset)
                strbuf_addch(dst, ';');

        /* ... followed by "A=B C=D ..." */
        for (i = 0; i < envs.nr; i++) {
                const char *var = envs.items[i].string;
                const char *val = envs.items[i].util;
                const char *oldval;

                if (!val)
                        continue;

                oldval = getenv(var);
                if (oldval && !strcmp(val, oldval))
                        continue;

                strbuf_addf(dst, " %s=", var);
                sq_quote_buf_pretty(dst, val);
        }
        string_list_clear(&envs, 0);
}

static void trace_run_command(const struct child_process *cp)
{
        struct strbuf buf = STRBUF_INIT;

        if (!trace_want(&trace_default_key))
                return;

        strbuf_addstr(&buf, "trace: run_command:");
        if (cp->dir) {
                strbuf_addstr(&buf, " cd ");
                sq_quote_buf_pretty(&buf, cp->dir);
                strbuf_addch(&buf, ';');
        }
        trace_add_env(&buf, cp->env.v);
        if (cp->git_cmd)
                strbuf_addstr(&buf, " git");
        sq_quote_argv_pretty(&buf, cp->args.v);

        trace_printf("%s", buf.buf);
        strbuf_release(&buf);
}

int start_command(struct child_process *cmd)
{
        int need_in, need_out, need_err;
        int fdin[2], fdout[2], fderr[2];
        int failed_errno;
        char *str;

        /*
         * In case of errors we must keep the promise to close FDs
         * that have been passed in via ->in and ->out.
         */

        need_in = !cmd->no_stdin && cmd->in < 0;
        if (need_in) {
                if (pipe(fdin) < 0) {
                        failed_errno = errno;
                        if (cmd->out > 0)
                                close(cmd->out);
                        str = "standard input";
                        goto fail_pipe;
                }
                cmd->in = fdin[1];
        }

        need_out = !cmd->no_stdout
                && !cmd->stdout_to_stderr
                && cmd->out < 0;
        if (need_out) {
                if (pipe(fdout) < 0) {
                        failed_errno = errno;
                        if (need_in)
                                close_pair(fdin);
                        else if (cmd->in)
                                close(cmd->in);
                        str = "standard output";
                        goto fail_pipe;
                }
                cmd->out = fdout[0];
        }

        need_err = !cmd->no_stderr && cmd->err < 0;
        if (need_err) {
                if (pipe(fderr) < 0) {
                        failed_errno = errno;
                        if (need_in)
                                close_pair(fdin);
                        else if (cmd->in)
                                close(cmd->in);
                        if (need_out)
                                close_pair(fdout);
                        else if (cmd->out)
                                close(cmd->out);
                        str = "standard error";
fail_pipe:
                        error("cannot create %s pipe for %s: %s",
                                str, cmd->args.v[0], strerror(failed_errno));
                        child_process_clear(cmd);
                        errno = failed_errno;
                        return -1;
                }
                cmd->err = fderr[0];
        }

        trace2_child_start(cmd);
        trace_run_command(cmd);

        fflush(NULL);

        if (cmd->close_object_store)
                close_object_store(the_repository->objects);

#ifndef GIT_WINDOWS_NATIVE
{
        int notify_pipe[2];
        int null_fd = -1;
        char **childenv;
        struct strvec argv = STRVEC_INIT;
        struct child_err cerr;
        struct atfork_state as;

        if (prepare_cmd(&argv, cmd) < 0) {
                failed_errno = errno;
                cmd->pid = -1;
                if (!cmd->silent_exec_failure)
                        error_errno("cannot run %s", cmd->args.v[0]);
                goto end_of_spawn;
        }

        if (pipe(notify_pipe))
                notify_pipe[0] = notify_pipe[1] = -1;

        if (cmd->no_stdin || cmd->no_stdout || cmd->no_stderr) {
                null_fd = xopen("/dev/null", O_RDWR | O_CLOEXEC);
                set_cloexec(null_fd);
        }

        childenv = prep_childenv(cmd->env.v);
        atfork_prepare(&as);

