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When a sub-command dies due to a signal, we encode the
signal number into the numeric exit status as "signal -
128". This is easy to identify (versus a regular positive
error code), and when cast to an unsigned integer (e.g., by
feeding it to exit), matches what a POSIX shell would return
when reporting a signal death in $? or through its own exit
code.
So we have a negative value inside the code, but once it
passes across an exit() barrier, it looks positive (and any
code we receive from a sub-shell will have the positive
form). E.g., death by SIGPIPE (signal 13) will look like
-115 to us in inside git, but will end up as 141 when we
call exit() with it. And a program killed by SIGPIPE but run
via the shell will come to us with an exit code of 141.
Unfortunately, this means that when the "use_shell" option
is set, we need to be on the lookout for _both_ forms. We
might or might not have actually invoked the shell (because
we optimize out some useless shell calls). If we didn't invoke
the shell, we will will see the sub-process's signal death
directly, and run-command converts it into a negative value.
But if we did invoke the shell, we will see the shell's
128+signal exit status. To be thorough, we would need to
check both, or cast the value to an unsigned char (after
checking that it is not -1, which is a magic error value).
Fortunately, most callsites do not care at all whether the
exit was from a code or from a signal; they merely check for
a non-zero status, and sometimes propagate the error via
exit(). But for the callers that do care, we can make life
slightly easier by just using the consistent positive form.
This actually fixes two minor bugs:
1. In launch_editor, we check whether the editor died from
SIGINT or SIGQUIT. But we checked only the negative
form, meaning that we would fail to notice a signal
death exit code which was propagated through the shell.
2. In handle_alias, we assume that a negative return value
from run_command means that errno tells us something
interesting (like a fork failure, or ENOENT).
Otherwise, we simply propagate the exit code. Negative
signal death codes confuse us, and we print a useless
"unable to run alias 'foo': Success" message. By
encoding signal deaths using the positive form, the
existing code just propagates it as it would a normal
non-zero exit code.
The downside is that callers of run_command can no longer
differentiate between a signal received directly by the
sub-process, and one propagated. However, no caller
currently cares, and since we already optimize out some
calls to the shell under the hood, that distinction is not
something that should be relied upon by callers.
Fix the same logic in t/test-terminal.perl for consistency [jc:
raised by Jonathan in the discussion].
Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
242 lines
8.1 KiB
Plaintext
242 lines
8.1 KiB
Plaintext
run-command API
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===============
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The run-command API offers a versatile tool to run sub-processes with
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redirected input and output as well as with a modified environment
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and an alternate current directory.
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A similar API offers the capability to run a function asynchronously,
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which is primarily used to capture the output that the function
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produces in the caller in order to process it.
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Functions
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---------
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`start_command`::
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Start a sub-process. Takes a pointer to a `struct child_process`
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that specifies the details and returns pipe FDs (if requested).
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See below for details.
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`finish_command`::
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Wait for the completion of a sub-process that was started with
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start_command().
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`run_command`::
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A convenience function that encapsulates a sequence of
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start_command() followed by finish_command(). Takes a pointer
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to a `struct child_process` that specifies the details.
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`run_command_v_opt`, `run_command_v_opt_cd_env`::
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Convenience functions that encapsulate a sequence of
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start_command() followed by finish_command(). The argument argv
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specifies the program and its arguments. The argument opt is zero
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or more of the flags `RUN_COMMAND_NO_STDIN`, `RUN_GIT_CMD`,
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`RUN_COMMAND_STDOUT_TO_STDERR`, or `RUN_SILENT_EXEC_FAILURE`
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that correspond to the members .no_stdin, .git_cmd,
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.stdout_to_stderr, .silent_exec_failure of `struct child_process`.
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The argument dir corresponds the member .dir. The argument env
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corresponds to the member .env.
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The functions above do the following:
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. If a system call failed, errno is set and -1 is returned. A diagnostic
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is printed.
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. If the program was not found, then -1 is returned and errno is set to
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ENOENT; a diagnostic is printed only if .silent_exec_failure is 0.
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. Otherwise, the program is run. If it terminates regularly, its exit
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code is returned. No diagnostic is printed, even if the exit code is
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non-zero.
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. If the program terminated due to a signal, then the return value is the
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signal number + 128, ie. the same value that a POSIX shell's $? would
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report. A diagnostic is printed.
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`start_async`::
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Run a function asynchronously. Takes a pointer to a `struct
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async` that specifies the details and returns a set of pipe FDs
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for communication with the function. See below for details.
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`finish_async`::
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Wait for the completion of an asynchronous function that was
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started with start_async().
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`run_hook`::
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Run a hook.
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The first argument is a pathname to an index file, or NULL
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if the hook uses the default index file or no index is needed.
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The second argument is the name of the hook.
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The further arguments correspond to the hook arguments.
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The last argument has to be NULL to terminate the arguments list.
