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`, or
        `RUN_COMMAND_STDOUT_TO_STDERR` that correspond to the members
        .no_stdin, .git_cmd, .stdout_to_stderr of `struct child_process`.
        The argument dir corresponds the member .dir. The argument env
        corresponds to the member .env.

`start_async`::

        Run a function asynchronously. Takes a pointer to a `struct
        async` that specifies the details and returns a pipe FD
        from which the caller reads. 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));) 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.

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 > 0 is not supported.

  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.


* `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 the data by reading from the fd in .out;
5. closes .out;
6. calls finish_async().

The function pointer in .proc has the following signature:

        int proc(int fd, void *data);

. fd specifies a writable file descriptor to which the function must
  write the data that it produces. The function *must* close this
  descriptor before it returns.

. 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 pipe to a forked process on
UNIX, but by a thread in the same address space on Windows:

. It cannot change the program's state (global variables, environment,
  etc.) in a way that the caller notices; in other words, .out is the
  only communication channel to the caller.

. It must not change the program's state that the caller of the
  facility also uses.