SubmittingPatches: reword awkward phrasing

[alt-git.git] / run-command.h

#ifndef RUN_COMMAND_H
#define RUN_COMMAND_H

#include "thread-utils.h"

#include "strvec.h"

/**
 * The run-command API offers a versatile tool to run sub-processes with
 * redirected input and output as well as with a modified environment
 * and an alternate current directory.
 *
 * A similar API offers the capability to run a function asynchronously,
 * which is primarily used to capture the output that the function
 * produces in the caller in order to process it.
 */


/**
 * This describes the arguments, redirections, and environment of a
 * command to run in a sub-process.
 *
 * The caller:
 *
 * 1. allocates and clears (using child_process_init() or
 *    CHILD_PROCESS_INIT) a struct child_process variable;
 * 2. initializes the members;
 * 3. calls start_command();
 * 4. processes the data;
 * 5. closes file descriptors (if necessary; see below);
 * 6. calls finish_command().
 *
 * Special forms of redirection are available by setting these members
 * to 1:
 *
 *  .no_stdin, .no_stdout, .no_stderr: The respective channel is
 *              redirected to /dev/null.
 *
 *      .stdout_to_stderr: stdout of the child is redirected to its
 *              stderr. This happens after stderr is itself redirected.
 *              So stdout will follow stderr to wherever it is
 *              redirected.
 */
struct child_process {

        /**
         * The .args is a `struct strvec', use that API to manipulate
         * it, e.g. strvec_pushv() to add an existing "const char **"
         * vector.
         *
         * If the command to run is a git command, set the first
         * element in the strvec to the command name without the
         * 'git-' prefix and set .git_cmd = 1.
         *
         * The memory in .args will be cleaned up automatically during
         * `finish_command` (or during `start_command` when it is unsuccessful).
         */
        struct strvec args;

        /**
         * Like .args the .env is a `struct strvec'.
         *
         * To modify the environment of the sub-process, specify an array of
         * environment settings. Each string in the array manipulates the
         * environment.
         *
         * - If the string is of the form "VAR=value", i.e. it contains '='
         *   the variable is added to the child process's environment.
         *
         * - If the string does not contain '=', it names an environment
         *   variable that will be removed from the child process's environment.
         *
         * The memory in .env will be cleaned up automatically during
         * `finish_command` (or during `start_command` when it is unsuccessful).
         */
        struct strvec env;
        pid_t pid;

        int trace2_child_id;
        uint64_t trace2_child_us_start;
        const char *trace2_child_class;
        const char *trace2_hook_name;

        /*
         * Using .in, .out, .err:
         * - Specify 0 for no redirections. No new file descriptor is allocated.
         * (child inherits stdin, stdout, stderr from parent).
         * - Specify -1 to have a pipe allocated as follows:
         *     .in: returns the writable pipe end; parent writes to it,
         *          the readable pipe end becomes child's stdin
         *     .out, .err: returns the readable pipe end; parent reads from
         *          it, the writable pipe end becomes child's stdout/stderr
         *   The caller of start_command() must close the returned FDs
         *   after it has completed reading from/writing to it!
         * - Specify > 0 to set a channel to a particular FD as follows:
         *     .in: a readable FD, becomes child's stdin
         *     .out: a writable FD, becomes child's stdout/stderr
         *     .err: a writable FD, becomes child's stderr
         *   The specified FD is closed by start_command(), even in case
         *   of errors!
         */
        int in;
        int out;
        int err;

        /**
         * To specify a new initial working directory for the sub-process,
         * specify it in the .dir member.
         */
        const char *dir;

        unsigned no_stdin:1;
        unsigned no_stdout:1;
        unsigned no_stderr:1;
        unsigned git_cmd:1; /* if this is to be git sub-command */

        /**
         * If the program cannot be found, the functions return -1 and set
         * errno to ENOENT. Normally, an error message is printed, but if
         * .silent_exec_failure is set to 1, no message is printed for this
         * special error condition.
         */
        unsigned silent_exec_failure:1;

        /**
         * Run the command from argv[0] using a shell (but note that we may
         * still optimize out the shell call if the command contains no
         * metacharacters). Note that further arguments to the command in
         * argv[1], etc, do not need to be shell-quoted.
         */
        unsigned use_shell:1;