        /*
         * NOTE: In order to prevent deadlocking when using threads special
         * care should be taken with the function calls made in between the
         * fork() and exec() calls.  No calls should be made to functions which
         * require acquiring a lock (e.g. malloc) as the lock could have been
         * held by another thread at the time of forking, causing the lock to
         * never be released in the child process.  This means only
         * Async-Signal-Safe functions are permitted in the child.
         */
        cmd->pid = fork();
        failed_errno = errno;
        if (!cmd->pid) {
                int sig;
                /*
                 * Ensure the default die/error/warn routines do not get
                 * called, they can take stdio locks and malloc.
                 */
                set_die_routine(child_die_fn);
                set_error_routine(child_error_fn);
                set_warn_routine(child_warn_fn);

                close(notify_pipe[0]);
                set_cloexec(notify_pipe[1]);
                child_notifier = notify_pipe[1];

                if (cmd->no_stdin)
                        child_dup2(null_fd, 0);
                else if (need_in) {
                        child_dup2(fdin[0], 0);
                        child_close_pair(fdin);
                } else if (cmd->in) {
                        child_dup2(cmd->in, 0);
                        child_close(cmd->in);
                }

                if (cmd->no_stderr)
                        child_dup2(null_fd, 2);
                else if (need_err) {
                        child_dup2(fderr[1], 2);
                        child_close_pair(fderr);
                } else if (cmd->err > 1) {
                        child_dup2(cmd->err, 2);
                        child_close(cmd->err);
                }

                if (cmd->no_stdout)
                        child_dup2(null_fd, 1);
                else if (cmd->stdout_to_stderr)
                        child_dup2(2, 1);
                else if (need_out) {
                        child_dup2(fdout[1], 1);
                        child_close_pair(fdout);
                } else if (cmd->out > 1) {
                        child_dup2(cmd->out, 1);
                        child_close(cmd->out);
                }

                if (cmd->dir && chdir(cmd->dir))
                        child_die(CHILD_ERR_CHDIR);

                /*
                 * restore default signal handlers here, in case
                 * we catch a signal right before execve below
                 */
                for (sig = 1; sig < NSIG; sig++) {
                        /* ignored signals get reset to SIG_DFL on execve */
                        if (signal(sig, SIG_DFL) == SIG_IGN)
                                signal(sig, SIG_IGN);
                }

                if (sigprocmask(SIG_SETMASK, &as.old, NULL) != 0)
                        child_die(CHILD_ERR_SIGPROCMASK);

                /*
                 * Attempt to exec using the command and arguments starting at
                 * argv.argv[1].  argv.argv[0] contains SHELL_PATH which will
                 * be used in the event exec failed with ENOEXEC at which point
                 * we will try to interpret the command using 'sh'.
                 */
                execve(argv.v[1], (char *const *) argv.v + 1,
                       (char *const *) childenv);
                if (errno == ENOEXEC)
                        execve(argv.v[0], (char *const *) argv.v,
                               (char *const *) childenv);

                if (errno == ENOENT) {
                        if (cmd->silent_exec_failure)
                                child_die(CHILD_ERR_SILENT);
                        child_die(CHILD_ERR_ENOENT);
                } else {
                        child_die(CHILD_ERR_ERRNO);
                }
        }
        atfork_parent(&as);
        if (cmd->pid < 0)
                error_errno("cannot fork() for %s", cmd->args.v[0]);
        else if (cmd->clean_on_exit)
                mark_child_for_cleanup(cmd->pid, cmd);

        /*
         * Wait for child's exec. If the exec succeeds (or if fork()
         * failed), EOF is seen immediately by the parent. Otherwise, the
         * child process sends a child_err struct.
         * Note that use of this infrastructure is completely advisory,
         * therefore, we keep error checks minimal.
         */
        close(notify_pipe[1]);
        if (xread(notify_pipe[0], &cerr, sizeof(cerr)) == sizeof(cerr)) {
                /*
                 * At this point we know that fork() succeeded, but exec()
                 * failed. Errors have been reported to our stderr.
                 */
                wait_or_whine(cmd->pid, cmd->args.v[0], 0);
                child_err_spew(cmd, &cerr);
                failed_errno = errno;
                cmd->pid = -1;
        }
        close(notify_pipe[0]);

        if (null_fd >= 0)
                close(null_fd);
        strvec_clear(&argv);
        free(childenv);
}
end_of_spawn:

#else
{
        int fhin = 0, fhout = 1, fherr = 2;
        const char **sargv = cmd->args.v;
        struct strvec nargv = STRVEC_INIT;

        if (cmd->no_stdin)
                fhin = open("/dev/null", O_RDWR);
        else if (need_in)
                fhin = dup(fdin[0]);
        else if (cmd->in)
                fhin = dup(cmd->in);

        if (cmd->no_stderr)
                fherr = open("/dev/null", O_RDWR);
        else if (need_err)
                fherr = dup(fderr[1]);
        else if (cmd->err > 2)
                fherr = dup(cmd->err);

        if (cmd->no_stdout)
                fhout = open("/dev/null", O_RDWR);
        else if (cmd->stdout_to_stderr)
                fhout = dup(fherr);
        else if (need_out)
                fhout = dup(fdout[1]);
        else if (cmd->out > 1)
                fhout = dup(cmd->out);

