index-pack: prefetch missing REF_DELTA bases

When fetching, the client sends "have" commit IDs indicating that the
server does not need to send any object referenced by those commits,
reducing network I/O. When the client is a partial clone, the client
still sends "have"s in this way, even if it does not have every object
referenced by a commit it sent as "have".

If a server omits such an object, it is fine: the client could lazily
fetch that object before this fetch, and it can still do so after.

The issue is when the server sends a thin pack containing an object that
is a REF_DELTA against such a missing object: index-pack fails to fix
the thin pack. When support for lazily fetching missing objects was
added in 8b4c0103a9 ("sha1_file: support lazily fetching missing
objects", 2017-12-08), support in index-pack was turned off in the
belief that it accesses the repo only to do hash collision checks.
However, this is not true: it also needs to access the repo to resolve
REF_DELTA bases.

Support for lazy fetching should still generally be turned off in
index-pack because it is used as part of the lazy fetching process
itself (if not, infinite loops may occur), but we do need to fetch the
REF_DELTA bases. (When fetching REF_DELTA bases, it is unlikely that
those are REF_DELTA themselves, because we do not send "have" when
making such fetches.)

To resolve this, prefetch all missing REF_DELTA bases before attempting
to resolve them. This both ensures that all bases are attempted to be
fetched, and ensures that we make only one request per index-pack
invocation, and not one request per missing object.

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

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index ccf4eb7e9b3361ee7eb209180d98ce8df757d92d..0d55f73b0b443b60dccc31096675d40691b3eb56 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -14,6 +14,7 @@
 #include "thread-utils.h"
 #include "packfile.h"
 #include "object-store.h"
 #include "thread-utils.h"
 #include "packfile.h"
 #include "object-store.h"

+#include "fetch-object.h"

 
 static const char index_pack_usage[] =
 "git index-pack [-v] [-o <index-file>] [--keep | --keep=<msg>] [--verify] [--strict] (<pack-file> | --stdin [--fix-thin] [<pack-file>])";
 
 static const char index_pack_usage[] =
 "git index-pack [-v] [-o <index-file>] [--keep | --keep=<msg>] [--verify] [--strict] (<pack-file> | --stdin [--fix-thin] [<pack-file>])";


@@ -1351,6 +1352,25 @@ static void fix_unresolved_deltas(struct hashfile *f)
                sorted_by_pos[i] = &ref_deltas[i];
        QSORT(sorted_by_pos, nr_ref_deltas, delta_pos_compare);
 
                sorted_by_pos[i] = &ref_deltas[i];
        QSORT(sorted_by_pos, nr_ref_deltas, delta_pos_compare);
 

+       if (repository_format_partial_clone) {
+               /*
+                * Prefetch the delta bases.
+                */
+               struct oid_array to_fetch = OID_ARRAY_INIT;
+               for (i = 0; i < nr_ref_deltas; i++) {
+                       struct ref_delta_entry *d = sorted_by_pos[i];
+                       if (!oid_object_info_extended(the_repository, &d->oid,
+                                                     NULL,
+                                                     OBJECT_INFO_FOR_PREFETCH))
+                               continue;
+                       oid_array_append(&to_fetch, &d->oid);
+               }
+               if (to_fetch.nr)
+                       fetch_objects(repository_format_partial_clone,
+                                     to_fetch.oid, to_fetch.nr);
+               oid_array_clear(&to_fetch);
+       }
+

        for (i = 0; i < nr_ref_deltas; i++) {
                struct ref_delta_entry *d = sorted_by_pos[i];
                enum object_type type;
        for (i = 0; i < nr_ref_deltas; i++) {
                struct ref_delta_entry *d = sorted_by_pos[i];
                enum object_type type;


@@ -1650,8 +1670,10 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
        int report_end_of_input = 0;
 
        /*
        int report_end_of_input = 0;
 
        /*

-        * index-pack never needs to fetch missing objects, since it only
-        * accesses the repo to do hash collision checks
+        * index-pack never needs to fetch missing objects except when
+        * REF_DELTA bases are missing (which are explicitly handled). It only
+        * accesses the repo to do hash collision checks and to check which
+        * REF_DELTA bases need to be fetched.

