update-server-info: avoid needless overwrites

Do not change the existing info/refs and objects/info/packs
files if they match the existing content on the filesystem.
This is intended to preserve mtime and make it easier for dumb
HTTP pollers to rely on the If-Modified-Since header.

Combined with stdio and kernel buffering; the kernel should be
able to avoid block layer writes and reduce wear for small files.

As a result, the --force option is no longer needed.  So stop
documenting it, but let it remain for compatibility (and
debugging, if necessary).

v3: perform incremental comparison while generating to avoid
    OOM with giant files.  Remove documentation for --force.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-server-info.txt

@@ -9,7 +9,7 @@ git-update-server-info - Update auxiliary info file to help dumb servers
 SYNOPSIS
 --------
 [verse]
-'git update-server-info' [--force]
+'git update-server-info'
 
 DESCRIPTION
 -----------
@@ -19,15 +19,6 @@ $GIT_OBJECT_DIRECTORY/info directories to help clients discover
 what references and packs the server has.  This command
 generates such auxiliary files.
 
-
-OPTIONS
--------
-
--f::
---force::
-	Update the info files from scratch.
-
-
 OUTPUT
 ------
 

server-info.c

@@ -6,82 +6,174 @@
 #include "tag.h"
 #include "packfile.h"
 #include "object-store.h"
+#include "strbuf.h"
+
+struct update_info_ctx {
+	FILE *cur_fp;
+	FILE *old_fp; /* becomes NULL if it differs from cur_fp */
+	struct strbuf cur_sb;
+	struct strbuf old_sb;
+};
+
+static void uic_mark_stale(struct update_info_ctx *uic)
+{
+	fclose(uic->old_fp);
+	uic->old_fp = NULL;
+}
+
+static int uic_is_stale(const struct update_info_ctx *uic)
+{
+	return uic->old_fp == NULL;
+}
+
+static int uic_printf(struct update_info_ctx *uic, const char *fmt, ...)
+{
+	va_list ap;
+	int ret = -1;
+
+	va_start(ap, fmt);
+
+	if (uic_is_stale(uic)) {
+		ret = vfprintf(uic->cur_fp, fmt, ap);
+	} else {
+		ssize_t r;
+		struct strbuf *cur = &uic->cur_sb;
+		struct strbuf *old = &uic->old_sb;
+
+		strbuf_reset(cur);
+		strbuf_vinsertf(cur, 0, fmt, ap);
+
+		strbuf_reset(old);
+		strbuf_grow(old, cur->len);
+		r = fread(old->buf, 1, cur->len, uic->old_fp);
+		if (r != cur->len || memcmp(old->buf, cur->buf, r))
+			uic_mark_stale(uic);
+
+		if (fwrite(cur->buf, 1, cur->len, uic->cur_fp) == cur->len)
+			ret = 0;
+	}
+
+	va_end(ap);
+
+	return ret;
+}
 
 /*
  * Create the file "path" by writing to a temporary file and renaming
  * it into place. The contents of the file come from "generate", which
  * should return non-zero if it encounters an error.
  */
-static int update_info_file(char *path, int (*generate)(FILE *))
+static int update_info_file(char *path,
+			int (*generate)(struct update_info_ctx *),
+			int force)
 {
 	char *tmp = mkpathdup("%s_XXXXXX", path);
 	int ret = -1;
 	int fd = -1;
-	FILE *fp = NULL, *to_close;
+	FILE *to_close;
+	struct update_info_ctx uic = {
+		.cur_fp = NULL,
+		.old_fp = NULL,
+		.cur_sb = STRBUF_INIT,
+		.old_sb = STRBUF_INIT
+	};
 
 	safe_create_leading_directories(path);
 	fd = git_mkstemp_mode(tmp, 0666);
 	if (fd < 0)
 		goto out;
-	to_close = fp = fdopen(fd, "w");
-	if (!fp)
+	to_close = uic.cur_fp = fdopen(fd, "w");
+	if (!uic.cur_fp)
 		goto out;
 	fd = -1;
-	ret = generate(fp);
+
+	/* no problem on ENOENT and old_fp == NULL, it's stale, now */
+	if (!force)
+		uic.old_fp = fopen_or_warn(path, "r");
+
+	/*
+	 * uic_printf will compare incremental comparison aginst old_fp
+	 * and mark uic as stale if needed
+	 */
+	ret = generate(&uic);
 	if (ret)
 		goto out;
-	fp = NULL;
+
+	/* new file may be shorter than the old one, check here */
+	if (!uic_is_stale(&uic)) {
+		struct stat st;
+		long new_len = ftell(uic.cur_fp);
+		int old_fd = fileno(uic.old_fp);
+
+		if (new_len < 0) {
+			ret = -1;
+			goto out;
+		}
+		if (fstat(old_fd, &st) || (st.st_size != (size_t)new_len))
+			uic_mark_stale(&uic);
+	}
+
+	uic.cur_fp = NULL;
 	if (fclose(to_close))
 		goto out;
-	if (adjust_shared_perm(tmp) < 0)
-		goto out;
-	if (rename(tmp, path) < 0)
-		goto out;
+
+	if (uic_is_stale(&uic)) {
+		if (adjust_shared_perm(tmp) < 0)
+			goto out;
+		if (rename(tmp, path) < 0)
+			goto out;
+	} else {
+		unlink(tmp);
+	}
 	ret = 0;
 
