fetch-pack: prepare updated shallow file before fetching the pack

index-pack --strict looks up and follows parent commits. If shallow
information is not ready by the time index-pack is run, index-pack may
be led to non-existent objects. Make fetch-pack save shallow file to
disk before invoking index-pack.

git learns new global option --shallow-file to pass on the alternate
shallow file path. Undocumented (and not even support --shallow-file=
syntax) because it's unlikely to be used again elsewhere.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -176,6 +176,8 @@ extern int for_each_commit_graft(each_commit_graft_fn, void *);
 extern int is_repository_shallow(void);
 extern struct commit_list *get_shallow_commits(struct object_array *heads,
 		int depth, int shallow_flag, int not_shallow_flag);
+extern void check_shallow_file_for_update(void);
+extern void set_alternate_shallow_file(const char *path);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);

fetch-pack.c

@@ -20,6 +20,8 @@ static int no_done;
 static int fetch_fsck_objects = -1;
 static int transfer_fsck_objects = -1;
 static int agent_supported;
+static struct lock_file shallow_lock;
+static const char *alternate_shallow_file;
 
 #define COMPLETE	(1U << 0)
 #define COMMON		(1U << 1)
@@ -683,7 +685,7 @@ static int get_pack(struct fetch_pack_args *args,
 		    int xd[2], char **pack_lockfile)
 {
 	struct async demux;
-	const char *argv[20];
+	const char *argv[22];
 	char keep_arg[256];
 	char hdr_arg[256];
 	const char **av;
@@ -724,6 +726,11 @@ static int get_pack(struct fetch_pack_args *args,
 			do_keep = 1;
 	}
 
+	if (alternate_shallow_file) {
+		*av++ = "--shallow-file";
+		*av++ = alternate_shallow_file;
+	}
+
 	if (do_keep) {
 		if (pack_lockfile)
 			cmd.out = -1;
@@ -779,6 +786,27 @@ static int cmp_ref_by_name(const void *a_, const void *b_)
 	return strcmp(a->name, b->name);
 }
 
+static void setup_alternate_shallow(void)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int fd;
+
+	check_shallow_file_for_update();
+	fd = hold_lock_file_for_update(&shallow_lock, git_path("shallow"),
+				       LOCK_DIE_ON_ERROR);
+	if (write_shallow_commits(&sb, 0)) {
+		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
+			die_errno("failed to write to %s", shallow_lock.filename);
+		alternate_shallow_file = shallow_lock.filename;
+	} else
+		/*
+		 * is_repository_shallow() sees empty string as "no
+		 * shallow file".
+		 */
+		alternate_shallow_file = "";
+	strbuf_release(&sb);
+}
+
 static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
@@ -858,6 +886,8 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
+	if (args->depth > 0)
+		setup_alternate_shallow();
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
@@ -936,15 +966,9 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       struct ref **sought, int nr_sought,
 		       char **pack_lockfile)
 {
-	struct stat st;
 	struct ref *ref_cpy;
 
 	fetch_pack_setup();
-	if (args->depth > 0) {
-		if (stat(git_path("shallow"), &st))
-			st.st_mtime = 0;
-	}
-
 	if (nr_sought)
 		nr_sought = remove_duplicates_in_refs(sought, nr_sought);
 
@@ -954,35 +978,12 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 	}
 	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought, pack_lockfile);
 
-	if (args->depth > 0) {
-		static struct lock_file lock;
-		struct cache_time mtime;
-		struct strbuf sb = STRBUF_INIT;
-		char *shallow = git_path("shallow");
-		int fd;
-
-		mtime.sec = st.st_mtime;
-		mtime.nsec = ST_MTIME_NSEC(st);
-		if (stat(shallow, &st)) {
-			if (mtime.sec)
-				die("shallow file was removed during fetch");
-		} else if (st.st_mtime != mtime.sec
-#ifdef USE_NSEC
-				|| ST_MTIME_NSEC(st) != mtime.nsec
-#endif
-			  )
-			die("shallow file was changed during fetch");
-
-		fd = hold_lock_file_for_update(&lock, shallow,
-					       LOCK_DIE_ON_ERROR);
-		if (!write_shallow_commits(&sb, 0)
-		 || write_in_full(fd, sb.buf, sb.len) != sb.len) {
-			unlink_or_warn(shallow);
-			rollback_lock_file(&lock);
-		} else {
-			commit_lock_file(&lock);
-		}
-		strbuf_release(&sb);
+	if (alternate_shallow_file) {
+		if (*alternate_shallow_file == '\0') { /* --unshallow */
+			unlink_or_warn(git_path("shallow"));
+			rollback_lock_file(&shallow_lock);
+		} else
+			commit_lock_file(&shallow_lock);
 	}
 
