Merge branch 'jc/sha1-name-more'

Teaches the object name parser things like a "git describe" output
is always a commit object, "A" in "git log A" must be a committish,
and "A" and "B" in "git log A...B" both must be committish, etc., to
prolong the lifetime of abbreviated object names.

* jc/sha1-name-more: (27 commits)
  t1512: match the "other" object names
  t1512: ignore whitespaces in wc -l output
  rev-parse --disambiguate=<prefix>
  rev-parse: A and B in "rev-parse A..B" refer to committish
  reset: the command takes committish
  commit-tree: the command wants a tree and commits
  apply: --build-fake-ancestor expects blobs
  sha1_name.c: add support for disambiguating other types
  revision.c: the "log" family, except for "show", takes committish
  revision.c: allow handle_revision_arg() to take other flags
  sha1_name.c: introduce get_sha1_committish()
  sha1_name.c: teach lookup context to get_sha1_with_context()
  sha1_name.c: many short names can only be committish
  sha1_name.c: get_sha1_1() takes lookup flags
  sha1_name.c: get_describe_name() by definition groks only commits
  sha1_name.c: teach get_short_sha1() a commit-only option
  sha1_name.c: allow get_short_sha1() to take other flags
  get_sha1(): fix error status regression
  sha1_name.c: restructure disambiguation of short names
  sha1_name.c: correct misnamed "canonical" and "res"
  ...

15 files changed, 714 insertions, 172 deletions

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 4cc3e9586..3c63561f0 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -101,6 +101,12 @@ OPTIONS
 	The option core.warnAmbiguousRefs is used to select the strict
 	abbreviation mode.
 
+--disambiguate=<prefix>::
+	Show every object whose name begins with the given prefix.
+	The <prefix> must be at least 4 hexadecimal digits long to
+	avoid listing each and every object in the repository by
+	mistake.
+
 --all::
 	Show all refs found in `refs/`.
 
diff --git a/builtin/apply.c b/builtin/apply.c
index ace04c453..069cf341b 100644
--- a/builtin/apply.c
+++ b/builtin/apply.c
@@ -3589,7 +3589,7 @@ static void build_fake_ancestor(struct patch *list, const char *filename)
 		name = patch->old_name ? patch->old_name : patch->new_name;
 		if (0 < patch->is_new)
 			continue;
-		else if (get_sha1(patch->old_sha1_prefix, sha1))
+		else if (get_sha1_blob(patch->old_sha1_prefix, sha1))
 			/* git diff has no index line for mode/type changes */
 			if (!patch->lines_added && !patch->lines_deleted) {
 				if (get_current_sha1(patch->old_name, sha1))
diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 36a910443..af74e775a 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -91,7 +91,7 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name)
 	unsigned long size;
 	struct object_context obj_context;
 
-	if (get_sha1_with_context(obj_name, sha1, &obj_context))
+	if (get_sha1_with_context(obj_name, 0, sha1, &obj_context))
 		die("Not a valid object name %s", obj_name);
 
 	buf = NULL;
diff --git a/builtin/commit-tree.c b/builtin/commit-tree.c
index 164b655df..cb982c550 100644
--- a/builtin/commit-tree.c
+++ b/builtin/commit-tree.c
@@ -48,8 +48,8 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 	if (argc < 2 || !strcmp(argv[1], "-h"))
 		usage(commit_tree_usage);
 
-	if (get_sha1(argv[1], tree_sha1))
-		die("Not a valid object name %s", argv[1]);
+	if (get_sha1_tree(argv[1], tree_sha1))
+		die("Not a valid tree object name %s", argv[1]);
 
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
@@ -57,7 +57,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			unsigned char sha1[20];
 			if (argc <= ++i)
 				usage(commit_tree_usage);
-			if (get_sha1(argv[i], sha1))
+			if (get_sha1_commit(argv[i], sha1))
 				die("Not a valid object name %s", argv[i]);
 			assert_sha1_type(sha1, OBJ_COMMIT);
 			new_parent(lookup_commit(sha1), &parents);
@@ -104,7 +104,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			continue;
 		}
 
