188 lines
6.0 KiB
Plaintext
188 lines
6.0 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`, or
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`RUN_COMMAND_STDOUT_TO_STDERR` that correspond to the members
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.no_stdin, .git_cmd, .stdout_to_stderr 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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`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 pipe FD
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from which the caller reads. 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 > 0 is not supported.
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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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* `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 the data by reading from the fd in .out;
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5. closes .out;
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6. calls finish_async().
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The function pointer in .proc has the following signature:
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int proc(int fd, void *data);
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. fd specifies a writable file descriptor to which the function must
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write the data that it produces. The function *must* close this
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descriptor before it returns.
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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 pipe to a forked process on
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UNIX, but by a thread in the same address space on Windows:
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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, .out is the
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only communication channel 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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