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911ec99b68
Something as simple as reading the stdout from a command
turns out to be rather hard to do right. Doing:
cmd.out = -1;
run_command(&cmd);
strbuf_read(&buf, cmd.out, 0);
can result in deadlock if the child process produces a large
amount of output. What happens is:
1. The parent spawns the child with its stdout connected
to a pipe, of which the parent is the sole reader.
2. The parent calls wait(), blocking until the child exits.
3. The child writes to stdout. If it writes more data than
the OS pipe buffer can hold, the write() call will
block.
This is a deadlock; the parent is waiting for the child to
exit, and the child is waiting for the parent to call
read().
So we might try instead:
start_command(&cmd);
strbuf_read(&buf, cmd.out, 0);
finish_command(&cmd);
But that is not quite right either. We are examining cmd.out
and running finish_command whether start_command succeeded
or not, which is wrong. Moreover, these snippets do not do
any error handling. If our read() fails, we must make sure
to still call finish_command (to reap the child process).
And both snippets failed to close the cmd.out descriptor,
which they must do (provided start_command succeeded).
Let's introduce a run-command helper that can make this a
bit simpler for callers to get right.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
117 lines
3.8 KiB
C
117 lines
3.8 KiB
C
#ifndef RUN_COMMAND_H
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#define RUN_COMMAND_H
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#ifndef NO_PTHREADS
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#include <pthread.h>
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#endif
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#include "argv-array.h"
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struct child_process {
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const char **argv;
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struct argv_array args;
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struct argv_array env_array;
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pid_t pid;
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/*
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* Using .in, .out, .err:
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* - Specify 0 for no redirections (child inherits stdin, stdout,
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* stderr from parent).
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* - Specify -1 to have a pipe allocated as follows:
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* .in: returns the writable pipe end; parent writes to it,
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* the readable pipe end becomes child's stdin
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* .out, .err: returns the readable pipe end; parent reads from
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* it, the writable pipe end becomes child's stdout/stderr
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* The caller of start_command() must close the returned FDs
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* after it has completed reading from/writing to it!
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* - Specify > 0 to set a channel to a particular FD as follows:
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* .in: a readable FD, becomes child's stdin
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* .out: a writable FD, becomes child's stdout/stderr
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* .err: a writable FD, becomes child's stderr
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* The specified FD is closed by start_command(), even in case
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* of errors!
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*/
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int in;
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int out;
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int err;
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const char *dir;
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const char *const *env;
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unsigned no_stdin:1;
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unsigned no_stdout:1;
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unsigned no_stderr:1;
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unsigned git_cmd:1; /* if this is to be git sub-command */
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unsigned silent_exec_failure:1;
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unsigned stdout_to_stderr:1;
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unsigned use_shell:1;
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unsigned clean_on_exit:1;
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};
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#define CHILD_PROCESS_INIT { NULL, ARGV_ARRAY_INIT, ARGV_ARRAY_INIT }
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void child_process_init(struct child_process *);
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int start_command(struct child_process *);
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int finish_command(struct child_process *);
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int run_command(struct child_process *);
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extern char *find_hook(const char *name);
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LAST_ARG_MUST_BE_NULL
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extern int run_hook_le(const char *const *env, const char *name, ...);
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extern int run_hook_ve(const char *const *env, const char *name, va_list args);
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#define RUN_COMMAND_NO_STDIN 1
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#define RUN_GIT_CMD 2 /*If this is to be git sub-command */
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#define RUN_COMMAND_STDOUT_TO_STDERR 4
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#define RUN_SILENT_EXEC_FAILURE 8
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#define RUN_USING_SHELL 16
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#define RUN_CLEAN_ON_EXIT 32
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int run_command_v_opt(const char **argv, int opt);
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/*
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* env (the environment) is to be formatted like environ: "VAR=VALUE".
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* To unset an environment variable use just "VAR".
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*/
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int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const char *const *env);
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/**
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* Execute the given command, capturing its stdout in the given strbuf.
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* Returns -1 if starting the command fails or reading fails, and otherwise
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* returns the exit code of the command. The output collected in the
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* buffer is kept even if the command returns a non-zero exit. The hint field
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* gives a starting size for the strbuf allocation.
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*
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* The fields of "cmd" should be set up as they would for a normal run_command
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* invocation. But note that there is no need to set cmd->out; the function
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* sets it up for the caller.
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*/
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int capture_command(struct child_process *cmd, struct strbuf *buf, size_t hint);
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/*
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* The purpose of the following functions is to feed a pipe by running
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* a function asynchronously and providing output that the caller reads.
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*
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* It is expected that no synchronization and mutual exclusion between
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* the caller and the feed function is necessary so that the function
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* can run in a thread without interfering with the caller.
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*/
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struct async {
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/*
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* proc reads from in; closes it before return
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* proc writes to out; closes it before return
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* returns 0 on success, non-zero on failure
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*/
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int (*proc)(int in, int out, void *data);
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void *data;
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int in; /* caller writes here and closes it */
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int out; /* caller reads from here and closes it */
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#ifdef NO_PTHREADS
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pid_t pid;
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#else
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pthread_t tid;
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int proc_in;
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int proc_out;
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#endif
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};
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int start_async(struct async *async);
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int finish_async(struct async *async);
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#endif
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