-		if (get_sha1(arg, tree_sha1))
+		if (get_sha1_tree(arg, tree_sha1))
 			die("Not a valid object name %s", arg);
 		if (got_tree)
 			die("Cannot give more than one trees");
diff --git a/builtin/log.c b/builtin/log.c
index adcbcf1f2..ecc279369 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -367,6 +367,7 @@ int cmd_whatchanged(int argc, const char **argv, const char *prefix)
 	rev.simplify_history = 0;
 	memset(&opt, 0, sizeof(opt));
 	opt.def = "HEAD";
+	opt.revarg_opt = REVARG_COMMITTISH;
 	cmd_log_init(argc, argv, prefix, &rev, &opt);
 	if (!rev.diffopt.output_format)
 		rev.diffopt.output_format = DIFF_FORMAT_RAW;
@@ -557,6 +558,7 @@ int cmd_log(int argc, const char **argv, const char *prefix)
 	rev.always_show_header = 1;
 	memset(&opt, 0, sizeof(opt));
 	opt.def = "HEAD";
+	opt.revarg_opt = REVARG_COMMITTISH;
 	cmd_log_init(argc, argv, prefix, &rev, &opt);
 	return cmd_log_walk(&rev);
 }
@@ -1132,6 +1134,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	rev.subject_prefix = fmt_patch_subject_prefix;
 	memset(&s_r_opt, 0, sizeof(s_r_opt));
 	s_r_opt.def = "HEAD";
+	s_r_opt.revarg_opt = REVARG_COMMITTISH;
 
 	if (default_attach) {
 		rev.mime_boundary = default_attach;
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index f3348208d..782e7d0c3 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2373,7 +2373,7 @@ static void get_object_list(int ac, const char **av)
 			}
 			die("not a rev '%s'", line);
 		}
-		if (handle_revision_arg(line, &revs, flags, 1))
+		if (handle_revision_arg(line, &revs, flags, REVARG_CANNOT_BE_FILENAME))
 			die("bad revision '%s'", line);
 	}
 
diff --git a/builtin/reset.c b/builtin/reset.c
index 4cc34c908..74442bd76 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -276,7 +276,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		 * Otherwise, argv[i] could be either <rev> or <paths> and
 		 * has to be unambiguous.
 		 */
-		else if (!get_sha1(argv[i], sha1)) {
+		else if (!get_sha1_committish(argv[i], sha1)) {
 			/*
 			 * Ok, argv[i] looks like a rev; it should not
 			 * be a filename.
@@ -289,9 +289,15 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (get_sha1(rev, sha1))
+	if (get_sha1_committish(rev, sha1))
 		die(_("Failed to resolve '%s' as a valid ref."), rev);
 
+	/*
+	 * NOTE: As "git reset $treeish -- $path" should be usable on
+	 * any tree-ish, this is not strictly correct. We are not
+	 * moving the HEAD to any commit; we are merely resetting the
+	 * entries in the index to that of a treeish.
+	 */
 	commit = lookup_commit_reference(sha1);
 	if (!commit)
 		die(_("Could not parse object '%s'."), rev);
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 13495b88f..32788a9f8 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -195,6 +195,12 @@ static int anti_reference(const char *refname, const unsigned char *sha1, int fl
 	return 0;
 }
 
+static int show_abbrev(const unsigned char *sha1, void *cb_data)
+{
+	show_rev(NORMAL, sha1, NULL);
+	return 0;
+}
+
 static void show_datestring(const char *flag, const char *datestr)
 {
 	static char buffer[100];
@@ -238,7 +244,7 @@ static int try_difference(const char *arg)
 		next = "HEAD";
 	if (dotdot == arg)
 		this = "HEAD";
-	if (!get_sha1(this, sha1) && !get_sha1(next, end)) {
+	if (!get_sha1_committish(this, sha1) && !get_sha1_committish(next, end)) {
 		show_rev(NORMAL, end, next);
 		show_rev(symmetric ? NORMAL : REVERSED, sha1, this);
 		if (symmetric) {
@@ -278,7 +284,7 @@ static int try_parent_shorthands(const char *arg)
 		return 0;
 
