daemon: send stderr of service programs to the syslog

If git-daemon is run with --detach or --inetd, then stderr is explicitly
redirected to /dev/null. But notice that the service programs were spawned
via execl_git_cmd(), in particular, the stderr channel is inherited from
the daemon. This means that errors that the programs wrote to stderr (for
example, via die()), went to /dev/null.

This patch arranges that the daemon does not merely exec the service
program, but forks it and monitors stderr of the child; it writes the
errors that it produces to the daemons log via logerror().

A consequence is that the daemon process remains in memory for the full
duration of the service program, but this cannot be avoided.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

daemon.c

@@ -1,6 +1,8 @@
 #include "cache.h"
 #include "pkt-line.h"
 #include "exec_cmd.h"
+#include "run-command.h"
+#include "strbuf.h"
 
 #include <syslog.h>
 
@@ -343,28 +345,66 @@ static int run_service(char *dir, struct daemon_service *service)
 	return service->fn();
 }
 
+static void copy_to_log(int fd)
+{
+	struct strbuf line = STRBUF_INIT;
+	FILE *fp;
+
+	fp = fdopen(fd, "r");
+	if (fp == NULL) {
+		logerror("fdopen of error channel failed");
+		close(fd);
+		return;
+	}
+
+	while (strbuf_getline(&line, fp, '\n') != EOF) {
+		logerror("%s", line.buf);
+		strbuf_setlen(&line, 0);
+	}
+
+	strbuf_release(&line);
+	fclose(fp);
+}
+
+static int run_service_command(const char **argv)
+{
+	struct child_process cld;
+
+	memset(&cld, 0, sizeof(cld));
+	cld.argv = argv;
+	cld.git_cmd = 1;
+	cld.err = -1;
+	if (start_command(&cld))
+		return -1;
+
+	close(0);
+	close(1);
+
+	copy_to_log(cld.err);
+
+	return finish_command(&cld);
+}
+
 static int upload_pack(void)
 {
 	/* Timeout as string */
 	char timeout_buf[64];
+	const char *argv[] = { "upload-pack", "--strict", timeout_buf, ".", NULL };
 
 	snprintf(timeout_buf, sizeof timeout_buf, "--timeout=%u", timeout);
-
-	/* git-upload-pack only ever reads stuff, so this is safe */
-	execl_git_cmd("upload-pack", "--strict", timeout_buf, ".", NULL);
-	return -1;
+	return run_service_command(argv);
 }
 
 static int upload_archive(void)
 {
-	execl_git_cmd("upload-archive", ".", NULL);
-	return -1;
+	static const char *argv[] = { "upload-archive", ".", NULL };
+	return run_service_command(argv);
 }
 
 static int receive_pack(void)
 {
-	execl_git_cmd("receive-pack", ".", NULL);
-	return -1;
+	static const char *argv[] = { "receive-pack", ".", NULL };
+	return run_service_command(argv);
 }
 
 static struct daemon_service daemon_service[] = {