argv-array: add detach function

The usual pattern for an argv array is to initialize it,
push in some strings, and then clear it when done. Very
occasionally, though, we must do other exotic things with
the memory, like freeing the list but keeping the strings.
Let's provide a detach function so that callers can make use
of our API to build up the array, and then take ownership of
it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/technical/api-argv-array.txt b/Documentation/technical/api-argv-array.txt
index 8076172a08..cfc063018c 100644
--- a/Documentation/technical/api-argv-array.txt
+++ b/Documentation/technical/api-argv-array.txt
@@ -56,3 +56,10 @@ Functions
 `argv_array_clear`::
 	Free all memory associated with the array and return it to the
 	initial, empty state.
+
+`argv_array_detach`::
+	Disconnect the `argv` member from the `argv_array` struct and
+	return it. The caller is responsible for freeing the memory used
+	by the array, and by the strings it references. After detaching,
+	the `argv_array` is in a reinitialized state and can be pushed
+	into again.
diff --git a/argv-array.c b/argv-array.c
index eaed47712b..5d370fa336 100644
--- a/argv-array.c
+++ b/argv-array.c
@@ -74,3 +74,14 @@ void argv_array_clear(struct argv_array *array)
 	}
 	argv_array_init(array);
 }
+
+const char **argv_array_detach(struct argv_array *array)
+{
+	if (array->argv == empty_argv)
+		return xcalloc(1, sizeof(const char *));
+	else {
+		const char **ret = array->argv;
+		argv_array_init(array);
+		return ret;
+	}
+}
diff --git a/argv-array.h b/argv-array.h
index a2fa0aa606..29056e49a1 100644
--- a/argv-array.h
+++ b/argv-array.h
@@ -20,5 +20,6 @@ void argv_array_pushl(struct argv_array *, ...);
 void argv_array_pushv(struct argv_array *, const char **);
 void argv_array_pop(struct argv_array *);
 void argv_array_clear(struct argv_array *);
+const char **argv_array_detach(struct argv_array *);
 
 #endif /* ARGV_ARRAY_H */