        /**
         * Release any open file handles to the object store before running
         * the command; This is necessary e.g. when the spawned process may
         * want to repack because that would delete `.pack` files (and on
         * Windows, you cannot delete files that are still in use).
         */
        unsigned close_object_store:1;

        unsigned stdout_to_stderr:1;
        unsigned clean_on_exit:1;
        unsigned wait_after_clean:1;
        void (*clean_on_exit_handler)(struct child_process *process);
};

#define CHILD_PROCESS_INIT { \
        .args = STRVEC_INIT, \
        .env = STRVEC_INIT, \
}

/**
 * The functions: start_command, finish_command, run_command do the following:
 *
 * - If a system call failed, errno is set and -1 is returned. A diagnostic
 *   is printed.
 *
 * - If the program was not found, then -1 is returned and errno is set to
 *   ENOENT; a diagnostic is printed only if .silent_exec_failure is 0.
 *
 * - Otherwise, the program is run. If it terminates regularly, its exit
 *   code is returned. No diagnostic is printed, even if the exit code is
 *   non-zero.
 *
 * - If the program terminated due to a signal, then the return value is the
 *   signal number + 128, ie. the same value that a POSIX shell's $? would
 *   report.  A diagnostic is printed.
 *
 */

/**
 * Initialize a struct child_process variable.
 */
void child_process_init(struct child_process *);

/**
 * Release the memory associated with the struct child_process.
 * Most users of the run-command API don't need to call this
 * function explicitly because `start_command` invokes it on
 * failure and `finish_command` calls it automatically already.
 */
void child_process_clear(struct child_process *);

int is_executable(const char *name);

/**
 * Check if the command exists on $PATH. 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 command contains no directory separators.
 *
 * Returns 1 if it is found in $PATH or 0 if the command could not be found.
 */
int exists_in_PATH(const char *command);

/**
 * Start a sub-process. Takes a pointer to a `struct child_process`
 * that specifies the details and returns pipe FDs (if requested).
 * See below for details.
 */
int start_command(struct child_process *);

/**
 * Wait for the completion of a sub-process that was started with
 * start_command().
 */
int finish_command(struct child_process *);

int finish_command_in_signal(struct child_process *);

/**
 * A convenience function that encapsulates a sequence of
 * start_command() followed by finish_command(). Takes a pointer
 * to a `struct child_process` that specifies the details.
 */
int run_command(struct child_process *);

/*
 * Trigger an auto-gc
 */
int run_auto_maintenance(int quiet);

/**
 * Execute the given command, sending "in" to its stdin, and capturing its
 * stdout and stderr in the "out" and "err" strbufs. Any of the three may
 * be NULL to skip processing.
 *
 * Returns -1 if starting the command fails or reading fails, and otherwise
 * returns the exit code of the command. Any output collected in the
 * buffers is kept even if the command returns a non-zero exit. The hint fields
 * gives starting sizes for the strbuf allocations.
 *
 * The fields of "cmd" should be set up as they would for a normal run_command
 * invocation. But note that there is no need to set the in, out, or err
 * fields; pipe_command handles that automatically.
 */
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);

/**
 * Convenience wrapper around pipe_command for the common case
 * of capturing only stdout.
 */
static inline int capture_command(struct child_process *cmd,
                                  struct strbuf *out,
                                  size_t hint)
{
        return pipe_command(cmd, NULL, 0, out, hint, NULL, 0);
}

/*
 * The purpose of the following functions is to feed a pipe by running
 * a function asynchronously and providing output that the caller reads.
 *
 * It is expected that no synchronization and mutual exclusion between
 * the caller and the feed function is necessary so that the function
 * can run in a thread without interfering with the caller.
 *
 * The caller:
 *
 * 1. allocates and clears (memset(&asy, 0, sizeof(asy));) a
 *    struct async variable;
 * 2. initializes .proc and .data;
 * 3. calls start_async();
 * 4. processes communicates with proc through .in and .out;
 * 5. closes .in and .out;
 * 6. calls finish_async().
 *
 * There are serious restrictions on what the asynchronous function can do
 * because this facility is implemented by a thread in the same address
 * space on most platforms (when pthreads is available), but by a pipe to
 * a forked process otherwise:
 *
 * - It cannot change the program's state (global variables, environment,
 *   etc.) in a way that the caller notices; in other words, .in and .out
 *   are the only communication channels to the caller.
 *
 * - It must not change the program's state that the caller of the
 *   facility also uses.
 *
 */
struct async {

