http-walker: verify remote packs

In c17fb6e ("Verify remote packs, speed up pending request queue"),
changes were made to index fetching in http-push.c, particularly the
methods fetch_index and setup_index. Since http-walker.c has similar
code for index fetching, these improvements should apply to
http-walker.c's fetch_index and setup_index.

Invocations of free() of string memory are reproduced as well.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-walker.c

@@ -384,24 +384,48 @@ static int fetch_index(struct walker *walker, struct alt_base *repo, unsigned ch
 	struct active_request_slot *slot;
 	struct slot_results results;
 
-	if (has_pack_index(sha1))
+	/* Don't use the index if the pack isn't there */
+	url = xmalloc(strlen(repo->base) + 64);
+	sprintf(url, "%s/objects/pack/pack-%s.pack", repo->base, hex);
+	slot = get_active_slot();
+	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	curl_easy_setopt(slot->curl, CURLOPT_NOBODY, 1);
+	if (start_active_slot(slot)) {
+		run_active_slot(slot);
+		if (results.curl_result != CURLE_OK) {
+			free(url);
+			return error("Unable to verify pack %s is available",
+				     hex);
+		}
+	} else {
+		free(url);
+		return error("Unable to start request");
+	}
+
+	if (has_pack_index(sha1)) {
+		free(url);
 		return 0;
+	}
 
 	if (walker->get_verbosely)
 		fprintf(stderr, "Getting index for pack %s\n", hex);
 
-	url = xmalloc(strlen(repo->base) + 64);
 	sprintf(url, "%s/objects/pack/pack-%s.idx", repo->base, hex);
 
 	filename = sha1_pack_index_name(sha1);
 	snprintf(tmpfile, sizeof(tmpfile), "%s.temp", filename);
 	indexfile = fopen(tmpfile, "a");
-	if (!indexfile)
+	if (!indexfile) {
+		free(url);
 		return error("Unable to open local file %s for pack index",
 			     tmpfile);
+	}
 
 	slot = get_active_slot();
 	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_NOBODY, 0);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPGET, 1);
 	curl_easy_setopt(slot->curl, CURLOPT_FILE, indexfile);
 	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite);
 	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
@@ -426,17 +450,20 @@ static int fetch_index(struct walker *walker, struct alt_base *repo, unsigned ch
 	if (start_active_slot(slot)) {
 		run_active_slot(slot);
 		if (results.curl_result != CURLE_OK) {
+			free(url);
 			fclose(indexfile);
 			slot->local = NULL;
 			return error("Unable to get pack index %s\n%s", url,
 				     curl_errorstr);
 		}
 	} else {
+		free(url);
 		fclose(indexfile);
 		slot->local = NULL;
 		return error("Unable to start request");
 	}
 
+	free(url);
 	fclose(indexfile);
 	slot->local = NULL;