graph: use SWAP macro

[git/raj.git] / run-command.c

#include "cache.h"
#include "run-command.h"
#include "exec_cmd.h"
#include "sigchain.h"
#include "argv-array.h"
#include "thread-utils.h"
#include "strbuf.h"

void child_process_init(struct child_process *child)
{
        memset(child, 0, sizeof(*child));
        argv_array_init(&child->args);
        argv_array_init(&child->env_array);
}

void child_process_clear(struct child_process *child)
{
        argv_array_clear(&child->args);
        argv_array_clear(&child->env_array);
}

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->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]);
}

#ifndef GIT_WINDOWS_NATIVE
static inline void dup_devnull(int to)
{
        int fd = open("/dev/null", O_RDWR);
        if (fd < 0)
                die_errno(_("open /dev/null failed"));
        if (dup2(fd, to) < 0)
                die_errno(_("dup2(%d,%d) failed"), fd, to);
        close(fd);
}
#endif

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 (!access(buf.buf, F_OK))
                        return strbuf_detach(&buf, NULL);

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

        strbuf_release(&buf);
        return NULL;
}

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

int sane_execvp(const char *file, char * const argv[])
{
        if (!execvp(file, argv))
                return 0; /* cannot happen ;-) */

        /*
         * 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 argv_array *out, const char **argv)
{
        if (!argv[0])
                die("BUG: shell command is empty");

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

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

        argv_array_pushv(out, argv);
        return out->argv;
}

#ifndef GIT_WINDOWS_NATIVE
static int execv_shell_cmd(const char **argv)
{
        struct argv_array nargv = ARGV_ARRAY_INIT;
        prepare_shell_cmd(&nargv, argv);
        trace_argv_printf(nargv.argv, "trace: exec:");
        sane_execvp(nargv.argv[0], (char **)nargv.argv);
        argv_array_clear(&nargv);
        return -1;
}
#endif

#ifndef GIT_WINDOWS_NATIVE
static int child_notifier = -1;

static void notify_parent(void)
{
        /*
         * execvp failed.  If possible, we'd like to let start_command
         * know, so failures like ENOENT can be handled right away; but
         * otherwise, finish_command will still report the error.
         */
        xwrite(child_notifier, "", 1);
}
#endif

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 (in_signal)
                return 0;

        if (waiting < 0) {
                failed_errno = errno;
                error_errno("waitpid for %s failed", argv0);
        } else if (waiting != pid) {
                error("waitpid is confused (%s)", argv0);
        } else if (WIFSIGNALED(status)) {
                code = WTERMSIG(status);
                if (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);
                /*
                 * Convert special exit code when execvp failed.
                 */
                if (code == 127) {
                        code = -1;
                        failed_errno = ENOENT;
                }
        } else {
                error("waitpid is confused (%s)", argv0);
        }

        clear_child_for_cleanup(pid);

        errno = failed_errno;
        return code;
}

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;

        if (!cmd->argv)
                cmd->argv = cmd->args.argv;
        if (!cmd->env)
                cmd->env = cmd->env_array.argv;

        /*
         * 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->argv[0], strerror(failed_errno));
                        child_process_clear(cmd);
                        errno = failed_errno;
                        return -1;
                }
                cmd->err = fderr[0];
        }

        trace_argv_printf(cmd->argv, "trace: run_command:");
        fflush(NULL);

#ifndef GIT_WINDOWS_NATIVE
{
        int notify_pipe[2];
        if (pipe(notify_pipe))
                notify_pipe[0] = notify_pipe[1] = -1;

        cmd->pid = fork();
        failed_errno = errno;
        if (!cmd->pid) {
                /*
                 * Redirect the channel to write syscall error messages to
                 * before redirecting the process's stderr so that all die()
                 * in subsequent call paths use the parent's stderr.
                 */
                if (cmd->no_stderr || need_err) {
                        int child_err = dup(2);
                        set_cloexec(child_err);
                        set_error_handle(fdopen(child_err, "w"));
                }