        /**
         * The function pointer in .proc has the following signature:
         *
         *      int proc(int in, int out, void *data);
         *
         * - in, out specifies a set of file descriptors to which the function
         *  must read/write the data that it needs/produces.  The function
         *  *must* close these descriptors before it returns.  A descriptor
         *  may be -1 if the caller did not configure a descriptor for that
         *  direction.
         *
         * - data is the value that the caller has specified in the .data member
         *  of struct async.
         *
         * - The return value of the function is 0 on success and non-zero
         *  on failure. If the function indicates failure, finish_async() will
         *  report failure as well.
         *
         */
        int (*proc)(int in, int out, void *data);

        void *data;

        /**
         * The members .in, .out are used to provide a set of fd's for
         * communication between the caller and the callee as follows:
         *
         * - Specify 0 to have no file descriptor passed.  The callee will
         *   receive -1 in the corresponding argument.
         *
         * - Specify < 0 to have a pipe allocated; start_async() replaces
         *   with the pipe FD in the following way:
         *
         *      .in: Returns the writable pipe end into which the caller
         *      writes; the readable end of the pipe becomes the function's
         *      in argument.
         *
         *      .out: Returns the readable pipe end from which the caller
         *      reads; the writable end of the pipe becomes the function's
         *      out argument.
         *
         *   The caller of start_async() must close the returned FDs after it
         *   has completed reading from/writing from them.
         *
         * - Specify a file descriptor > 0 to be used by the function:
         *
         *      .in: The FD must be readable; it becomes the function's in.
         *      .out: The FD must be writable; it becomes the function's out.
         *
         *   The specified FD is closed by start_async(), even if it fails to
         *   run the function.
         */
        int in;         /* caller writes here and closes it */
        int out;        /* caller reads from here and closes it */
#ifdef NO_PTHREADS
        pid_t pid;
#else
        pthread_t tid;
        int proc_in;
        int proc_out;
#endif
        int isolate_sigpipe;
};

/**
 * Run a function asynchronously. Takes a pointer to a `struct
 * async` that specifies the details and returns a set of pipe FDs
 * for communication with the function. See below for details.
 */
int start_async(struct async *async);

/**
 * Wait for the completion of an asynchronous function that was
 * started with start_async().
 */
int finish_async(struct async *async);

int in_async(void);
int async_with_fork(void);
void check_pipe(int err);

/**
 * This callback should initialize the child process and preload the
 * error channel if desired. The preloading of is useful if you want to
 * have a message printed directly before the output of the child process.
 * pp_cb is the callback cookie as passed to run_processes_parallel.
 * You can store a child process specific callback cookie in pp_task_cb.
 *
 * See run_processes_parallel() below for a discussion of the "struct
 * strbuf *out" parameter.
 *
 * Even after returning 0 to indicate that there are no more processes,
 * this function will be called again until there are no more running
 * child processes.
 *
 * Return 1 if the next child is ready to run.
 * Return 0 if there are currently no more tasks to be processed.
 * To send a signal to other child processes for abortion,
 * return the negative signal number.
 */
typedef int (*get_next_task_fn)(struct child_process *cp,
                                struct strbuf *out,
                                void *pp_cb,
                                void **pp_task_cb);

/**
 * This callback is called whenever there are problems starting
 * a new process.
 *
 * See run_processes_parallel() below for a discussion of the "struct
 * strbuf *out" parameter.
 *
 * pp_cb is the callback cookie as passed into run_processes_parallel,
 * pp_task_cb is the callback cookie as passed into get_next_task_fn.
 *
 * Return 0 to continue the parallel processing. To abort return non zero.
 * To send a signal to other child processes for abortion, return
 * the negative signal number.
 */
typedef int (*start_failure_fn)(struct strbuf *out,
                                void *pp_cb,
                                void *pp_task_cb);

/**
 * This callback is called on every child process that finished processing.
 *
 * See run_processes_parallel() below for a discussion of the "struct
 * strbuf *out" parameter.
 *
 * pp_cb is the callback cookie as passed into run_processes_parallel,
 * pp_task_cb is the callback cookie as passed into get_next_task_fn.
 *
 * Return 0 to continue the parallel processing.  To abort return non zero.
 * To send a signal to other child processes for abortion, return
 * the negative signal number.
 */
typedef int (*task_finished_fn)(int result,
                                struct strbuf *out,
                                void *pp_cb,
                                void *pp_task_cb);

