run-command: offer to close the object store before running

Especially on Windows, where files cannot be deleted if _any_ process
holds an open file handle to them, it is important to close the object
store (releasing all handles to all `.pack` files) before running a
command that might spawn a garbage collection.

This scenario is so common that we frequently see the pattern of closing
the object store before running auto maintenance or another Git command.

Let's make this much more convenient by teaching the `run_command()`
machinery a new flag to release the object store before spawning the
process.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -8,6 +8,7 @@
 #include "string-list.h"
 #include "quote.h"
 #include "config.h"
+#include "packfile.h"
 
 void child_process_init(struct child_process *child)
 {
@@ -740,6 +741,9 @@ int start_command(struct child_process *cmd)
 
 	fflush(NULL);
 
+	if (cmd->close_object_store)
+		close_object_store(the_repository->objects);
+
 #ifndef GIT_WINDOWS_NATIVE
 {
 	int notify_pipe[2];
@@ -1044,6 +1048,7 @@ int run_command_v_opt_cd_env_tr2(const char **argv, int opt, const char *dir,
 	cmd.use_shell = opt & RUN_USING_SHELL ? 1 : 0;
 	cmd.clean_on_exit = opt & RUN_CLEAN_ON_EXIT ? 1 : 0;
 	cmd.wait_after_clean = opt & RUN_WAIT_AFTER_CLEAN ? 1 : 0;
+	cmd.close_object_store = opt & RUN_CLOSE_OBJECT_STORE ? 1 : 0;
 	cmd.dir = dir;
 	cmd.env = env;
 	cmd.trace2_child_class = tr2_class;

run-command.h

@@ -134,6 +134,14 @@ struct child_process {
 	 */
 	unsigned use_shell:1;
 
+	/**
+	 * Release any open file handles to the object store before running
+	 * the command; This is necessary e.g. when the spawned process may
+	 * want to repack because that would delete `.pack` files (and on
+	 * Windows, you cannot delete files that are still in use).
+	 */
+	unsigned close_object_store:1;
+
 	unsigned stdout_to_stderr:1;
 	unsigned clean_on_exit:1;
 	unsigned wait_after_clean:1;
@@ -240,6 +248,7 @@ int run_auto_maintenance(int quiet);
 #define RUN_USING_SHELL			(1<<4)
 #define RUN_CLEAN_ON_EXIT		(1<<5)
 #define RUN_WAIT_AFTER_CLEAN		(1<<6)
+#define RUN_CLOSE_OBJECT_STORE		(1<<7)
 
 /**
  * Convenience functions that encapsulate a sequence of