        if (cmd->git_cmd)
                cmd->args.v = prepare_git_cmd(&nargv, sargv);
        else if (cmd->use_shell)
                cmd->args.v = prepare_shell_cmd(&nargv, sargv);

        cmd->pid = mingw_spawnvpe(cmd->args.v[0], cmd->args.v,
                                  (char**) cmd->env.v,
                                  cmd->dir, fhin, fhout, fherr);
        failed_errno = errno;
        if (cmd->pid < 0 && (!cmd->silent_exec_failure || errno != ENOENT))
                error_errno("cannot spawn %s", cmd->args.v[0]);
        if (cmd->clean_on_exit && cmd->pid >= 0)
                mark_child_for_cleanup(cmd->pid, cmd);

        strvec_clear(&nargv);
        cmd->args.v = sargv;
        if (fhin != 0)
                close(fhin);
        if (fhout != 1)
                close(fhout);
        if (fherr != 2)
                close(fherr);
}
#endif

        if (cmd->pid < 0) {
                trace2_child_exit(cmd, -1);

                if (need_in)
                        close_pair(fdin);
                else if (cmd->in)
                        close(cmd->in);
                if (need_out)
                        close_pair(fdout);
                else if (cmd->out)
                        close(cmd->out);
                if (need_err)
                        close_pair(fderr);
                else if (cmd->err)
                        close(cmd->err);
                child_process_clear(cmd);
                errno = failed_errno;
                return -1;
        }

        if (need_in)
                close(fdin[0]);
        else if (cmd->in)
                close(cmd->in);

        if (need_out)
                close(fdout[1]);
        else if (cmd->out)
                close(cmd->out);

        if (need_err)
                close(fderr[1]);
        else if (cmd->err)
                close(cmd->err);

        return 0;
}

int finish_command(struct child_process *cmd)
{
        int ret = wait_or_whine(cmd->pid, cmd->args.v[0], 0);
        trace2_child_exit(cmd, ret);
        child_process_clear(cmd);
        invalidate_lstat_cache();
        return ret;
}

int finish_command_in_signal(struct child_process *cmd)
{
        int ret = wait_or_whine(cmd->pid, cmd->args.v[0], 1);
        if (ret != -1)
                trace2_child_exit(cmd, ret);
        return ret;
}


int run_command(struct child_process *cmd)
{
        int code;

        if (cmd->out < 0 || cmd->err < 0)
                BUG("run_command with a pipe can cause deadlock");

        code = start_command(cmd);
        if (code)
                return code;
        return finish_command(cmd);
}

#ifndef NO_PTHREADS
static pthread_t main_thread;
static int main_thread_set;
static pthread_key_t async_key;
static pthread_key_t async_die_counter;

static void *run_thread(void *data)
{
        struct async *async = data;
        intptr_t ret;

        if (async->isolate_sigpipe) {
                sigset_t mask;
                sigemptyset(&mask);
                sigaddset(&mask, SIGPIPE);
                if (pthread_sigmask(SIG_BLOCK, &mask, NULL)) {
                        ret = error("unable to block SIGPIPE in async thread");
                        return (void *)ret;
                }
        }

        pthread_setspecific(async_key, async);
        ret = async->proc(async->proc_in, async->proc_out, async->data);
        return (void *)ret;
}

static NORETURN void die_async(const char *err, va_list params)
{
        report_fn die_message_fn = get_die_message_routine();

        die_message_fn(err, params);

        if (in_async()) {
                struct async *async = pthread_getspecific(async_key);
                if (async->proc_in >= 0)
                        close(async->proc_in);
                if (async->proc_out >= 0)
                        close(async->proc_out);
                pthread_exit((void *)128);
        }

        exit(128);
}

static int async_die_is_recursing(void)
{
        void *ret = pthread_getspecific(async_die_counter);
        pthread_setspecific(async_die_counter, &async_die_counter); /* set to any non-NULL valid pointer */
        return ret != NULL;
}

int in_async(void)
{
        if (!main_thread_set)
                return 0; /* no asyncs started yet */
        return !pthread_equal(main_thread, pthread_self());
}

static void NORETURN async_exit(int code)
{
        pthread_exit((void *)(intptr_t)code);
}

#else

static struct {
        void (**handlers)(void);
        size_t nr;
        size_t alloc;
} git_atexit_hdlrs;

static int git_atexit_installed;

static void git_atexit_dispatch(void)
{
        size_t i;

        for (i=git_atexit_hdlrs.nr ; i ; i--)
                git_atexit_hdlrs.handlers[i-1]();
}

static void git_atexit_clear(void)
{
        free(git_atexit_hdlrs.handlers);
        memset(&git_atexit_hdlrs, 0, sizeof(git_atexit_hdlrs));
        git_atexit_installed = 0;
}

