make pack-objects a bit more resilient to repo corruption

Right now, packing valid objects could fail when creating a thin pack
simply because a pack edge object used as a preferred base is corrupted.
Since preferred base objects are not strictly needed to produce a valid
pack, let's not consider the inability to read them as a fatal error.
Delta compression may well be attempted against other objects in the
search window.  To avoid warning storms (we are in the inner loop of
the delta search window) a warning is emitted only on the first
occurrence.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -1298,9 +1298,23 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 		read_lock();
 		src->data = read_sha1_file(src_entry->idx.sha1, &type, &sz);
 		read_unlock();
-		if (!src->data)
+		if (!src->data) {
+			if (src_entry->preferred_base) {
+				static int warned = 0;
+				if (!warned++)
+					warning("object %s cannot be read",
+						sha1_to_hex(src_entry->idx.sha1));
+				/*
+				 * Those objects are not included in the
+				 * resulting pack.  Be resilient and ignore
+				 * them if they can't be read, in case the
+				 * pack could be created nevertheless.
+				 */
+				return 0;
+			}
 			die("object %s cannot be read",
 			    sha1_to_hex(src_entry->idx.sha1));
+		}
 		if (sz != src_size)
 			die("object %s inconsistent object length (%lu vs %lu)",
 			    sha1_to_hex(src_entry->idx.sha1), sz, src_size);