[PATCH v3] 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.

Daemonized gc now saves stderr to $GIT_DIR/gc.log. Following gc runs
will not be daemonized and gc.log printed out until the user removes
gc.log.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Let's try again. Compared to v2 [1], this version does not delete
 gc.log automatically any more. The user needs to take action, then
 delete gc.log to bring it background again.

 [1] http://thread.gmane.org/gmane.comp.version-control.git/266182/focus=266320

 builtin/gc.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index bcc75d9..00a83e1 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -43,9 +43,20 @@ static struct argv_array prune_worktrees = ARGV_ARRAY_INIT;
 static struct argv_array rerere = ARGV_ARRAY_INIT;
 
 static char *pidfile;
+static struct strbuf log_filename = STRBUF_INIT;
+static int daemonized;
 
 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);
 }
@@ -330,13 +341,24 @@ 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:\n"
+					  "%s\n"
+					  "running in foreground until %s is removed"),
+					sb.buf, git_path("gc.log"));
+				detach_auto = 0;
+			}
+			strbuf_release(&sb);
+		}
+		if (detach_auto) {
 			if (gc_before_repack())
 				return -1;
 			/*
 			 * failure to daemonize is ok, we'll continue
 			 * in foreground
 			 */
-			daemonize();
+			daemonized = !daemonize();
 		}
 	} else
 		add_repack_all_option();
@@ -349,6 +371,18 @@ 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;
 
-- 
2.3.0.rc1.137.g477eb31