         */
        fetch_if_missing = 0;
 
         */
        fetch_if_missing = 0;
 

diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 7cc0c71556171c807ca53045bcc85bebace3b3b6..f1baf83502093c338c7fe9bec97e69ff25f373ed 100755 (executable)
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -339,4 +339,65 @@ test_expect_success 'when partial cloning, tolerate server not sending target of
        ! test -e "$HTTPD_ROOT_PATH/one-time-sed"
 '
 
        ! test -e "$HTTPD_ROOT_PATH/one-time-sed"
 '
 

+test_expect_success 'tolerate server sending REF_DELTA against missing promisor objects' '
+       SERVER="$HTTPD_DOCUMENT_ROOT_PATH/server" &&
+       rm -rf "$SERVER" repo &&
+       test_create_repo "$SERVER" &&
+       test_config -C "$SERVER" uploadpack.allowfilter 1 &&
+       test_config -C "$SERVER" uploadpack.allowanysha1inwant 1 &&
+
+       # Create a commit with a blob to be used as a delta base.
+       for i in $(test_seq 10)
+       do
+               echo "this is a line" >>"$SERVER/foo.txt"
+       done &&
+       git -C "$SERVER" add foo.txt &&
+       git -C "$SERVER" commit -m bar &&
+       git -C "$SERVER" rev-parse HEAD:foo.txt >deltabase &&
+
+       git -c protocol.version=2 clone --no-checkout \
+               --filter=blob:none $HTTPD_URL/one_time_sed/server repo &&
+
+       # Sanity check to ensure that the client does not have that blob.
+       git -C repo rev-list --objects --exclude-promisor-objects \
+               -- $(cat deltabase) >objlist &&
+       test_line_count = 0 objlist &&
+
+       # Another commit. This commit will be fetched by the client.
+       echo "abcdefghijklmnopqrstuvwxyz" >>"$SERVER/foo.txt" &&
+       git -C "$SERVER" add foo.txt &&
+       git -C "$SERVER" commit -m baz &&
+
+       # Pack a thin pack containing, among other things, HEAD:foo.txt
+       # delta-ed against HEAD^:foo.txt.
+       printf "%s\n--not\n%s\n" \
+               $(git -C "$SERVER" rev-parse HEAD) \
+               $(git -C "$SERVER" rev-parse HEAD^) |
+               git -C "$SERVER" pack-objects --thin --stdout >thin.pack &&
+
+       # Ensure that the pack contains one delta against HEAD^:foo.txt. Since
+       # the delta contains at least 26 novel characters, the size cannot be
+       # contained in 4 bits, so the object header will take up 2 bytes. The
+       # most significant nybble of the first byte is 0b1111 (0b1 to indicate
+       # that the header continues, and 0b111 to indicate REF_DELTA), followed
+       # by any 3 nybbles, then the OID of the delta base.
+       git -C "$SERVER" rev-parse HEAD^:foo.txt >deltabase &&
+       printf "f.,..%s" $(intersperse "," <deltabase) >want &&
+       hex_unpack <thin.pack | intersperse "," >have &&
+       grep $(cat want) have &&
+
+       replace_packfile thin.pack &&
+
+       # Use protocol v2 because the sed command looks for the "packfile"
+       # section header.
+       test_config -C "$SERVER" protocol.version 2 &&
+
+       # Fetch the thin pack and ensure that index-pack is able to handle the
+       # REF_DELTA object with a missing promisor delta base.
+       git -C repo -c protocol.version=2 fetch &&
+
+       # Ensure that the one-time-sed script was used.
+       ! test -e "$HTTPD_ROOT_PATH/one-time-sed"
+'
+

 test_done
 test_done