Add argv_array_detach and argv_array_free_detached

Allow detaching of ownership of the argv_array's contents and add a
function to free those detached argv_arrays later.

This makes it possible to use argv_array efficiently with the exiting
struct child_process which only contains a member char **argv.

Add to documentation.

Signed-off-by: Florian Achleitner <florian.achleitner.2.6.31@gmail.com>
Acked-by: David Michael Barr <b@rr-dav.id.au>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-argv-array.txt

@@ -49,3 +49,11 @@ Functions
 `argv_array_clear`::
 	Free all memory associated with the array and return it to the
 	initial, empty state.
+
+`argv_array_detach`::
+	Detach the argv array from the `struct argv_array`, transfering
+	ownership of the allocated array and strings.
+
+`argv_array_free_detached`::
+	Free the memory allocated by a `struct argv_array` that was later
+	detached and is now no longer needed.

argv-array.c

@@ -59,3 +59,23 @@ void argv_array_clear(struct argv_array *array)
 	}
 	argv_array_init(array);
 }
+
+const char **argv_array_detach(struct argv_array *array, int *argc)
+{
+	const char **argv =
+		array->argv == empty_argv || array->argc == 0 ? NULL : array->argv;
+	if (argc)
+		*argc = array->argc;
+	argv_array_init(array);
+	return argv;
+}
+
+void argv_array_free_detached(const char **argv)
+{
+	if (argv) {
+		int i;
+		for (i = 0; argv[i]; i++)
+			free((char **)argv[i]);
+		free(argv);
+	}
+}

argv-array.h

@@ -17,5 +17,7 @@ __attribute__((format (printf,2,3)))
 void argv_array_pushf(struct argv_array *, const char *fmt, ...);
 void argv_array_pushl(struct argv_array *, ...);
 void argv_array_clear(struct argv_array *);
+const char **argv_array_detach(struct argv_array *array, int *argc);
+void argv_array_free_detached(const char **argv);
 
 #endif /* ARGV_ARRAY_H */