#undef atexit
int git_atexit(void (*handler)(void))
{
        ALLOC_GROW(git_atexit_hdlrs.handlers, git_atexit_hdlrs.nr + 1, git_atexit_hdlrs.alloc);
        git_atexit_hdlrs.handlers[git_atexit_hdlrs.nr++] = handler;
        if (!git_atexit_installed) {
                if (atexit(&git_atexit_dispatch))
                        return -1;
                git_atexit_installed = 1;
        }
        return 0;
}
#define atexit git_atexit

static int process_is_async;
int in_async(void)
{
        return process_is_async;
}

static void NORETURN async_exit(int code)
{
        exit(code);
}

#endif

void check_pipe(int err)
{
        if (err == EPIPE) {
                if (in_async())
                        async_exit(141);

                signal(SIGPIPE, SIG_DFL);
                raise(SIGPIPE);
                /* Should never happen, but just in case... */
                exit(141);
        }
}

int start_async(struct async *async)
{
        int need_in, need_out;
        int fdin[2], fdout[2];
        int proc_in, proc_out;

        need_in = async->in < 0;
        if (need_in) {
                if (pipe(fdin) < 0) {
                        if (async->out > 0)
                                close(async->out);
                        return error_errno("cannot create pipe");
                }
                async->in = fdin[1];
        }

        need_out = async->out < 0;
        if (need_out) {
                if (pipe(fdout) < 0) {
                        if (need_in)
                                close_pair(fdin);
                        else if (async->in)
                                close(async->in);
                        return error_errno("cannot create pipe");
                }
                async->out = fdout[0];
        }

        if (need_in)
                proc_in = fdin[0];
        else if (async->in)
                proc_in = async->in;
        else
                proc_in = -1;

        if (need_out)
                proc_out = fdout[1];
        else if (async->out)
                proc_out = async->out;
        else
                proc_out = -1;

#ifdef NO_PTHREADS
        /* Flush stdio before fork() to avoid cloning buffers */
        fflush(NULL);

        async->pid = fork();
        if (async->pid < 0) {
                error_errno("fork (async) failed");
                goto error;
        }
        if (!async->pid) {
                if (need_in)
                        close(fdin[1]);
                if (need_out)
                        close(fdout[0]);
                git_atexit_clear();
                process_is_async = 1;
                exit(!!async->proc(proc_in, proc_out, async->data));
        }

        mark_child_for_cleanup(async->pid, NULL);

        if (need_in)
                close(fdin[0]);
        else if (async->in)
                close(async->in);

        if (need_out)
                close(fdout[1]);
        else if (async->out)
                close(async->out);
#else
        if (!main_thread_set) {
                /*
                 * We assume that the first time that start_async is called
                 * it is from the main thread.
                 */
                main_thread_set = 1;
                main_thread = pthread_self();
                pthread_key_create(&async_key, NULL);
                pthread_key_create(&async_die_counter, NULL);
                set_die_routine(die_async);
                set_die_is_recursing_routine(async_die_is_recursing);
        }

        if (proc_in >= 0)
                set_cloexec(proc_in);
        if (proc_out >= 0)
                set_cloexec(proc_out);
        async->proc_in = proc_in;
        async->proc_out = proc_out;
        {
                int err = pthread_create(&async->tid, NULL, run_thread, async);
                if (err) {
                        error(_("cannot create async thread: %s"), strerror(err));
                        goto error;
                }
        }
#endif
        return 0;

error:
        if (need_in)
                close_pair(fdin);
        else if (async->in)
                close(async->in);

        if (need_out)
                close_pair(fdout);
        else if (async->out)
                close(async->out);
        return -1;
}

int finish_async(struct async *async)
{
#ifdef NO_PTHREADS
        int ret = wait_or_whine(async->pid, "child process", 0);

        invalidate_lstat_cache();

        return ret;
#else
        void *ret = (void *)(intptr_t)(-1);

        if (pthread_join(async->tid, &ret))
                error("pthread_join failed");
        invalidate_lstat_cache();
        return (int)(intptr_t)ret;

#endif
}

int async_with_fork(void)
{
#ifdef NO_PTHREADS
        return 1;
#else
        return 0;
#endif
}

struct io_pump {
        /* initialized by caller */
        int fd;
        int type; /* POLLOUT or POLLIN */
        union {
                struct {
                        const char *buf;
                        size_t len;
                } out;
                struct {
                        struct strbuf *buf;
                        size_t hint;
                } in;
        } u;

        /* returned by pump_io */
        int error; /* 0 for success, otherwise errno */