/**
 * Option used by run_processes_parallel(), { 0 }-initialized means no
 * options.
 */
struct run_process_parallel_opts
{
        /**
         * tr2_category & tr2_label: sets the trace2 category and label for
         * logging. These must either be unset, or both of them must be set.
         */
        const char *tr2_category;
        const char *tr2_label;

        /**
         * processes: see 'processes' in run_processes_parallel() below.
         */
        size_t processes;

        /**
         * ungroup: see 'ungroup' in run_processes_parallel() below.
         */
        unsigned int ungroup:1;

        /**
         * get_next_task: See get_next_task_fn() above. This must be
         * specified.
         */
        get_next_task_fn get_next_task;

        /**
         * start_failure: See start_failure_fn() above. This can be
         * NULL to omit any special handling.
         */
        start_failure_fn start_failure;

        /**
         * task_finished: See task_finished_fn() above. This can be
         * NULL to omit any special handling.
         */
        task_finished_fn task_finished;

        /**
         * data: user data, will be passed as "pp_cb" to the callback
         * parameters.
         */
        void *data;
};

/**
 * Options are passed via the "struct run_process_parallel_opts" above.
 *
 * Runs N 'processes' at the same time. Whenever a process can be
 * started, the callback opts.get_next_task is called to obtain the data
 * required to start another child process.
 *
 * The children started via this function run in parallel. Their output
 * (both stdout and stderr) is routed to stderr in a manner that output
 * from different tasks does not interleave (but see "ungroup" below).
 *
 * If the "ungroup" option isn't specified, the API will set the
 * "stdout_to_stderr" parameter in "struct child_process" and provide
 * the callbacks with a "struct strbuf *out" parameter to write output
 * to. In this case the callbacks must not write to stdout or
 * stderr as such output will mess up the output of the other parallel
 * processes. If "ungroup" option is specified callbacks will get a
 * NULL "struct strbuf *out" parameter, and are responsible for
 * emitting their own output, including dealing with any race
 * conditions due to writing in parallel to stdout and stderr.
 */
void run_processes_parallel(const struct run_process_parallel_opts *opts);

/**
 * Convenience function which prepares env for a command to be run in a
 * new repo. This adds all GIT_* environment variables to env with the
 * exception of GIT_CONFIG_PARAMETERS and GIT_CONFIG_COUNT (which cause the
 * corresponding environment variables to be unset in the subprocess) and adds
 * an environment variable pointing to new_git_dir. See local_repo_env in
 * cache.h for more information.
 */
void prepare_other_repo_env(struct strvec *env, const char *new_git_dir);

/**
 * Possible return values for start_bg_command().
 */
enum start_bg_result {
        /* child process is "ready" */
        SBGR_READY = 0,

        /* child process could not be started */
        SBGR_ERROR,

        /* callback error when testing for "ready" */
        SBGR_CB_ERROR,

        /* timeout expired waiting for child to become "ready" */
        SBGR_TIMEOUT,

        /* child process exited or was signalled before becomming "ready" */
        SBGR_DIED,
};

/**
 * Callback used by start_bg_command() to ask whether the
 * child process is ready or needs more time to become "ready".
 *
 * The callback will receive the cmd and cb_data arguments given to
 * start_bg_command().
 *
 * Returns 1 is child needs more time (subject to the requested timeout).
 * Returns 0 if child is "ready".
 * Returns -1 on any error and cause start_bg_command() to also error out.
 */
typedef int(start_bg_wait_cb)(const struct child_process *cmd, void *cb_data);

/**
 * Start a command in the background.  Wait long enough for the child
 * to become "ready" (as defined by the provided callback).  Capture
 * immediate errors (like failure to start) and any immediate exit
 * status (such as a shutdown/signal before the child became "ready")
 * and return this like start_command().
 *
 * We run a custom wait loop using the provided callback to wait for
 * the child to start and become "ready".  This is limited by the given
 * timeout value.
 *
 * If the child does successfully start and become "ready", we orphan
 * it into the background.
 *
 * The caller must not call finish_command().
 *
 * The opaque cb_data argument will be forwarded to the callback for
 * any instance data that it might require.  This may be NULL.
 */
enum start_bg_result start_bg_command(struct child_process *cmd,
                                      start_bg_wait_cb *wait_cb,
                                      void *cb_data,
                                      unsigned int timeout_sec);

#endif