diff: restrict when prefetching occurs

Commit 7fbbcb21b1 ("diff: batch fetching of missing blobs", 2019-04-08)
optimized "diff" by prefetching blobs in a partial clone, but there are
some cases wherein blobs do not need to be prefetched. In these cases,
any command that uses the diff machinery will unnecessarily fetch blobs.

diffcore_std() may read blobs when it calls the following functions:
 (1) diffcore_skip_stat_unmatch() (controlled by the config variable
     diff.autorefreshindex)
 (2) diffcore_break() and diffcore_merge_broken() (for break-rewrite
     detection)
 (3) diffcore_rename() (for rename detection)
 (4) diffcore_pickaxe() (for detecting addition/deletion of specified
     string)

Instead of always prefetching blobs, teach diffcore_skip_stat_unmatch(),
diffcore_break(), and diffcore_rename() to prefetch blobs upon the first
read of a missing object. This covers (1), (2), and (3): to cover the
rest, teach diffcore_std() to prefetch if the output type is one that
includes blob data (and hence blob data will be required later anyway),
or if it knows that (4) will be run.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/diffcore-break.c b/diffcore-break.c
index e8f6322c6a..0d4a14964d 100644
--- a/diffcore-break.c
+++ b/diffcore-break.c
@@ -4,6 +4,7 @@
 #include "cache.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "promisor-remote.h"
 
 static int should_break(struct repository *r,
 			struct diff_filespec *src,
@@ -49,6 +50,8 @@ static int should_break(struct repository *r,
 	unsigned long delta_size, max_size;
 	unsigned long src_copied, literal_added, src_removed;
 
+	struct diff_populate_filespec_options options = { 0 };
+
 	*merge_score_p = 0; /* assume no deletion --- "do not break"
 			     * is the default.
 			     */
@@ -62,8 +65,13 @@ static int should_break(struct repository *r,
 	    oideq(&src->oid, &dst->oid))
 		return 0; /* they are the same */
 
-	if (diff_populate_filespec(r, src, NULL) ||
-	    diff_populate_filespec(r, dst, NULL))
+	if (r == the_repository && has_promisor_remote()) {
+		options.missing_object_cb = diff_queued_diff_prefetch;
+		options.missing_object_data = r;
+	}
+
+	if (diff_populate_filespec(r, src, &options) ||
+	    diff_populate_filespec(r, dst, &options))
 		return 0; /* error but caught downstream */
 
 	max_size = ((src->size > dst->size) ? src->size : dst->size);