        /* internal use */
        struct pollfd *pfd;
};

static int pump_io_round(struct io_pump *slots, int nr, struct pollfd *pfd)
{
        int pollsize = 0;
        int i;

        for (i = 0; i < nr; i++) {
                struct io_pump *io = &slots[i];
                if (io->fd < 0)
                        continue;
                pfd[pollsize].fd = io->fd;
                pfd[pollsize].events = io->type;
                io->pfd = &pfd[pollsize++];
        }

        if (!pollsize)
                return 0;

        if (poll(pfd, pollsize, -1) < 0) {
                if (errno == EINTR)
                        return 1;
                die_errno("poll failed");
        }

        for (i = 0; i < nr; i++) {
                struct io_pump *io = &slots[i];

                if (io->fd < 0)
                        continue;

                if (!(io->pfd->revents & (POLLOUT|POLLIN|POLLHUP|POLLERR|POLLNVAL)))
                        continue;

                if (io->type == POLLOUT) {
                        ssize_t len;

                        /*
                         * Don't use xwrite() here. It loops forever on EAGAIN,
                         * and we're in our own poll() loop here.
                         *
                         * Note that we lose xwrite()'s handling of MAX_IO_SIZE
                         * and EINTR, so we have to implement those ourselves.
                         */
                        len = write(io->fd, io->u.out.buf,
                                    io->u.out.len <= MAX_IO_SIZE ?
                                    io->u.out.len : MAX_IO_SIZE);
                        if (len < 0) {
                                if (errno != EINTR && errno != EAGAIN &&
                                    errno != ENOSPC) {
                                        io->error = errno;
                                        close(io->fd);
                                        io->fd = -1;
                                }
                        } else {
                                io->u.out.buf += len;
                                io->u.out.len -= len;
                                if (!io->u.out.len) {
                                        close(io->fd);
                                        io->fd = -1;
                                }
                        }
                }

                if (io->type == POLLIN) {
                        ssize_t len = strbuf_read_once(io->u.in.buf,
                                                       io->fd, io->u.in.hint);
                        if (len < 0)
                                io->error = errno;
                        if (len <= 0) {
                                close(io->fd);
                                io->fd = -1;
                        }
                }
        }

        return 1;
}

static int pump_io(struct io_pump *slots, int nr)
{
        struct pollfd *pfd;
        int i;

        for (i = 0; i < nr; i++)
                slots[i].error = 0;

        ALLOC_ARRAY(pfd, nr);
        while (pump_io_round(slots, nr, pfd))
                ; /* nothing */
        free(pfd);

        /* There may be multiple errno values, so just pick the first. */
        for (i = 0; i < nr; i++) {
                if (slots[i].error) {
                        errno = slots[i].error;
                        return -1;
                }
        }
        return 0;
}


int pipe_command(struct child_process *cmd,
                 const char *in, size_t in_len,
                 struct strbuf *out, size_t out_hint,
                 struct strbuf *err, size_t err_hint)
{
        struct io_pump io[3];
        int nr = 0;

        if (in)
                cmd->in = -1;
        if (out)
                cmd->out = -1;
        if (err)
                cmd->err = -1;

        if (start_command(cmd) < 0)
                return -1;

        if (in) {
                if (enable_pipe_nonblock(cmd->in) < 0) {
                        error_errno("unable to make pipe non-blocking");
                        close(cmd->in);
                        if (out)
                                close(cmd->out);
                        if (err)
                                close(cmd->err);
                        return -1;
                }
                io[nr].fd = cmd->in;
                io[nr].type = POLLOUT;
                io[nr].u.out.buf = in;
                io[nr].u.out.len = in_len;
                nr++;
        }
        if (out) {
                io[nr].fd = cmd->out;
                io[nr].type = POLLIN;
                io[nr].u.in.buf = out;
                io[nr].u.in.hint = out_hint;
                nr++;
        }
        if (err) {
                io[nr].fd = cmd->err;
                io[nr].type = POLLIN;
                io[nr].u.in.buf = err;
                io[nr].u.in.hint = err_hint;
                nr++;
        }

        if (pump_io(io, nr) < 0) {
                finish_command(cmd); /* throw away exit code */
                return -1;
        }

        return finish_command(cmd);
}

enum child_state {
        GIT_CP_FREE,
        GIT_CP_WORKING,
        GIT_CP_WAIT_CLEANUP,
};

struct parallel_processes {
        size_t nr_processes;

        struct {
                enum child_state state;
                struct child_process process;
                struct strbuf err;
                void *data;
        } *children;
        /*
         * The struct pollfd is logically part of *children,
         * but the system call expects it as its own array.
         */
        struct pollfd *pfd;

        unsigned shutdown : 1;

