pack-objects: Allow missing base objects when creating thin packs

If we are building a thin pack and one of the base objects we would
consider for deltification is missing its OK, the other side already
has that base object.  We may be able to get a delta from another
object, or we can simply send the new object whole (no delta).

This change allows a shallow clone to store only the objects which
are unique to it, as well as the boundary commit and its trees, but
avoids storing the boundary blobs.  This special form of a shallow
clone is able to represent just the difference between two trees.

Pack objects change suggested by Nicolas Pitre.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1096,9 +1096,12 @@ static void check_object(struct object_entry *entry)
 	}
 
 	entry->type = sha1_object_info(entry->idx.sha1, &entry->size);
-	if (entry->type < 0)
-		die("unable to get type of object %s",
-		    sha1_to_hex(entry->idx.sha1));
+	/*
+	 * The error condition is checked in prepare_pack().  This is
+	 * to permit a missing preferred base object to be ignored
+	 * as a preferred base.  Doing so can result in a larger
+	 * pack file, but the transfer will still take place.
+	 */
 }
 
 static int pack_offset_sort(const void *_a, const void *_b)
@@ -1722,8 +1725,12 @@ static void prepare_pack(int window, int depth)
 		if (entry->no_try_delta)
 			continue;
 
-		if (!entry->preferred_base)
+		if (!entry->preferred_base) {
 			nr_deltas++;
+			if (entry->type < 0)
+				die("unable to get type of object %s",
+				    sha1_to_hex(entry->idx.sha1));
+		}
 
 		delta_list[n++] = entry;
 	}

t/t5306-pack-nobase.sh

@@ -0,0 +1,80 @@
+#!/bin/sh
+#
+# Copyright (c) 2008 Google Inc.
+#
+
+test_description='git-pack-object with missing base
+
+'
+. ./test-lib.sh
+
+# Create A-B chain
+#
+test_expect_success \
+    'setup base' \
+    'for a in a b c d e f g h i; do echo $a >>text; done &&
+     echo side >side &&
+     git update-index --add text side &&
+     A=$(echo A | git commit-tree $(git write-tree)) &&
+
+     echo m >>text &&
+     git update-index text &&
+     B=$(echo B | git commit-tree $(git write-tree) -p $A) &&
+     git update-ref HEAD $B
+    '
+
+# Create repository with C whose parent is B.
+# Repository contains C, C^{tree}, C:text, B, B^{tree}.
+# Repository is missing B:text (best delta base for C:text).
+# Repository is missing A (parent of B).
+# Repository is missing A:side.
+#
+test_expect_success \
+    'setup patch_clone' \
+    'base_objects=$(pwd)/.git/objects &&
+     (mkdir patch_clone &&
+      cd patch_clone &&
+      git init &&
+      echo "$base_objects" >.git/objects/info/alternates &&
+      echo q >>text &&
+      git read-tree $B &&
+      git update-index text &&
+      git update-ref HEAD $(echo C | git commit-tree $(git write-tree) -p $B) &&
+      rm .git/objects/info/alternates &&
+
+      git --git-dir=../.git cat-file commit $B |
+      git hash-object -t commit -w --stdin &&
+
+      git --git-dir=../.git cat-file tree "$B^{tree}" |
+      git hash-object -t tree -w --stdin
+     ) &&
+     C=$(git --git-dir=patch_clone/.git rev-parse HEAD)
+    '
+
+# Clone patch_clone indirectly by cloning base and fetching.
+#
+test_expect_success \
+    'indirectly clone patch_clone' \
+    '(mkdir user_clone &&
+      cd user_clone &&
+      git init &&
+      git pull ../.git &&
+      test $(git rev-parse HEAD) = $B &&
+
+      git pull ../patch_clone/.git &&
+      test $(git rev-parse HEAD) = $C
+     )
+    '
+
+# Cloning the patch_clone directly should fail.
+#
+test_expect_success \
+    'clone of patch_clone is incomplete' \
+    '(mkdir user_direct &&
+      cd user_direct &&
+      git init &&
+      test_must_fail git fetch ../patch_clone/.git
+     )
+    '
+
+test_done