Merge branch 'jk/fsck-doc'

"git fsck --connectivity-only" omits computation necessary to sift
the objects that are not reachable from any of the refs into
unreachable and dangling.  This is now enabled when dangling
objects are requested (which is done by default, but can be
overridden with the "--no-dangling" option).

* jk/fsck-doc:
  fsck: always compute USED flags for unreachable objects
  doc/fsck: clarify --connectivity-only behavior

Documentation/git-fsck.txt

@@ -62,9 +62,17 @@ index file, all SHA-1 references in `refs` namespace, and all reflogs
 	with --no-full.
 
 --connectivity-only::
-	Check only the connectivity of tags, commits and tree objects. By
-	avoiding to unpack blobs, this speeds up the operation, at the
-	expense of missing corrupt objects or other problematic issues.
+	Check only the connectivity of reachable objects, making sure
+	that any objects referenced by a reachable tag, commit, or tree
+	is present. This speeds up the operation by avoiding reading
+	blobs entirely (though it does still check that referenced blobs
+	exist). This will detect corruption in commits and trees, but
+	not do any semantic checks (e.g., for format errors). Corruption
+	in blob objects will not be detected at all.
++
+Unreachable tags, commits, and trees will also be accessed to find the
+tips of dangling segments of history. Use `--no-dangling` if you don't
+care about this output and want to speed it up further.
 
 --strict::
 	Enable more strict checking, namely to catch a file mode

builtin/fsck.c

@@ -235,6 +235,48 @@ static int mark_used(struct object *obj, int type, void *data, struct fsck_optio
 	return 0;
 }
 
+static void mark_unreachable_referents(const struct object_id *oid)
+{
+	struct fsck_options options = FSCK_OPTIONS_DEFAULT;
+	struct object *obj = lookup_object(the_repository, oid->hash);
+
+	if (!obj || !(obj->flags & HAS_OBJ))
+		return; /* not part of our original set */
+	if (obj->flags & REACHABLE)
+		return; /* reachable objects already traversed */
+
+	/*
+	 * Avoid passing OBJ_NONE to fsck_walk, which will parse the object
+	 * (and we want to avoid parsing blobs).
+	 */
+	if (obj->type == OBJ_NONE) {
+		enum object_type type = oid_object_info(the_repository,
+							&obj->oid, NULL);
+		if (type > 0)
+			object_as_type(the_repository, obj, type, 0);
+	}
+
+	options.walk = mark_used;
+	fsck_walk(obj, NULL, &options);
+}
+
+static int mark_loose_unreachable_referents(const struct object_id *oid,
+					    const char *path,
+					    void *data)
+{
+	mark_unreachable_referents(oid);
+	return 0;
+}
+
+static int mark_packed_unreachable_referents(const struct object_id *oid,
+					     struct packed_git *pack,
+					     uint32_t pos,
+					     void *data)
+{
+	mark_unreachable_referents(oid);
+	return 0;
+}
+
 /*
  * Check a single reachable object
  */
@@ -347,6 +389,26 @@ static void check_connectivity(void)
 	/* Traverse the pending reachable objects */
 	traverse_reachable();
 
+	/*
+	 * With --connectivity-only, we won't have actually opened and marked
+	 * unreachable objects with USED. Do that now to make --dangling, etc
+	 * accurate.
+	 */
+	if (connectivity_only && (show_dangling || write_lost_and_found)) {
+		/*
+		 * Even though we already have a "struct object" for each of
+		 * these in memory, we must not iterate over the internal
+		 * object hash as we do below. Our loop would potentially
+		 * resize the hash, making our iteration invalid.
+		 *
+		 * Instead, we'll just go back to the source list of objects,
+		 * and ignore any that weren't present in our earlier
+		 * traversal.
+		 */
+		for_each_loose_object(mark_loose_unreachable_referents, NULL, 0);
+		for_each_packed_object(mark_packed_unreachable_referents, NULL, 0);
+	}
+
 	/* Look up all the requirements, warn about missing objects.. */
 	max = get_max_object_index();
 	if (verbose)

t/t1450-fsck.sh

@@ -740,7 +740,7 @@ test_expect_success 'fsck detects truncated loose object' '
 # for each of type, we have one version which is referenced by another object
 # (and so while unreachable, not dangling), and another variant which really is
 # dangling.
-test_expect_success 'fsck notices dangling objects' '
+test_expect_success 'create dangling-object repository' '
 	git init dangling &&
 	(
 		cd dangling &&
@@ -751,19 +751,34 @@ test_expect_success 'fsck notices dangling objects' '
 		commit=$(git commit-tree $tree) &&
 		dcommit=$(git commit-tree -p $commit $tree) &&
 
-		cat >expect <<-EOF &&
+		cat >expect <<-EOF
 		dangling blob $dblob
 		dangling commit $dcommit
 		dangling tree $dtree
 		EOF
+	)
+'
 
+test_expect_success 'fsck notices dangling objects' '
+	(
+		cd dangling &&
 		git fsck >actual &&
 		# the output order is non-deterministic, as it comes from a hash
 		sort <actual >actual.sorted &&
 		test_i18ncmp expect actual.sorted
 	)
 '
 
+test_expect_success 'fsck --connectivity-only notices dangling objects' '
+	(
+		cd dangling &&
+		git fsck --connectivity-only >actual &&
+		# the output order is non-deterministic, as it comes from a hash
+		sort <actual >actual.sorted &&
+		test_i18ncmp expect actual.sorted
+	)
+'
+
 test_expect_success 'fsck $name notices bogus $name' '
 	test_must_fail git fsck bogus &&
 	test_must_fail git fsck $ZERO_OID