        size_t output_owner;
        struct strbuf buffered_output; /* of finished children */
};

struct parallel_processes_for_signal {
        const struct run_process_parallel_opts *opts;
        const struct parallel_processes *pp;
};

static void kill_children(const struct parallel_processes *pp,
                          const struct run_process_parallel_opts *opts,
                          int signo)
{
        for (size_t i = 0; i < opts->processes; i++)
                if (pp->children[i].state == GIT_CP_WORKING)
                        kill(pp->children[i].process.pid, signo);
}

static void kill_children_signal(const struct parallel_processes_for_signal *pp_sig,
                                 int signo)
{
        kill_children(pp_sig->pp, pp_sig->opts, signo);
}

static struct parallel_processes_for_signal *pp_for_signal;

static void handle_children_on_signal(int signo)
{
        kill_children_signal(pp_for_signal, signo);
        sigchain_pop(signo);
        raise(signo);
}

static void pp_init(struct parallel_processes *pp,
                    const struct run_process_parallel_opts *opts,
                    struct parallel_processes_for_signal *pp_sig)
{
        const size_t n = opts->processes;

        if (!n)
                BUG("you must provide a non-zero number of processes!");

        trace_printf("run_processes_parallel: preparing to run up to %"PRIuMAX" tasks",
                     (uintmax_t)n);

        if (!opts->get_next_task)
                BUG("you need to specify a get_next_task function");

        CALLOC_ARRAY(pp->children, n);
        if (!opts->ungroup)
                CALLOC_ARRAY(pp->pfd, n);

        for (size_t i = 0; i < n; i++) {
                strbuf_init(&pp->children[i].err, 0);
                child_process_init(&pp->children[i].process);
                if (pp->pfd) {
                        pp->pfd[i].events = POLLIN | POLLHUP;
                        pp->pfd[i].fd = -1;
                }
        }

        pp_sig->pp = pp;
        pp_sig->opts = opts;
        pp_for_signal = pp_sig;
        sigchain_push_common(handle_children_on_signal);
}

static void pp_cleanup(struct parallel_processes *pp,
                       const struct run_process_parallel_opts *opts)
{
        trace_printf("run_processes_parallel: done");
        for (size_t i = 0; i < opts->processes; i++) {
                strbuf_release(&pp->children[i].err);
                child_process_clear(&pp->children[i].process);
        }

        free(pp->children);
        free(pp->pfd);

        /*
         * When get_next_task added messages to the buffer in its last
         * iteration, the buffered output is non empty.
         */
        strbuf_write(&pp->buffered_output, stderr);
        strbuf_release(&pp->buffered_output);

        sigchain_pop_common();
}

/* returns
 *  0 if a new task was started.
 *  1 if no new jobs was started (get_next_task ran out of work, non critical
 *    problem with starting a new command)
 * <0 no new job was started, user wishes to shutdown early. Use negative code
 *    to signal the children.
 */
static int pp_start_one(struct parallel_processes *pp,
                        const struct run_process_parallel_opts *opts)
{
        size_t i;
        int code;

        for (i = 0; i < opts->processes; i++)
                if (pp->children[i].state == GIT_CP_FREE)
                        break;
        if (i == opts->processes)
                BUG("bookkeeping is hard");

        code = opts->get_next_task(&pp->children[i].process,
                                   opts->ungroup ? NULL : &pp->children[i].err,
                                   opts->data,
                                   &pp->children[i].data);
        if (!code) {
                if (!opts->ungroup) {
                        strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
                        strbuf_reset(&pp->children[i].err);
                }
                return 1;
        }
        if (!opts->ungroup) {
                pp->children[i].process.err = -1;
                pp->children[i].process.stdout_to_stderr = 1;
        }
        pp->children[i].process.no_stdin = 1;

        if (start_command(&pp->children[i].process)) {
                if (opts->start_failure)
                        code = opts->start_failure(opts->ungroup ? NULL :
                                                   &pp->children[i].err,
                                                   opts->data,
                                                   pp->children[i].data);
                else
                        code = 0;

                if (!opts->ungroup) {
                        strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
                        strbuf_reset(&pp->children[i].err);
                }
                if (code)
                        pp->shutdown = 1;
                return code;
        }

        pp->nr_processes++;
        pp->children[i].state = GIT_CP_WORKING;
        if (pp->pfd)
                pp->pfd[i].fd = pp->children[i].process.err;
        return 0;
}

static void pp_buffer_stderr(struct parallel_processes *pp,
                             const struct run_process_parallel_opts *opts,
                             int output_timeout)
{
        int i;

        while ((i = poll(pp->pfd, opts->processes, output_timeout) < 0)) {
                if (errno == EINTR)
                        continue;
                pp_cleanup(pp, opts);
                die_errno("poll");
        }

