[RFC PATCH] http: use --stdin and --keep when downloading pack

[Jonathan Tan <jonathantanmy@google.com>]

When Git fetches a pack using dumb HTTP, it does at least 2 things
differently from when it fetches using fetch-pack or receive-pack: (1)
it reuses the server's name for the packfile (which incorporates a hash)
for the packfile, and (2) it does not create a .keep file to avoid race
conditions with repack.

A subsequent patch will allow downloading packs over HTTP(S) as part of
a fetch. These downloads will not necessarily be from a Git repository,
and thus may not have a hash as part of its name. Also, generating a
.keep file will be necessary to avoid race conditions with repack (until
the fetch has successfully written the new refs).

Thus, teach http to pass --stdin and --keep to index-pack, the former so
that we have no reliance on the server's name for the packfile, and the
latter so that we have the .keep file.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
This is part of the work of CDN offloading of fetch responses.

I have plans to use the http_pack_request suite of functions to
implement the part where we download from CDN over HTTP(S), but need
this change to be able to do so. I think it's better from the code
quality perspective to reuse these functions, but this necessitates a
behavior change in that we no longer use the filename as declared by the
server, so I'm sending this as RFC to see what the community thinks.
---
 http-push.c   |  7 ++++++-
 http-walker.c |  5 ++++-
 http.c        | 42 ++++++++++++++++++++----------------------
 http.h        |  2 +-
 4 files changed, 31 insertions(+), 25 deletions(-)

diff --git a/http-push.c b/http-push.c
index b22c7caea0..409b266b0c 100644
--- a/http-push.c
+++ b/http-push.c
@@ -586,11 +586,16 @@ static void finish_request(struct transfer_request *request)
 			fprintf(stderr, "Unable to get pack file %s\n%s",
 				request->url, curl_errorstr);
 		} else {
+			char *lockfile;
+
 			preq = (struct http_pack_request *)request->userData;
 
 			if (preq) {
-				if (finish_http_pack_request(preq) == 0)
+				if (finish_http_pack_request(preq,
+							     &lockfile) == 0) {
+					unlink(lockfile);
 					fail = 0;
+				}
 				release_http_pack_request(preq);
 			}
 		}
diff --git a/http-walker.c b/http-walker.c
index 8ae5d76c6a..804dc82304 100644
--- a/http-walker.c
+++ b/http-walker.c
@@ -425,6 +425,7 @@ static int http_fetch_pack(struct walker *walker, struct alt_base *repo, unsigne
 	int ret;
 	struct slot_results results;
 	struct http_pack_request *preq;
+	char *lockfile;
 
 	if (fetch_indices(walker, repo))
 		return -1;
@@ -457,7 +458,9 @@ static int http_fetch_pack(struct walker *walker, struct alt_base *repo, unsigne
 		goto abort;
 	}
 
-	ret = finish_http_pack_request(preq);
+	ret = finish_http_pack_request(preq, &lockfile);
+	if (!ret)
+		unlink(lockfile);
 	release_http_pack_request(preq);
 	if (ret)
 		return ret;
diff --git a/http.c b/http.c
index a32ad36ddf..5f8e602cd2 100644
--- a/http.c
+++ b/http.c
@@ -2200,13 +2200,13 @@ void release_http_pack_request(struct http_pack_request *preq)
 	free(preq);
 }
 
-int finish_http_pack_request(struct http_pack_request *preq)
+int finish_http_pack_request(struct http_pack_request *preq, char **lockfile)
 {
 	struct packed_git **lst;
 	struct packed_git *p = preq->target;
-	char *tmp_idx;
-	size_t len;
 	struct child_process ip = CHILD_PROCESS_INIT;
+	int tmpfile_fd;
+	int ret = 0;
 
 	close_pack_index(p);
 
@@ -2218,35 +2218,33 @@ int finish_http_pack_request(struct http_pack_request *preq)
 		lst = &((*lst)->next);
 	*lst = (*lst)->next;
 
-	if (!strip_suffix(preq->tmpfile.buf, ".pack.temp", &len))
-		BUG("pack tmpfile does not end in .pack.temp?");
-	tmp_idx = xstrfmt("%.*s.idx.temp", (int)len, preq->tmpfile.buf);
+	tmpfile_fd = xopen(preq->tmpfile.buf, O_RDONLY);
 
 	argv_array_push(&ip.args, "index-pack");
-	argv_array_pushl(&ip.args, "-o", tmp_idx, NULL);
-	argv_array_push(&ip.args, preq->tmpfile.buf);
+	argv_array_push(&ip.args, "--stdin");
+	argv_array_pushf(&ip.args, "--keep=git %"PRIuMAX, (uintmax_t)getpid());
 	ip.git_cmd = 1;
-	ip.no_stdin = 1;
-	ip.no_stdout = 1;
+	ip.in = tmpfile_fd;
+	ip.out = -1;
 
-	if (run_command(&ip)) {
-		unlink(preq->tmpfile.buf);
-		unlink(tmp_idx);
-		free(tmp_idx);
-		return -1;
+	if (start_command(&ip)) {
+		ret = -1;
+		goto cleanup;
 	}
 
-	unlink(sha1_pack_index_name(p->sha1));
+	*lockfile = index_pack_lockfile(ip.out);
+	close(ip.out);
 
-	if (finalize_object_file(preq->tmpfile.buf, sha1_pack_name(p->sha1))
-	 || finalize_object_file(tmp_idx, sha1_pack_index_name(p->sha1))) {
-		free(tmp_idx);
-		return -1;
+	if (finish_command(&ip)) {
+		ret = -1;
+		goto cleanup;
 	}
 
 	install_packed_git(the_repository, p);
-	free(tmp_idx);
-	return 0;
+cleanup:
+	close(tmpfile_fd);
+	unlink(preq->tmpfile.buf);
+	return ret;
 }
 
 struct http_pack_request *new_http_pack_request(
diff --git a/http.h b/http.h
index 4eb4e808e5..20d1c85d0b 100644
--- a/http.h
+++ b/http.h
@@ -212,7 +212,7 @@ struct http_pack_request {
 
 extern struct http_pack_request *new_http_pack_request(
 	struct packed_git *target, const char *base_url);
-extern int finish_http_pack_request(struct http_pack_request *preq);
+int finish_http_pack_request(struct http_pack_request *preq, char **lockfile);
 extern void release_http_pack_request(struct http_pack_request *preq);
 
 /* Helpers for fetching object */
-- 
2.19.0.271.gfe8321ec05.dirty