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-run-command API
-===============
-
-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.
-
-
-Functions
----------
-
-`child_process_init`::
-
- Initialize a struct child_process variable.
-
-`start_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.
-
-`finish_command`::
-
- Wait for the completion of a sub-process that was started with
- start_command().
-
-`run_command`::
-
- 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.
-
-`run_command_v_opt`, `run_command_v_opt_cd_env`::
-
- Convenience functions that encapsulate a sequence of
- start_command() followed by finish_command(). The argument argv
- specifies the program and its arguments. The argument opt is zero
- or more of the flags `RUN_COMMAND_NO_STDIN`, `RUN_GIT_CMD`,
- `RUN_COMMAND_STDOUT_TO_STDERR`, or `RUN_SILENT_EXEC_FAILURE`
- that correspond to the members .no_stdin, .git_cmd,
- .stdout_to_stderr, .silent_exec_failure of `struct child_process`.
- The argument dir corresponds the member .dir. The argument env
- corresponds to the member .env.
-
-`child_process_clear`::
-
- 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.
-
-The functions above 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.
-
-
-`start_async`::
-
- 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.
-
-`finish_async`::
-
- Wait for the completion of an asynchronous function that was
- started with start_async().
-
-`run_hook`::
-
- Run a hook.
- The first argument is a pathname to an index file, or NULL
- if the hook uses the default index file or no index is needed.
- The second argument is the name of the hook.
- The further arguments correspond to the hook arguments.
- The last argument has to be NULL to terminate the arguments list.
- If the hook does not exist or is not executable, the return
- value will be zero.
- If it is executable, the hook will be executed and the exit
- status of the hook is returned.
- On execution, .stdout_to_stderr and .no_stdin will be set.
- (See below.)
-
-
-Data structures
----------------
-
-* `struct child_process`
-
-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().
-
-The .argv member is set up as an array of string pointers (NULL
-terminated), of which .argv[0] is the program name to run (usually
-without a path). If the command to run is a git command, set argv[0] to
-the command name without the 'git-' prefix and set .git_cmd = 1.
-
-Note that the ownership of the memory pointed to by .argv stays with the
-caller, but it should survive until `finish_command` completes. If the
-.argv member is NULL, `start_command` will point it at the .args
-`argv_array` (so you may use one or the other, but you must use exactly
-one). The memory in .args will be cleaned up automatically during
-`finish_command` (or during `start_command` when it is unsuccessful).
-
-The members .in, .out, .err are used to redirect stdin, stdout,
-stderr as follows:
-
-. Specify 0 to request no special redirection. No new file descriptor
- is allocated. The child process simply inherits the channel from the
- parent.
-
-. Specify -1 to have a pipe allocated; start_command() replaces -1
- by 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 child's stdin.
-
- .out, .err: Returns the readable pipe end from which the caller
- reads; the writable end of the pipe end becomes child's
- stdout/stderr.
-
- The caller of start_command() must close the so returned FDs
- after it has completed reading from/writing to it!
-
-. Specify a file descriptor > 0 to be used by the child:
-
- .in: The FD must be readable; it becomes child's stdin.
- .out: The FD must be writable; it becomes child's stdout.
- .err: The FD must be writable; it becomes child's stderr.
-
- The specified FD is closed by start_command(), even if it fails to
- run the sub-process!
-
-. 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.
-
-To modify the environment of the sub-process, specify an array of
-string pointers (NULL terminated) in .env:
-
-. 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.
-
-If the .env member is NULL, `start_command` will point it at the
-.env_array `argv_array` (so you may use one or the other, but not both).
-The memory in .env_array will be cleaned up automatically during
-`finish_command` (or during `start_command` when it is unsuccessful).
-
-To specify a new initial working directory for the sub-process,
-specify it in the .dir member.
-
-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.
-
-
-* `struct async`
-
-This describes a function to run asynchronously, whose purpose is
-to produce output that the caller reads.
-
-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().
-
-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.
-
-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.
-
-
-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.