sha1_file: support lazily fetching missing objects

Teach sha1_file to fetch objects from the remote configured in
extensions.partialclone whenever an object is requested but missing.

The fetching of objects can be suppressed through a global variable.
This is used by fsck and index-pack.

However, by default, such fetching is not suppressed. This is meant as a
temporary measure to ensure that all Git commands work in such a
situation. Future patches will update some commands to either tolerate
missing objects (without fetching them) or be more efficient in fetching
them.

In order to determine the code changes in sha1_file.c necessary, I
investigated the following:
 (1) functions in sha1_file.c that take in a hash, without the user
     regarding how the object is stored (loose or packed)
 (2) functions in packfile.c (because I need to check callers that know
     about the loose/packed distinction and operate on both differently,
     and ensure that they can handle the concept of objects that are
     neither loose nor packed)

(1) is handled by the modification to sha1_object_info_extended().

For (2), I looked at for_each_packed_object and others.  For
for_each_packed_object, the callers either already work or are fixed in
this patch:
 - reachable - only to find recent objects
 - builtin/fsck - already knows about missing objects
 - builtin/cat-file - warning message added in this commit

Callers of the other functions do not need to be changed:
 - parse_pack_index
   - http - indirectly from http_get_info_packs
   - find_pack_entry_one
     - this searches a single pack that is provided as an argument; the
       caller already knows (through other means) that the sought object
       is in a specific pack
 - find_sha1_pack
   - fast-import - appears to be an optimization to not store a file if
     it is already in a pack
   - http-walker - to search through a struct alt_base
   - http-push - to search through remote packs
 - has_sha1_pack
   - builtin/fsck - already knows about promisor objects
   - builtin/count-objects - informational purposes only (check if loose
     object is also packed)
   - builtin/prune-packed - check if object to be pruned is packed (if
     not, don't prune it)
   - revision - used to exclude packed objects if requested by user
   - diff - just for optimization

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 24 insertions, 8 deletions

diff --git a/sha1_file.c b/sha1_file.c
index 10c3a00..dd956e2 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -29,6 +29,7 @@
 #include "mergesort.h"
 #include "quote.h"
 #include "packfile.h"
+#include "fetch-object.h"
 
 const unsigned char null_sha1[GIT_MAX_RAWSZ];
 const struct object_id null_oid;
@@ -1144,6 +1145,8 @@ static int sha1_loose_object_info(const unsigned char *sha1,
 	return (status < 0) ? status : 0;
 }
 
+int fetch_if_missing = 1;
+
 int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi, unsigned flags)
 {
 	static struct object_info blank_oi = OBJECT_INFO_INIT;
@@ -1152,6 +1155,7 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 	const unsigned char *real = (flags & OBJECT_INFO_LOOKUP_REPLACE) ?
 				    lookup_replace_object(sha1) :
 				    sha1;
+	int already_retried = 0;
 
 	if (!oi)
 		oi = &blank_oi;
@@ -1176,19 +1180,32 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 		}
 	}
 
-	if (!find_pack_entry(real, &e)) {
+	while (1) {
+		if (find_pack_entry(real, &e))
+			break;
+
 		/* Most likely it's a loose object. */
 		if (!sha1_loose_object_info(real, oi, flags))
 			return 0;
 
 		/* Not a loose object; someone else may have just packed it. */
-		if (flags & OBJECT_INFO_QUICK) {
-			return -1;
-		} else {
-			reprepare_packed_git();
-			if (!find_pack_entry(real, &e))
-				return -1;
+		reprepare_packed_git();
+		if (find_pack_entry(real, &e))
+			break;
+
+		/* Check if it is a missing object */
+		if (fetch_if_missing && repository_format_partial_clone &&
+		    !already_retried) {
+			/*
+			 * TODO Investigate haveing fetch_object() return
+			 * TODO error/success and stopping the music here.
+			 */
+			fetch_object(repository_format_partial_clone, real);
+			already_retried = 1;
+			continue;
 		}
+
+		return -1;
 	}
 
 	if (oi == &blank_oi)
@@ -1197,7 +1214,6 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 		 * information below, so return early.
 		 */
 		return 0;
-
 	rtype = packed_object_info(e.p, e.offset, oi);
 	if (rtype < 0) {
 		mark_bad_packed_object(e.p, real);