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, 21 insertions, 0 deletions

diff --git a/diffcore.h b/diffcore.h
index 3b2020c..d2a63c5 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -65,9 +65,22 @@ void free_filespec(struct diff_filespec *);
 void fill_filespec(struct diff_filespec *, const struct object_id *,
 		   int, unsigned short);
 
+/*
+ * Prefetch the entries in diff_queued_diff. The parameter is a pointer to a
+ * struct repository.
+ */
+void diff_queued_diff_prefetch(void *repository);
+
 struct diff_populate_filespec_options {
 	unsigned check_size_only : 1;
 	unsigned check_binary : 1;
+
+	/*
+	 * If an object is missing, diff_populate_filespec() will invoke this
+	 * callback before attempting to read that object again.
+	 */
+	void (*missing_object_cb)(void *);
+	void *missing_object_data;
 };
 int diff_populate_filespec(struct repository *, struct diff_filespec *,
 			   const struct diff_populate_filespec_options *);
@@ -185,4 +198,12 @@ int diffcore_count_changes(struct repository *r,
 			   unsigned long *src_copied,
 			   unsigned long *literal_added);
 
+/*
+ * If filespec contains an OID and if that object is missing from the given
+ * repository, add that OID to to_fetch.
+ */
+void diff_add_if_missing(struct repository *r,
+			 struct oid_array *to_fetch,
+			 const struct diff_filespec *filespec);
+
 #endif