 	reprepare_packed_git();

git.c

@@ -4,6 +4,7 @@
 #include "help.h"
 #include "quote.h"
 #include "run-command.h"
+#include "commit.h"
 
 const char git_usage_string[] =
 	"git [--version] [--help] [-c name=value]\n"
@@ -146,6 +147,12 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 			setenv(GIT_LITERAL_PATHSPECS_ENVIRONMENT, "0", 1);
 			if (envchanged)
 				*envchanged = 1;
+		} else if (!strcmp(cmd, "--shallow-file")) {
+			(*argv)++;
+			(*argc)--;
+			set_alternate_shallow_file((*argv)[0]);
+			if (envchanged)
+				*envchanged = 1;
 		} else {
 			fprintf(stderr, "Unknown option: %s\n", cmd);
 			usage(git_usage_string);

shallow.c

@@ -3,6 +3,16 @@
 #include "tag.h"
 
 static int is_shallow = -1;
+static struct stat shallow_stat;
+static char *alternate_shallow_file;
+
+void set_alternate_shallow_file(const char *path)
+{
+	if (is_shallow != -1)
+		die("BUG: is_repository_shallow must not be called before set_alternate_shallow_file");
+	free(alternate_shallow_file);
+	alternate_shallow_file = path ? xstrdup(path) : NULL;
+}
 
 int register_shallow(const unsigned char *sha1)
 {
@@ -21,12 +31,21 @@ int is_repository_shallow(void)
 {
 	FILE *fp;
 	char buf[1024];
+	const char *path = alternate_shallow_file;
 
 	if (is_shallow >= 0)
 		return is_shallow;
 
-	fp = fopen(git_path("shallow"), "r");
-	if (!fp) {
+	if (!path)
+		path = git_path("shallow");
+	/*
+	 * fetch-pack sets '--shallow-file ""' as an indicator that no
+	 * shallow file should be used. We could just open it and it
+	 * will likely fail. But let's do an explicit check instead.
+	 */
+	if (!*path ||
+	    stat(path, &shallow_stat) ||
+	    (fp = fopen(path, "r")) == NULL) {
 		is_shallow = 0;
 		return is_shallow;
 	}
@@ -108,3 +127,22 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 
 	return result;
 }
+
+void check_shallow_file_for_update(void)
+{
+	struct stat st;
+
+	if (!is_shallow)
+		return;
+	else if (is_shallow == -1)
+		die("BUG: shallow must be initialized by now");
+
+	if (stat(git_path("shallow"), &st))
+		die("shallow file was removed during fetch");
+	else if (st.st_mtime != shallow_stat.st_mtime
+#ifdef USE_NSEC
+		 || ST_MTIME_NSEC(st) != ST_MTIME_NSEC(shallow_stat)
+#endif
+		   )
+		die("shallow file was changed during fetch");
+}

t/t5500-fetch-pack.sh

@@ -135,6 +135,13 @@ test_expect_success 'clone shallow depth 1' '
 	test "`git --git-dir=shallow0/.git rev-list --count HEAD`" = 1
 '
 
+test_expect_success 'clone shallow depth 1 with fsck' '
+	git config --global fetch.fsckobjects true &&
+	git clone --no-single-branch --depth 1 "file://$(pwd)/." shallow0fsck &&
+	test "`git --git-dir=shallow0fsck/.git rev-list --count HEAD`" = 1 &&
+	git config --global --unset fetch.fsckobjects
+'
+
 test_expect_success 'clone shallow' '
 	git clone --no-single-branch --depth 2 "file://$(pwd)/." shallow
 '