 	*dotdot = 0;
-	if (get_sha1(arg, sha1))
+	if (get_sha1_committish(arg, sha1))
 		return 0;
 
 	if (!parents_only)
@@ -589,6 +595,10 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				for_each_ref(show_reference, NULL);
 				continue;
 			}
+			if (!prefixcmp(arg, "--disambiguate=")) {
+				for_each_abbrev(arg + 15, show_abbrev, NULL);
+				continue;
+			}
 			if (!strcmp(arg, "--bisect")) {
 				for_each_ref_in("refs/bisect/bad", show_reference, NULL);
 				for_each_ref_in("refs/bisect/good", anti_reference, NULL);
diff --git a/cache.h b/cache.h
index 6a1aff5e2..2997b918e 100644
--- a/cache.h
+++ b/cache.h
@@ -790,17 +790,25 @@ struct object_context {
 	unsigned mode;
 };
 
+#define GET_SHA1_QUIETLY        01
+#define GET_SHA1_COMMIT         02
+#define GET_SHA1_COMMITTISH     04
+#define GET_SHA1_TREE          010
+#define GET_SHA1_TREEISH       020
+#define GET_SHA1_BLOB	       040
+#define GET_SHA1_ONLY_TO_DIE 04000
+
 extern int get_sha1(const char *str, unsigned char *sha1);
-extern int get_sha1_with_mode_1(const char *str, unsigned char *sha1, unsigned *mode, int only_to_die, const char *prefix);
-static inline int get_sha1_with_mode(const char *str, unsigned char *sha1, unsigned *mode)
-{
-	return get_sha1_with_mode_1(str, sha1, mode, 0, NULL);
-}
-extern int get_sha1_with_context_1(const char *name, unsigned char *sha1, struct object_context *orc, int only_to_die, const char *prefix);
-static inline int get_sha1_with_context(const char *str, unsigned char *sha1, struct object_context *orc)
-{
-	return get_sha1_with_context_1(str, sha1, orc, 0, NULL);
-}
+extern int get_sha1_commit(const char *str, unsigned char *sha1);
+extern int get_sha1_committish(const char *str, unsigned char *sha1);
+extern int get_sha1_tree(const char *str, unsigned char *sha1);
+extern int get_sha1_treeish(const char *str, unsigned char *sha1);
+extern int get_sha1_blob(const char *str, unsigned char *sha1);
+extern void maybe_die_on_misspelt_object_name(const char *name, const char *prefix);
+extern int get_sha1_with_context(const char *str, unsigned flags, unsigned char *sha1, struct object_context *orc);
+
+typedef int each_abbrev_fn(const unsigned char *sha1, void *);
+extern int for_each_abbrev(const char *prefix, each_abbrev_fn, void *);
 
 /*
  * Try to read a SHA1 in hexadecimal format from the 40 characters
diff --git a/commit.c b/commit.c
index 8248a994a..42af4c1f2 100644
--- a/commit.c
+++ b/commit.c
@@ -68,7 +68,7 @@ struct commit *lookup_commit_reference_by_name(const char *name)
 	unsigned char sha1[20];
 	struct commit *commit;
 
-	if (get_sha1(name, sha1))
+	if (get_sha1_committish(name, sha1))
 		return NULL;
 	commit = lookup_commit_reference(sha1);
 	if (!commit || parse_commit(commit))
diff --git a/revision.c b/revision.c
index 5b81a92e3..c64bf68fa 100644
--- a/revision.c
+++ b/revision.c
@@ -1000,7 +1000,7 @@ static int add_parents_only(struct rev_info *revs, const char *arg_, int flags)
 		flags ^= UNINTERESTING;
 		arg++;
 	}
-	if (get_sha1(arg, sha1))
+	if (get_sha1_committish(arg, sha1))
 		return 0;
 	while (1) {
 		it = get_reference(revs, arg, sha1, 0);
@@ -1114,16 +1114,16 @@ static void prepare_show_merge(struct rev_info *revs)
 	revs->limited = 1;
 }
 
-int handle_revision_arg(const char *arg_, struct rev_info *revs,
-			int flags,
-			int cant_be_filename)
+int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsigned revarg_opt)
 {
-	unsigned mode;
+	struct object_context oc;
 	char *dotdot;
 	struct object *object;
 	unsigned char sha1[20];
 	int local_flags;
 	const char *arg = arg_;
+	int cant_be_filename = revarg_opt & REVARG_CANNOT_BE_FILENAME;
+	unsigned get_sha1_flags = 0;
 
 	dotdot = strstr(arg, "..");
 	if (dotdot) {
@@ -1141,8 +1141,8 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs,
 			next = "HEAD";
 		if (dotdot == arg)
 			this = "HEAD";
-		if (!get_sha1(this, from_sha1) &&
-		    !get_sha1(next, sha1)) {
+		if (!get_sha1_committish(this, from_sha1) &&
+		    !get_sha1_committish(next, sha1)) {
 			struct commit *a, *b;
 			struct commit_list *exclude;
 
