Technical documentation of the run-command API.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-run-command.txt

@@ -1,10 +1,171 @@
 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_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 completeion 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 environement
+  variable that will be removed from the child process's envionment.
+
+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.