 out:
 	if (ret) {
 		error_errno("unable to update %s", path);
-		if (fp)
-			fclose(fp);
+		if (uic.cur_fp)
+			fclose(uic.cur_fp);
 		else if (fd >= 0)
 			close(fd);
 		unlink(tmp);
 	}
 	free(tmp);
+	if (uic.old_fp)
+		fclose(uic.old_fp);
+	strbuf_release(&uic.old_sb);
+	strbuf_release(&uic.cur_sb);
 	return ret;
 }
 
 static int add_info_ref(const char *path, const struct object_id *oid,
 			int flag, void *cb_data)
 {
-	FILE *fp = cb_data;
+	struct update_info_ctx *uic = cb_data;
 	struct object *o = parse_object(the_repository, oid);
 	if (!o)
 		return -1;
 
-	if (fprintf(fp, "%s	%s\n", oid_to_hex(oid), path) < 0)
+	if (uic_printf(uic, "%s	%s\n", oid_to_hex(oid), path) < 0)
 		return -1;
 
 	if (o->type == OBJ_TAG) {
 		o = deref_tag(the_repository, o, path, 0);
 		if (o)
-			if (fprintf(fp, "%s	%s^{}\n",
+			if (uic_printf(uic, "%s	%s^{}\n",
 				oid_to_hex(&o->oid), path) < 0)
 				return -1;
 	}
 	return 0;
 }
 
-static int generate_info_refs(FILE *fp)
+static int generate_info_refs(struct update_info_ctx *uic)
 {
-	return for_each_ref(add_info_ref, fp);
+	return for_each_ref(add_info_ref, uic);
 }
 
-static int update_info_refs(void)
+static int update_info_refs(int force)
 {
 	char *path = git_pathdup("info/refs");
-	int ret = update_info_file(path, generate_info_refs);
+	int ret = update_info_file(path, generate_info_refs, force);
 	free(path);
 	return ret;
 }
@@ -236,14 +328,14 @@ static void free_pack_info(void)
 	free(info);
 }
 
-static int write_pack_info_file(FILE *fp)
+static int write_pack_info_file(struct update_info_ctx *uic)
 {
 	int i;
 	for (i = 0; i < num_pack; i++) {
-		if (fprintf(fp, "P %s\n", pack_basename(info[i]->p)) < 0)
+		if (uic_printf(uic, "P %s\n", pack_basename(info[i]->p)) < 0)
 			return -1;
 	}
-	if (fputc('\n', fp) == EOF)
+	if (uic_printf(uic, "\n") < 0)
 		return -1;
 	return 0;
 }
@@ -254,7 +346,7 @@ static int update_info_packs(int force)
 	int ret;
 
 	init_pack_info(infofile, force);
-	ret = update_info_file(infofile, write_pack_info_file);
+	ret = update_info_file(infofile, write_pack_info_file, force);
 	free_pack_info();
 	free(infofile);
 	return ret;
@@ -269,7 +361,7 @@ int update_server_info(int force)
 	 */
 	int errs = 0;
 
-	errs = errs | update_info_refs();
+	errs = errs | update_info_refs(force);
 	errs = errs | update_info_packs(force);
 
 	/* remove leftover rev-cache file if there is any */

t/t5200-update-server-info.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+
+test_description='Test git update-server-info'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' 'test_commit file'
+
+test_expect_success 'create info/refs' '
+	git update-server-info &&
+	test_path_is_file .git/info/refs
+'
+
+test_expect_success 'modify and store mtime' '
+	test-tool chmtime =0 .git/info/refs &&
+	test-tool chmtime --get .git/info/refs >a
+'
+
+test_expect_success 'info/refs is not needlessly overwritten' '
+	git update-server-info &&
+	test-tool chmtime --get .git/info/refs >b &&
+	test_cmp a b
+'
+
+test_expect_success 'info/refs can be forced to update' '
+	git update-server-info -f &&
+	test-tool chmtime --get .git/info/refs >b &&
+	! test_cmp a b
+'
+
+test_expect_success 'info/refs updates when changes are made' '
+	test-tool chmtime =0 .git/info/refs &&
+	test-tool chmtime --get .git/info/refs >b &&
+	test_cmp a b &&
+	git update-ref refs/heads/foo HEAD &&
+	git update-server-info &&
+	test-tool chmtime --get .git/info/refs >b &&
+	! test_cmp a b
+'
+
+test_done