@@ -1201,13 +1201,17 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs,
 		local_flags = UNINTERESTING;
 		arg++;
 	}
-	if (get_sha1_with_mode(arg, sha1, &mode))
+
+	if (revarg_opt & REVARG_COMMITTISH)
+		get_sha1_flags = GET_SHA1_COMMITTISH;
+
+	if (get_sha1_with_context(arg, get_sha1_flags, sha1, &oc))
 		return revs->ignore_missing ? 0 : -1;
 	if (!cant_be_filename)
 		verify_non_filename(revs->prefix, arg);
 	object = get_reference(revs, arg, sha1, flags ^ local_flags);
 	add_rev_cmdline(revs, object, arg_, REV_CMD_REV, flags ^ local_flags);
-	add_pending_object_with_mode(revs, object, arg, mode);
+	add_pending_object_with_mode(revs, object, arg, oc.mode);
 	return 0;
 }
 
@@ -1257,7 +1261,7 @@ static void read_revisions_from_stdin(struct rev_info *revs,
 			}
 			die("options not supported in --stdin mode");
 		}
-		if (handle_revision_arg(sb.buf, revs, 0, 1))
+		if (handle_revision_arg(sb.buf, revs, 0, REVARG_CANNOT_BE_FILENAME))
 			die("bad revision '%s'", sb.buf);
 	}
 	if (seen_dashdash)
@@ -1708,7 +1712,7 @@ static int handle_revision_pseudo_opt(const char *submodule,
  */
 int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct setup_revision_opt *opt)
 {
-	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0;
+	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0, revarg_opt;
 	struct cmdline_pathspec prune_data;
 	const char *submodule = NULL;
 
@@ -1736,6 +1740,9 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 
 	/* Second, deal with arguments and options */
 	flags = 0;
+	revarg_opt = opt ? opt->revarg_opt : 0;
+	if (seen_dashdash)
+		revarg_opt |= REVARG_CANNOT_BE_FILENAME;
 	read_from_stdin = 0;
 	for (left = i = 1; i < argc; i++) {
 		const char *arg = argv[i];
@@ -1771,7 +1778,8 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 			continue;
 		}
 
-		if (handle_revision_arg(arg, revs, flags, seen_dashdash)) {
+
+		if (handle_revision_arg(arg, revs, flags, revarg_opt)) {
 			int j;
 			if (seen_dashdash || *arg == '^')
 				die("bad revision '%s'", arg);
@@ -1822,11 +1830,11 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 	if (revs->def && !revs->pending.nr && !got_rev_arg) {
 		unsigned char sha1[20];
 		struct object *object;
-		unsigned mode;
-		if (get_sha1_with_mode(revs->def, sha1, &mode))
+		struct object_context oc;
+		if (get_sha1_with_context(revs->def, 0, sha1, &oc))
 			die("bad default revision '%s'", revs->def);
 		object = get_reference(revs, revs->def, sha1, 0);
-		add_pending_object_with_mode(revs, object, revs->def, mode);
+		add_pending_object_with_mode(revs, object, revs->def, oc.mode);
 	}
 
 	/* Did the user ask for any diff output? Run the diff! */
diff --git a/revision.h b/revision.h
index 863f4f645..cb5ab3513 100644
--- a/revision.h
+++ b/revision.h
@@ -184,6 +184,7 @@ struct setup_revision_opt {
 	void (*tweak)(struct rev_info *, struct setup_revision_opt *);
 	const char *submodule;
 	int assume_dashdash;
+	unsigned revarg_opt;
 };
 
