run-command API
===============
-Talk about <run-command.h>, and things like:
+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.
-* Environment the command runs with (e.g. GIT_DIR);
-* File descriptors and pipes;
-* Exit status;
+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.
-(Hannes, Dscho, Shawn)
+
+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.