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

                if (cmd->no_stdin)
                        dup_devnull(0);
                else if (need_in) {
                        dup2(fdin[0], 0);
                        close_pair(fdin);
                } else if (cmd->in) {
                        dup2(cmd->in, 0);
                        close(cmd->in);
                }

                if (cmd->no_stderr)
                        dup_devnull(2);
                else if (need_err) {
                        dup2(fderr[1], 2);
                        close_pair(fderr);
                } else if (cmd->err > 1) {
                        dup2(cmd->err, 2);
                        close(cmd->err);
                }

                if (cmd->no_stdout)
                        dup_devnull(1);
                else if (cmd->stdout_to_stderr)
                        dup2(2, 1);
                else if (need_out) {
                        dup2(fdout[1], 1);
                        close_pair(fdout);
                } else if (cmd->out > 1) {
                        dup2(cmd->out, 1);
                        close(cmd->out);
                }

                if (cmd->dir && chdir(cmd->dir))
                        die_errno("exec '%s': cd to '%s' failed", cmd->argv[0],
                            cmd->dir);
                if (cmd->env) {
                        for (; *cmd->env; cmd->env++) {
                                if (strchr(*cmd->env, '='))
                                        putenv((char *)*cmd->env);
                                else
                                        unsetenv(*cmd->env);
                        }
                }
                if (cmd->git_cmd)
                        execv_git_cmd(cmd->argv);
                else if (cmd->use_shell)
                        execv_shell_cmd(cmd->argv);
                else
                        sane_execvp(cmd->argv[0], (char *const*) cmd->argv);
                if (errno == ENOENT) {
                        if (!cmd->silent_exec_failure)
                                error("cannot run %s: %s", cmd->argv[0],
                                        strerror(ENOENT));
                        exit(127);
                } else {
                        die_errno("cannot exec '%s'", cmd->argv[0]);
                }
        }
        if (cmd->pid < 0)
                error_errno("cannot fork() for %s", cmd->argv[0]);
        else if (cmd->clean_on_exit)
                mark_child_for_cleanup(cmd->pid, cmd);

        /*
         * Wait for child's execvp. If the execvp succeeds (or if fork()
         * failed), EOF is seen immediately by the parent. Otherwise, the
         * child process sends a single byte.
         * Note that use of this infrastructure is completely advisory,
         * therefore, we keep error checks minimal.
         */
        close(notify_pipe[1]);
        if (read(notify_pipe[0], &notify_pipe[1], 1) == 1) {
                /*
                 * At this point we know that fork() succeeded, but execvp()
                 * failed. Errors have been reported to our stderr.
                 */
                wait_or_whine(cmd->pid, cmd->argv[0], 0);
                failed_errno = errno;
                cmd->pid = -1;
        }
        close(notify_pipe[0]);
}
#else
{
        int fhin = 0, fhout = 1, fherr = 2;
        const char **sargv = cmd->argv;
        struct argv_array nargv = ARGV_ARRAY_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->argv = prepare_git_cmd(&nargv, cmd->argv);
        else if (cmd->use_shell)
                cmd->argv = prepare_shell_cmd(&nargv, cmd->argv);

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

        argv_array_clear(&nargv);
        cmd->argv = sargv;
        if (fhin != 0)
                close(fhin);
        if (fhout != 1)
                close(fhout);
        if (fherr != 2)
                close(fherr);
}
#endif

        if (cmd->pid < 0) {
                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->argv[0], 0);
        child_process_clear(cmd);
        return ret;
}

int finish_command_in_signal(struct child_process *cmd)
{
        return wait_or_whine(cmd->pid, cmd->argv[0], 1);
}


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

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

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

int run_command_v_opt(const char **argv, int opt)
{
        return run_command_v_opt_cd_env(argv, opt, NULL, NULL);
}

int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const char *const *env)
{
        struct child_process cmd = CHILD_PROCESS_INIT;
        cmd.argv = argv;
        cmd.no_stdin = opt & RUN_COMMAND_NO_STDIN ? 1 : 0;
        cmd.git_cmd = opt & RUN_GIT_CMD ? 1 : 0;
        cmd.stdout_to_stderr = opt & RUN_COMMAND_STDOUT_TO_STDERR ? 1 : 0;
        cmd.silent_exec_failure = opt & RUN_SILENT_EXEC_FAILURE ? 1 : 0;
        cmd.use_shell = opt & RUN_USING_SHELL ? 1 : 0;
        cmd.clean_on_exit = opt & RUN_CLEAN_ON_EXIT ? 1 : 0;
        cmd.dir = dir;
        cmd.env = env;
        return run_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) < 0) {
                        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)
{
        vreportf("fatal: ", 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, (void *)1);
        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_errno("cannot create thread");
                        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
        return wait_or_whine(async->pid, "child process", 0);
#else
        void *ret = (void *)(intptr_t)(-1);

