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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.