        /* Buffer output from all pipes. */
        for (size_t i = 0; i < opts->processes; i++) {
                if (pp->children[i].state == GIT_CP_WORKING &&
                    pp->pfd[i].revents & (POLLIN | POLLHUP)) {
                        int n = strbuf_read_once(&pp->children[i].err,
                                                 pp->children[i].process.err, 0);
                        if (n == 0) {
                                close(pp->children[i].process.err);
                                pp->children[i].state = GIT_CP_WAIT_CLEANUP;
                        } else if (n < 0)
                                if (errno != EAGAIN)
                                        die_errno("read");
                }
        }
}

static void pp_output(const struct parallel_processes *pp)
{
        size_t i = pp->output_owner;

        if (pp->children[i].state == GIT_CP_WORKING &&
            pp->children[i].err.len) {
                strbuf_write(&pp->children[i].err, stderr);
                strbuf_reset(&pp->children[i].err);
        }
}

static int pp_collect_finished(struct parallel_processes *pp,
                               const struct run_process_parallel_opts *opts)
{
        int code;
        size_t i;
        int result = 0;

        while (pp->nr_processes > 0) {
                for (i = 0; i < opts->processes; i++)
                        if (pp->children[i].state == GIT_CP_WAIT_CLEANUP)
                                break;
                if (i == opts->processes)
                        break;

                code = finish_command(&pp->children[i].process);

                if (opts->task_finished)
                        code = opts->task_finished(code, opts->ungroup ? NULL :
                                                   &pp->children[i].err, opts->data,
                                                   pp->children[i].data);
                else
                        code = 0;

                if (code)
                        result = code;
                if (code < 0)
                        break;

                pp->nr_processes--;
                pp->children[i].state = GIT_CP_FREE;
                if (pp->pfd)
                        pp->pfd[i].fd = -1;
                child_process_init(&pp->children[i].process);

                if (opts->ungroup) {
                        ; /* no strbuf_*() work to do here */
                } else if (i != pp->output_owner) {
                        strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
                        strbuf_reset(&pp->children[i].err);
                } else {
                        const size_t n = opts->processes;

                        strbuf_write(&pp->children[i].err, stderr);
                        strbuf_reset(&pp->children[i].err);

                        /* Output all other finished child processes */
                        strbuf_write(&pp->buffered_output, stderr);
                        strbuf_reset(&pp->buffered_output);

                        /*
                         * Pick next process to output live.
                         * NEEDSWORK:
                         * For now we pick it randomly by doing a round
                         * robin. Later we may want to pick the one with
                         * the most output or the longest or shortest
                         * running process time.
                         */
                        for (i = 0; i < n; i++)
                                if (pp->children[(pp->output_owner + i) % n].state == GIT_CP_WORKING)
                                        break;
                        pp->output_owner = (pp->output_owner + i) % n;
                }
        }
        return result;
}

void run_processes_parallel(const struct run_process_parallel_opts *opts)
{
        int i, code;
        int output_timeout = 100;
        int spawn_cap = 4;
        struct parallel_processes_for_signal pp_sig;
        struct parallel_processes pp = {
                .buffered_output = STRBUF_INIT,
        };
        /* options */
        const char *tr2_category = opts->tr2_category;
        const char *tr2_label = opts->tr2_label;
        const int do_trace2 = tr2_category && tr2_label;

        if (do_trace2)
                trace2_region_enter_printf(tr2_category, tr2_label, NULL,
                                           "max:%d", opts->processes);

        pp_init(&pp, opts, &pp_sig);
        while (1) {
                for (i = 0;
                    i < spawn_cap && !pp.shutdown &&
                    pp.nr_processes < opts->processes;
                    i++) {
                        code = pp_start_one(&pp, opts);
                        if (!code)
                                continue;
                        if (code < 0) {
                                pp.shutdown = 1;
                                kill_children(&pp, opts, -code);
                        }
                        break;
                }
                if (!pp.nr_processes)
                        break;
                if (opts->ungroup) {
                        for (size_t i = 0; i < opts->processes; i++)
                                pp.children[i].state = GIT_CP_WAIT_CLEANUP;
                } else {
                        pp_buffer_stderr(&pp, opts, output_timeout);
                        pp_output(&pp);
                }
                code = pp_collect_finished(&pp, opts);
                if (code) {
                        pp.shutdown = 1;
                        if (code < 0)
                                kill_children(&pp, opts,-code);
                }
        }

        pp_cleanup(&pp, opts);

        if (do_trace2)
                trace2_region_leave(tr2_category, tr2_label, NULL);
}

