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_dir`, `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().


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 the
                parent's stderr (i.e. *not* to what .err or
                .no_stderr specify).

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.