 extern void init_revisions(struct rev_info *revs, const char *prefix);
@@ -191,7 +192,9 @@ extern int setup_revisions(int argc, const char **argv, struct rev_info *revs, s
 extern void parse_revision_opt(struct rev_info *revs, struct parse_opt_ctx_t *ctx,
 				 const struct option *options,
 				 const char * const usagestr[]);
-extern int handle_revision_arg(const char *arg, struct rev_info *revs,int flags,int cant_be_filename);
+#define REVARG_CANNOT_BE_FILENAME 01
+#define REVARG_COMMITTISH 02
+extern int handle_revision_arg(const char *arg, struct rev_info *revs, int flags, unsigned revarg_opt);
 
 extern void reset_revision_walk(void);
 extern int prepare_revision_walk(struct rev_info *revs);
diff --git a/setup.c b/setup.c
index e11497720..9139beefc 100644
--- a/setup.c
+++ b/setup.c
@@ -77,9 +77,6 @@ static void NORETURN die_verify_filename(const char *prefix,
 					 const char *arg,
 					 int diagnose_misspelt_rev)
 {
-	unsigned char sha1[20];
-	unsigned mode;
-
 	if (!diagnose_misspelt_rev)
 		die("%s: no such path in the working tree.\n"
 		    "Use '-- <path>...' to specify paths that do not exist locally.",
@@ -88,11 +85,10 @@ static void NORETURN die_verify_filename(const char *prefix,
 	 * Saying "'(icase)foo' does not exist in the index" when the
 	 * user gave us ":(icase)foo" is just stupid.  A magic pathspec
 	 * begins with a colon and is followed by a non-alnum; do not
-	 * let get_sha1_with_mode_1(only_to_die=1) to even trigger.
+	 * let maybe_die_on_misspelt_object_name() even trigger.
 	 */
 	if (!(arg[0] == ':' && !isalnum(arg[1])))
-		/* try a detailed diagnostic ... */
-		get_sha1_with_mode_1(arg, sha1, &mode, 1, prefix);
+		maybe_die_on_misspelt_object_name(arg, prefix);
 
 	/* ... or fall back the most general message. */
 	die("ambiguous argument '%s': unknown revision or path not in the working tree.\n"
diff --git a/sha1_name.c b/sha1_name.c
index 5d81ea056..95003c77e 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -9,14 +9,82 @@
 
 static int get_sha1_oneline(const char *, unsigned char *, struct commit_list *);
 
-static int find_short_object_filename(int len, const char *name, unsigned char *sha1)
+typedef int (*disambiguate_hint_fn)(const unsigned char *, void *);
+
+struct disambiguate_state {
+	disambiguate_hint_fn fn;
+	void *cb_data;
+	unsigned char candidate[20];
+	unsigned candidate_exists:1;
+	unsigned candidate_checked:1;
+	unsigned candidate_ok:1;
+	unsigned disambiguate_fn_used:1;
+	unsigned ambiguous:1;
+	unsigned always_call_fn:1;
+};
+
+static void update_candidates(struct disambiguate_state *ds, const unsigned char *current)
+{
+	if (ds->always_call_fn) {
+		ds->ambiguous = ds->fn(current, ds->cb_data) ? 1 : 0;
+		return;
+	}
+	if (!ds->candidate_exists) {
+		/* this is the first candidate */
+		hashcpy(ds->candidate, current);
+		ds->candidate_exists = 1;
+		return;
+	} else if (!hashcmp(ds->candidate, current)) {
+		/* the same as what we already have seen */
+		return;
+	}
+
+	if (!ds->fn) {
+		/* cannot disambiguate between ds->candidate and current */
+		ds->ambiguous = 1;
+		return;
+	}
+
+	if (!ds->candidate_checked) {
+		ds->candidate_ok = ds->fn(ds->candidate, ds->cb_data);
+		ds->disambiguate_fn_used = 1;
+		ds->candidate_checked = 1;
+	}
+
+	if (!ds->candidate_ok) {
+		/* discard the candidate; we know it does not satisify fn */
+		hashcpy(ds->candidate, current);
+		ds->candidate_checked = 0;
+		return;
+	}
+
+	/* if we reach this point, we know ds->candidate satisfies fn */
+	if (ds->fn(current, ds->cb_data)) {
+		/*
+		 * if both current and candidate satisfy fn, we cannot
+		 * disambiguate.
+		 */
+		ds->candidate_ok = 0;
+		ds->ambiguous = 1;
+	}
+
+	/* otherwise, current can be discarded and candidate is still good */
+}
+
+static void find_short_object_filename(int len, const char *hex_pfx, struct disambiguate_state *ds)
 {
 	struct alternate_object_database *alt;
 	char hex[40];
-	int found = 0;
 	static struct alternate_object_database *fakeent;
 
