[PATCH] gc: save log from daemonized gc --auto and print it next time

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

While commit 9f673f9 (gc: config option for running --auto in
background - 2014-02-08) helps reduce some complaints about 'gc
--auto' hogging the terminal, it creates another set of problems.

The latest in this set is, as the result of daemonizing, stderr is
closed and all warnings are lost. This warning at the end of cmd_gc()
is particularly important because it tells the user how to avoid "gc
--auto" running repeatedly. Because stderr is closed, the user does
not know, naturally they complain about 'gc --auto' wasting CPU.

Besides reverting 9f673f9 and looking at the problem from another
angle, we could save the stderr in $GIT_DIR/gc.log. Next time, 'gc
--auto' will print the saved warnings, delete gc.log and exit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 builtin/gc.c  | 37 ++++++++++++++++++++++++++++++++++++-
 t/t6500-gc.sh | 20 ++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 5c634af..07769a9 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -32,6 +32,8 @@ static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
 static int detach_auto = 1;
+static struct strbuf log_filename = STRBUF_INIT;
+static int daemonized;
 static const char *prune_expire = "2.weeks.ago";
 
 static struct argv_array pack_refs_cmd = ARGV_ARRAY_INIT;
@@ -44,6 +46,15 @@ static char *pidfile;
 
 static void remove_pidfile(void)
 {
+	if (daemonized && log_filename.len) {
+		struct stat st;
+
+		close(2);
+		if (stat(log_filename.buf, &st) ||
+		    !st.st_size ||
+		    rename(log_filename.buf, git_path("gc.log")))
+			unlink(log_filename.buf);
+	}
 	if (pidfile)
 		unlink(pidfile);
 }
@@ -324,13 +335,25 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			fprintf(stderr, _("See \"git help gc\" for manual housekeeping.\n"));
 		}
 		if (detach_auto) {
+			struct strbuf sb = STRBUF_INIT;
+
+			if (strbuf_read_file(&sb, git_path("gc.log"), 0) > 0) {
+				warning(_("Last gc run reported the following, gc skipped"));
+				fputs(sb.buf, stderr);
+				strbuf_release(&sb);
+				/* let the next gc --auto run as usual */
+				unlink(git_path("gc.log"));
+				return 0;
+			}
+
 			if (gc_before_repack())
 				return -1;
 			/*
 			 * failure to daemonize is ok, we'll continue
 			 * in foreground
 			 */
-			daemonize();
+			if (!daemonize())
+				daemonized = 1;
 		}
 	} else
 		add_repack_all_option();
@@ -343,6 +366,18 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		    name, (uintmax_t)pid);
 	}
 
+	if (daemonized) {
+		int fd;
+
+		strbuf_addstr(&log_filename, git_path("gc.log_XXXXXX"));
+		fd = xmkstemp(log_filename.buf);
+		if (fd >= 0) {
+			dup2(fd, 2);
+			close(fd);
+		} else
+			strbuf_release(&log_filename);
+	}
+
 	if (gc_before_repack())
 		return -1;
 
diff --git a/t/t6500-gc.sh b/t/t6500-gc.sh
index 63194d8..54bc9c4 100755
--- a/t/t6500-gc.sh
+++ b/t/t6500-gc.sh
@@ -30,4 +30,24 @@ test_expect_success 'gc -h with invalid configuration' '
 	test_i18ngrep "[Uu]sage" broken/usage
 '
 
+test_expect_success !MINGW 'gc --auto and logging' '
+	git init abc &&
+	(
+		cd abc &&
+		# These create blobs starting with the magic number "17"
+		for i in 901 944; do
+			echo $i >test && git hash-object -w test >/dev/null
+		done &&
+		git config gc.auto 1 &&
+		LANG=C git gc --auto &&
+		sleep 1 && # give it time to daemonize
+		while test -f .git/gc.pid; do sleep 1; done &&
+		grep "too many unreachable loose objects" .git/gc.log &&
+		LANG=C git gc --auto 2>error &&
+		grep skipped error &&
+		grep "too many unreachable loose objects" error &&
+		! test -f .git/gc.log
+	)
+'
+
 test_done
-- 
2.3.0.rc1.137.g477eb31