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If the hook does not exist or is not executable, the return
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value will be zero.
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If it is executable, the hook will be executed and the exit
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status of the hook is returned.
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On execution, .stdout_to_stderr and .no_stdin will be set.
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(See below.)
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Data structures
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---------------
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* `struct child_process`
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This describes the arguments, redirections, and environment of a
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command to run in a sub-process.
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The caller:
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1. allocates and clears (memset(&chld, 0, sizeof(chld));) a
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struct child_process variable;
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2. initializes the members;
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3. calls start_command();
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4. processes the data;
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5. closes file descriptors (if necessary; see below);
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6. calls finish_command().
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The .argv member is set up as an array of string pointers (NULL
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terminated), of which .argv[0] is the program name to run (usually
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without a path). If the command to run is a git command, set argv[0] to
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the command name without the 'git-' prefix and set .git_cmd = 1.
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The members .in, .out, .err are used to redirect stdin, stdout,
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stderr as follows:
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. Specify 0 to request no special redirection. No new file descriptor
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is allocated. The child process simply inherits the channel from the
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parent.
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. Specify -1 to have a pipe allocated; start_command() replaces -1
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by the pipe FD in the following way:
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.in: Returns the writable pipe end into which the caller writes;
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the readable end of the pipe becomes the child's stdin.
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.out, .err: Returns the readable pipe end from which the caller
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reads; the writable end of the pipe end becomes child's
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stdout/stderr.
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The caller of start_command() must close the so returned FDs
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after it has completed reading from/writing to it!
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. Specify a file descriptor > 0 to be used by the child:
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.in: The FD must be readable; it becomes child's stdin.
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.out: The FD must be writable; it becomes child's stdout.
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.err: The FD must be writable; it becomes child's stderr.
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The specified FD is closed by start_command(), even if it fails to
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run the sub-process!
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. Special forms of redirection are available by setting these members
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to 1:
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.no_stdin, .no_stdout, .no_stderr: The respective channel is
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redirected to /dev/null.
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.stdout_to_stderr: stdout of the child is redirected to its
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stderr. This happens after stderr is itself redirected.
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So stdout will follow stderr to wherever it is
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redirected.
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To modify the environment of the sub-process, specify an array of
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string pointers (NULL terminated) in .env:
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. If the string is of the form "VAR=value", i.e. it contains '='
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the variable is added to the child process's environment.
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. If the string does not contain '=', it names an environment
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variable that will be removed from the child process's environment.
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To specify a new initial working directory for the sub-process,
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specify it in the .dir member.
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If the program cannot be found, the functions return -1 and set
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errno to ENOENT. Normally, an error message is printed, but if
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.silent_exec_failure is set to 1, no message is printed for this
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special error condition.
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* `struct async`
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This describes a function to run asynchronously, whose purpose is
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to produce output that the caller reads.
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The caller:
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1. allocates and clears (memset(&asy, 0, sizeof(asy));) a
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struct async variable;
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2. initializes .proc and .data;
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3. calls start_async();
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4. processes communicates with proc through .in and .out;
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5. closes .in and .out;
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6. calls finish_async().
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The members .in, .out are used to provide a set of fd's for
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communication between the caller and the callee as follows:
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. Specify 0 to have no file descriptor passed. The callee will
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receive -1 in the corresponding argument.
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. Specify < 0 to have a pipe allocated; start_async() replaces
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with the pipe FD in the following way:
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.in: Returns the writable pipe end into which the caller
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writes; the readable end of the pipe becomes the function's
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in argument.
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.out: Returns the readable pipe end from which the caller
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reads; the writable end of the pipe becomes the function's
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out argument.
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The caller of start_async() must close the returned FDs after it
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has completed reading from/writing from them.
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. Specify a file descriptor > 0 to be used by the function:
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.in: The FD must be readable; it becomes the function's in.
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.out: The FD must be writable; it becomes the function's out.
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The specified FD is closed by start_async(), even if it fails to
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run the function.
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The function pointer in .proc has the following signature:
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int proc(int in, int out, void *data);
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. in, out specifies a set of file descriptors to which the function
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must read/write the data that it needs/produces. The function
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*must* close these descriptors before it returns. A descriptor
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may be -1 if the caller did not configure a descriptor for that
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direction.
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. data is the value that the caller has specified in the .data member
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of struct async.
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. The return value of the function is 0 on success and non-zero
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on failure. If the function indicates failure, finish_async() will
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report failure as well.
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There are serious restrictions on what the asynchronous function can do
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because this facility is implemented by a thread in the same address
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space on most platforms (when pthreads is available), but by a pipe to
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a forked process otherwise:
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. It cannot change the program's state (global variables, environment,
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etc.) in a way that the caller notices; in other words, .in and .out
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are the only communication channels to the caller.
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. It must not change the program's state that the caller of the
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facility also uses.
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