 	if (!fakeent) {
+		/*
+		 * Create a "fake" alternate object database that
+		 * points to our own object database, to make it
+		 * easier to get a temporary working space in
+		 * alt->name/alt->base while iterating over the
+		 * object databases including our own.
+		 */
 		const char *objdir = get_object_directory();
 		int objdir_len = strlen(objdir);
 		int entlen = objdir_len + 43;
@@ -27,33 +95,28 @@ static int find_short_object_filename(int len, const char *name, unsigned char *
 	}
 	fakeent->next = alt_odb_list;
 
-	sprintf(hex, "%.2s", name);
-	for (alt = fakeent; alt && found < 2; alt = alt->next) {
+	sprintf(hex, "%.2s", hex_pfx);
+	for (alt = fakeent; alt && !ds->ambiguous; alt = alt->next) {
 		struct dirent *de;
 		DIR *dir;
-		sprintf(alt->name, "%.2s/", name);
+		sprintf(alt->name, "%.2s/", hex_pfx);
 		dir = opendir(alt->base);
 		if (!dir)
 			continue;
-		while ((de = readdir(dir)) != NULL) {
+
+		while (!ds->ambiguous && (de = readdir(dir)) != NULL) {
+			unsigned char sha1[20];
+
 			if (strlen(de->d_name) != 38)
 				continue;
-			if (memcmp(de->d_name, name + 2, len - 2))
+			if (memcmp(de->d_name, hex_pfx + 2, len - 2))
 				continue;
-			if (!found) {
-				memcpy(hex + 2, de->d_name, 38);
-				found++;
-			}
-			else if (memcmp(hex + 2, de->d_name, 38)) {
-				found = 2;
-				break;
-			}
+			memcpy(hex + 2, de->d_name, 38);
+			if (!get_sha1_hex(hex, sha1))
+				update_candidates(ds, sha1);
 		}
 		closedir(dir);
 	}
-	if (found == 1)
-		return get_sha1_hex(hex, sha1) == 0;
-	return found;
 }
 
 static int match_sha(unsigned len, const unsigned char *a, const unsigned char *b)
@@ -71,103 +134,157 @@ static int match_sha(unsigned len, const unsigned char *a, const unsigned char *
 	return 1;
 }
 
-static int find_short_packed_object(int len, const unsigned char *match, unsigned char *sha1)
+static void unique_in_pack(int len,
+			  const unsigned char *bin_pfx,
+			   struct packed_git *p,
+			   struct disambiguate_state *ds)
 {
-	struct packed_git *p;
-	const unsigned char *found_sha1 = NULL;
-	int found = 0;
-
-	prepare_packed_git();
-	for (p = packed_git; p && found < 2; p = p->next) {
-		uint32_t num, last;
-		uint32_t first = 0;
-		open_pack_index(p);
-		num = p->num_objects;
-		last = num;
-		while (first < last) {
-			uint32_t mid = (first + last) / 2;
-			const unsigned char *now;
-			int cmp;
-
-			now = nth_packed_object_sha1(p, mid);
-			cmp = hashcmp(match, now);
-			if (!cmp) {
-				first = mid;
-				break;
-			}
-			if (cmp > 0) {
-				first = mid+1;
-				continue;
-			}
-			last = mid;
+	uint32_t num, last, i, first = 0;
+	const unsigned char *current = NULL;
+
+	open_pack_index(p);
+	num = p->num_objects;
+	last = num;
+	while (first < last) {
+		uint32_t mid = (first + last) / 2;
+		const unsigned char *current;
+		int cmp;
+
+		current = nth_packed_object_sha1(p, mid);
+		cmp = hashcmp(bin_pfx, current);
+		if (!cmp) {
+			first = mid;
+			break;
 		}
-		if (first < num) {
-			const unsigned char *now, *next;
-		       now = nth_packed_object_sha1(p, first);
-			if (match_sha(len, match, now)) {
-				next = nth_packed_object_sha1(p, first+1);
-			       if (!next|| !match_sha(len, match, next)) {
-					/* unique within this pack */
-					if (!found) {
-						found_sha1 = now;
-						found++;
-					}
-					else if (hashcmp(found_sha1, now)) {
-						found = 2;
-						break;
-					}
-				}
-				else {
-					/* not even unique within this pack */
-					found = 2;
-					break;
-				}
-			}
+		if (cmp > 0) {
+			first = mid+1;
+			continue;
 		}
+		last = mid;
+	}
+
+	/*
+	 * At this point, "first" is the location of the lowest object
+	 * with an object name that could match "bin_pfx".  See if we have
+	 * 0, 1 or more objects that actually match(es).
+	 */
+	for (i = first; i < num && !ds->ambiguous; i++) {
+		current = nth_packed_object_sha1(p, i);
+		if (!match_sha(len, bin_pfx, current))
+			break;
+		update_candidates(ds, current);
 	}
-	if (found == 1)
-		hashcpy(sha1, found_sha1);
-	return found;
+}
+
+static void find_short_packed_object(int len, const unsigned char *bin_pfx,
+				     struct disambiguate_state *ds)
+{
+	struct packed_git *p;
+
+	prepare_packed_git();
+	for (p = packed_git; p && !ds->ambiguous; p = p->next)
+		unique_in_pack(len, bin_pfx, p, ds);
 }
 
