Merge branch 'nd/gc-auto-background-fix' into maint

When "git gc --auto" is backgrounded, its diagnosis message is
lost.  Save it to a file in $GIT_DIR and show it next time the "gc
--auto" is run.

* nd/gc-auto-background-fix:
  gc: save log from daemonized gc --auto and print it next time

builtin/gc.c

@@ -44,6 +44,7 @@ static struct argv_array prune_worktrees = ARGV_ARRAY_INIT;
 static struct argv_array rerere = ARGV_ARRAY_INIT;
 
 static struct tempfile pidfile;
+static struct lock_file log_lock;
 
 static void git_config_date_string(const char *key, const char **output)
 {
@@ -56,6 +57,28 @@ static void git_config_date_string(const char *key, const char **output)
 	}
 }
 
+static void process_log_file(void)
+{
+	struct stat st;
+	if (!fstat(get_lock_file_fd(&log_lock), &st) && st.st_size)
+		commit_lock_file(&log_lock);
+	else
+		rollback_lock_file(&log_lock);
+}
+
+static void process_log_file_at_exit(void)
+{
+	fflush(stderr);
+	process_log_file();
+}
+
+static void process_log_file_on_signal(int signo)
+{
+	process_log_file();
+	sigchain_pop(signo);
+	raise(signo);
+}
+
 static void gc_config(void)
 {
 	const char *value;
@@ -241,6 +264,24 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 	return NULL;
 }
 
+static int report_last_gc_error(void)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int ret;
+
+	ret = strbuf_read_file(&sb, git_path("gc.log"), 0);
+	if (ret > 0)
+		return error(_("The last gc run reported the following. "
+			       "Please correct the root cause\n"
+			       "and remove %s.\n"
+			       "Automatic cleanup will not be performed "
+			       "until the file is removed.\n\n"
+			       "%s"),
+			     git_path("gc.log"), sb.buf);
+	strbuf_release(&sb);
+	return 0;
+}
+
 static int gc_before_repack(void)
 {
 	if (pack_refs && run_command_v_opt(pack_refs_cmd.argv, RUN_GIT_CMD))
@@ -262,6 +303,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	int force = 0;
 	const char *name;
 	pid_t pid;
+	int daemonized = 0;
 
 	struct option builtin_gc_options[] = {
 		OPT__QUIET(&quiet, N_("suppress progress reporting")),
@@ -318,13 +360,16 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			fprintf(stderr, _("See \"git help gc\" for manual housekeeping.\n"));
 		}
 		if (detach_auto) {
+			if (report_last_gc_error())
+				return -1;
+
 			if (gc_before_repack())
 				return -1;
 			/*
 			 * failure to daemonize is ok, we'll continue
 			 * in foreground
 			 */
-			daemonize();
+			daemonized = !daemonize();
 		}
 	} else
 		add_repack_all_option();
@@ -337,6 +382,15 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		    name, (uintmax_t)pid);
 	}
 
+	if (daemonized) {
+		hold_lock_file_for_update(&log_lock,
+					  git_path("gc.log"),
+					  LOCK_DIE_ON_ERROR);
+		dup2(get_lock_file_fd(&log_lock), 2);
+		sigchain_push_common(process_log_file_on_signal);
+		atexit(process_log_file_at_exit);
+	}
+
 	if (gc_before_repack())
 		return -1;