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

const char *find_hook(const char *name)
{
        static struct strbuf path = STRBUF_INIT;

        strbuf_reset(&path);
        strbuf_git_path(&path, "hooks/%s", name);
        if (access(path.buf, X_OK) < 0)
                return NULL;
        return path.buf;
}

int run_hook_ve(const char *const *env, const char *name, va_list args)
{
        struct child_process hook = CHILD_PROCESS_INIT;
        const char *p;

        p = find_hook(name);
        if (!p)
                return 0;

        argv_array_push(&hook.args, p);
        while ((p = va_arg(args, const char *)))
                argv_array_push(&hook.args, p);
        hook.env = env;
        hook.no_stdin = 1;
        hook.stdout_to_stderr = 1;

        return run_command(&hook);
}

int run_hook_le(const char *const *env, const char *name, ...)
{
        va_list args;
        int ret;

        va_start(args, name);
        ret = run_hook_ve(env, name, args);
        va_end(args);

        return ret;
}

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 = xwrite(io->fd,
                                             io->u.out.buf, io->u.out.len);
                        if (len < 0) {
                                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) {
                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 {
        void *data;

        int max_processes;
        int nr_processes;

        get_next_task_fn get_next_task;
        start_failure_fn start_failure;
        task_finished_fn task_finished;

        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;

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

static int default_start_failure(struct strbuf *out,
                                 void *pp_cb,
                                 void *pp_task_cb)
{
        return 0;
}

static int default_task_finished(int result,
                                 struct strbuf *out,
                                 void *pp_cb,
                                 void *pp_task_cb)
{
        return 0;
}

static void kill_children(struct parallel_processes *pp, int signo)
{
        int i, n = pp->max_processes;

        for (i = 0; i < n; i++)
                if (pp->children[i].state == GIT_CP_WORKING)
                        kill(pp->children[i].process.pid, signo);
}

static struct parallel_processes *pp_for_signal;

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

static void pp_init(struct parallel_processes *pp,
                    int n,
                    get_next_task_fn get_next_task,
                    start_failure_fn start_failure,
                    task_finished_fn task_finished,
                    void *data)
{
        int i;

        if (n < 1)
                n = online_cpus();

        pp->max_processes = n;

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

        pp->data = data;
        if (!get_next_task)
                die("BUG: you need to specify a get_next_task function");
        pp->get_next_task = get_next_task;

        pp->start_failure = start_failure ? start_failure : default_start_failure;
        pp->task_finished = task_finished ? task_finished : default_task_finished;

        pp->nr_processes = 0;
        pp->output_owner = 0;
        pp->shutdown = 0;
        pp->children = xcalloc(n, sizeof(*pp->children));
        pp->pfd = xcalloc(n, sizeof(*pp->pfd));
        strbuf_init(&pp->buffered_output, 0);

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

        pp_for_signal = pp;
        sigchain_push_common(handle_children_on_signal);
}

static void pp_cleanup(struct parallel_processes *pp)
{
        int i;

        trace_printf("run_processes_parallel: done");
        for (i = 0; i < pp->max_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)
{
        int i, code;

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

        code = pp->get_next_task(&pp->children[i].process,
                                 &pp->children[i].err,
                                 pp->data,
                                 &pp->children[i].data);
        if (!code) {
                strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
                strbuf_reset(&pp->children[i].err);
                return 1;
        }
        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)) {
                code = pp->start_failure(&pp->children[i].err,
                                         pp->data,
                                         &pp->children[i].data);
                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;
        pp->pfd[i].fd = pp->children[i].process.err;
        return 0;
}

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

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

        /* Buffer output from all pipes. */
        for (i = 0; i < pp->max_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(struct parallel_processes *pp)
{
        int 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)
{
        int i, code;
        int n = pp->max_processes;
        int result = 0;

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

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

                code = pp->task_finished(code,
                                         &pp->children[i].err, pp->data,
                                         &pp->children[i].data);

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

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

                if (i != pp->output_owner) {
                        strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
                        strbuf_reset(&pp->children[i].err);
                } else {
                        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;
}

int run_processes_parallel(int n,
                           get_next_task_fn get_next_task,
                           start_failure_fn start_failure,
                           task_finished_fn task_finished,
                           void *pp_cb)
{
        int i, code;
        int output_timeout = 100;
        int spawn_cap = 4;
        struct parallel_processes pp;

        pp_init(&pp, n, get_next_task, start_failure, task_finished, pp_cb);
        while (1) {
                for (i = 0;
                    i < spawn_cap && !pp.shutdown &&
                    pp.nr_processes < pp.max_processes;
                    i++) {
                        code = pp_start_one(&pp);
                        if (!code)
                                continue;
                        if (code < 0) {
                                pp.shutdown = 1;
                                kill_children(&pp, -code);
                        }
                        break;
                }
                if (!pp.nr_processes)
                        break;
                pp_buffer_stderr(&pp, output_timeout);
                pp_output(&pp);
                code = pp_collect_finished(&pp);
                if (code) {
                        pp.shutdown = 1;
                        if (code < 0)
                                kill_children(&pp, -code);
                }
        }

        pp_cleanup(&pp);
        return 0;
}