 #define SHORT_NAME_NOT_FOUND (-1)
 #define SHORT_NAME_AMBIGUOUS (-2)
 
-static int find_unique_short_object(int len, char *canonical,
-				    unsigned char *res, unsigned char *sha1)
+static int finish_object_disambiguation(struct disambiguate_state *ds,
+					unsigned char *sha1)
 {
-	int has_unpacked, has_packed;
-	unsigned char unpacked_sha1[20], packed_sha1[20];
+	if (ds->ambiguous)
+		return SHORT_NAME_AMBIGUOUS;
 
-	prepare_alt_odb();
-	has_unpacked = find_short_object_filename(len, canonical, unpacked_sha1);
-	has_packed = find_short_packed_object(len, res, packed_sha1);
-	if (!has_unpacked && !has_packed)
+	if (!ds->candidate_exists)
 		return SHORT_NAME_NOT_FOUND;
-	if (1 < has_unpacked || 1 < has_packed)
+
+	if (!ds->candidate_checked)
+		/*
+		 * If this is the only candidate, there is no point
+		 * calling the disambiguation hint callback.
+		 *
+		 * On the other hand, if the current candidate
+		 * replaced an earlier candidate that did _not_ pass
+		 * the disambiguation hint callback, then we do have
+		 * more than one objects that match the short name
+		 * given, so we should make sure this one matches;
+		 * otherwise, if we discovered this one and the one
+		 * that we previously discarded in the reverse order,
+		 * we would end up showing different results in the
+		 * same repository!
+		 */
+		ds->candidate_ok = (!ds->disambiguate_fn_used ||
+				    ds->fn(ds->candidate, ds->cb_data));
+
+	if (!ds->candidate_ok)
 		return SHORT_NAME_AMBIGUOUS;
-	if (has_unpacked != has_packed) {
-		hashcpy(sha1, (has_packed ? packed_sha1 : unpacked_sha1));
+
+	hashcpy(sha1, ds->candidate);
+	return 0;
+}
+
+static int disambiguate_commit_only(const unsigned char *sha1, void *cb_data_unused)
+{
+	int kind = sha1_object_info(sha1, NULL);
+	return kind == OBJ_COMMIT;
+}
+
+static int disambiguate_committish_only(const unsigned char *sha1, void *cb_data_unused)
+{
+	struct object *obj;
+	int kind;
+
+	kind = sha1_object_info(sha1, NULL);
+	if (kind == OBJ_COMMIT)
+		return 1;
+	if (kind != OBJ_TAG)
 		return 0;
-	}
-	/* Both have unique ones -- do they match? */
-	if (hashcmp(packed_sha1, unpacked_sha1))
-		return SHORT_NAME_AMBIGUOUS;
-	hashcpy(sha1, packed_sha1);
+
+	/* We need to do this the hard way... */
+	obj = deref_tag(lookup_object(sha1), NULL, 0);
+	if (obj && obj->type == OBJ_COMMIT)
+		return 1;
 	return 0;
 }
 