int run_auto_maintenance(int quiet)
{
        int enabled;
        struct child_process maint = CHILD_PROCESS_INIT;

        if (!git_config_get_bool("maintenance.auto", &enabled) &&
            !enabled)
                return 0;

        maint.git_cmd = 1;
        maint.close_object_store = 1;
        strvec_pushl(&maint.args, "maintenance", "run", "--auto", NULL);
        strvec_push(&maint.args, quiet ? "--quiet" : "--no-quiet");

        return run_command(&maint);
}

void prepare_other_repo_env(struct strvec *env, const char *new_git_dir)
{
        const char * const *var;

        for (var = local_repo_env; *var; var++) {
                if (strcmp(*var, CONFIG_DATA_ENVIRONMENT) &&
                    strcmp(*var, CONFIG_COUNT_ENVIRONMENT))
                        strvec_push(env, *var);
        }
        strvec_pushf(env, "%s=%s", GIT_DIR_ENVIRONMENT, new_git_dir);
}

enum start_bg_result start_bg_command(struct child_process *cmd,
                                      start_bg_wait_cb *wait_cb,
                                      void *cb_data,
                                      unsigned int timeout_sec)
{
        enum start_bg_result sbgr = SBGR_ERROR;
        int ret;
        int wait_status;
        pid_t pid_seen;
        time_t time_limit;

        /*
         * We do not allow clean-on-exit because the child process
         * should persist in the background and possibly/probably
         * after this process exits.  So we don't want to kill the
         * child during our atexit routine.
         */
        if (cmd->clean_on_exit)
                BUG("start_bg_command() does not allow non-zero clean_on_exit");

        if (!cmd->trace2_child_class)
                cmd->trace2_child_class = "background";

        ret = start_command(cmd);
        if (ret) {
                /*
                 * We assume that if `start_command()` fails, we
                 * either get a complete `trace2_child_start() /
                 * trace2_child_exit()` pair or it fails before the
                 * `trace2_child_start()` is emitted, so we do not
                 * need to worry about it here.
                 *
                 * We also assume that `start_command()` does not add
                 * us to the cleanup list.  And that it calls
                 * `child_process_clear()`.
                 */
                sbgr = SBGR_ERROR;
                goto done;
        }

        time(&time_limit);
        time_limit += timeout_sec;

wait:
        pid_seen = waitpid(cmd->pid, &wait_status, WNOHANG);

        if (!pid_seen) {
                /*
                 * The child is currently running.  Ask the callback
                 * if the child is ready to do work or whether we
                 * should keep waiting for it to boot up.
                 */
                ret = (*wait_cb)(cmd, cb_data);
                if (!ret) {
                        /*
                         * The child is running and "ready".
                         */
                        trace2_child_ready(cmd, "ready");
                        sbgr = SBGR_READY;
                        goto done;
                } else if (ret > 0) {
                        /*
                         * The callback said to give it more time to boot up
                         * (subject to our timeout limit).
                         */
                        time_t now;

                        time(&now);
                        if (now < time_limit)
                                goto wait;

                        /*
                         * Our timeout has expired.  We don't try to
                         * kill the child, but rather let it continue
                         * (hopefully) trying to startup.
                         */
                        trace2_child_ready(cmd, "timeout");
                        sbgr = SBGR_TIMEOUT;
                        goto done;
                } else {
                        /*
                         * The cb gave up on this child.  It is still running,
                         * but our cb got an error trying to probe it.
                         */
                        trace2_child_ready(cmd, "error");
                        sbgr = SBGR_CB_ERROR;
                        goto done;
                }
        }

        else if (pid_seen == cmd->pid) {
                int child_code = -1;

                /*
                 * The child started, but exited or was terminated
                 * before becoming "ready".
                 *
                 * We try to match the behavior of `wait_or_whine()`
                 * WRT the handling of WIFSIGNALED() and WIFEXITED()
                 * and convert the child's status to a return code for
                 * tracing purposes and emit the `trace2_child_exit()`
                 * event.
                 *
                 * We do not want the wait_or_whine() error message
                 * because we will be called by client-side library
                 * routines.
                 */
                if (WIFEXITED(wait_status))
                        child_code = WEXITSTATUS(wait_status);
                else if (WIFSIGNALED(wait_status))
                        child_code = WTERMSIG(wait_status) + 128;
                trace2_child_exit(cmd, child_code);

                sbgr = SBGR_DIED;
                goto done;
        }

        else if (pid_seen < 0 && errno == EINTR)
                goto wait;

        trace2_child_exit(cmd, -1);
        sbgr = SBGR_ERROR;

done:
        child_process_clear(cmd);
        invalidate_lstat_cache();
        return sbgr;
}