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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
---------
 
`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.
 
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 (memset(&chld, 0, sizeof(chld)); or
   using 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.
 
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.