-static int get_short_sha1(const char *name, int len, unsigned char *sha1,
-			  int quietly)
+static int disambiguate_tree_only(const unsigned char *sha1, void *cb_data_unused)
 {
-	int i, status;
-	char canonical[40];
-	unsigned char res[20];
+	int kind = sha1_object_info(sha1, NULL);
+	return kind == OBJ_TREE;
+}
 
-	if (len < MINIMUM_ABBREV || len > 40)
-		return -1;
-	hashclr(res);
-	memset(canonical, 'x', 40);
+static int disambiguate_treeish_only(const unsigned char *sha1, void *cb_data_unused)
+{
+	struct object *obj;
+	int kind;
+
+	kind = sha1_object_info(sha1, NULL);
+	if (kind == OBJ_TREE || kind == OBJ_COMMIT)
+		return 1;
+	if (kind != OBJ_TAG)
+		return 0;
+
+	/* We need to do this the hard way... */
+	obj = deref_tag(lookup_object(sha1), NULL, 0);
+	if (obj && (obj->type == OBJ_TREE || obj->type == OBJ_COMMIT))
+		return 1;
+	return 0;
+}
+
+static int disambiguate_blob_only(const unsigned char *sha1, void *cb_data_unused)
+{
+	int kind = sha1_object_info(sha1, NULL);
+	return kind == OBJ_BLOB;
+}
+
+static int prepare_prefixes(const char *name, int len,
+			    unsigned char *bin_pfx,
+			    char *hex_pfx)
+{
+	int i;
+
+	hashclr(bin_pfx);
+	memset(hex_pfx, 'x', 40);
 	for (i = 0; i < len ;i++) {
 		unsigned char c = name[i];
 		unsigned char val;
@@ -181,18 +298,76 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 		}
 		else
 			return -1;
-		canonical[i] = c;
+		hex_pfx[i] = c;
 		if (!(i & 1))
 			val <<= 4;
-		res[i >> 1] |= val;
+		bin_pfx[i >> 1] |= val;
 	}
+	return 0;
+}
+
+static int get_short_sha1(const char *name, int len, unsigned char *sha1,
+			  unsigned flags)
+{
+	int status;
+	char hex_pfx[40];
+	unsigned char bin_pfx[20];
+	struct disambiguate_state ds;
+	int quietly = !!(flags & GET_SHA1_QUIETLY);
+
+	if (len < MINIMUM_ABBREV || len > 40)
+		return -1;
+	if (prepare_prefixes(name, len, bin_pfx, hex_pfx) < 0)
+		return -1;
+
+	prepare_alt_odb();
+
+	memset(&ds, 0, sizeof(ds));
+	if (flags & GET_SHA1_COMMIT)
+		ds.fn = disambiguate_commit_only;
+	else if (flags & GET_SHA1_COMMITTISH)
+		ds.fn = disambiguate_committish_only;
+	else if (flags & GET_SHA1_TREE)
+		ds.fn = disambiguate_tree_only;
+	else if (flags & GET_SHA1_TREEISH)
+		ds.fn = disambiguate_treeish_only;
+	else if (flags & GET_SHA1_BLOB)
+		ds.fn = disambiguate_blob_only;
+
+	find_short_object_filename(len, hex_pfx, &ds);
+	find_short_packed_object(len, bin_pfx, &ds);
+	status = finish_object_disambiguation(&ds, sha1);
 
-	status = find_unique_short_object(i, canonical, res, sha1);
 	if (!quietly && (status == SHORT_NAME_AMBIGUOUS))
-		return error("short SHA1 %.*s is ambiguous.", len, canonical);
+		return error("short SHA1 %.*s is ambiguous.", len, hex_pfx);
 	return status;
 }
 
+
+int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
+{
+	char hex_pfx[40];
+	unsigned char bin_pfx[20];
+	struct disambiguate_state ds;
+	int len = strlen(prefix);
+
+	if (len < MINIMUM_ABBREV || len > 40)
+		return -1;
+	if (prepare_prefixes(prefix, len, bin_pfx, hex_pfx) < 0)
+		return -1;
+
+	prepare_alt_odb();
+
+	memset(&ds, 0, sizeof(ds));
+	ds.always_call_fn = 1;
+	ds.cb_data = cb_data;
+	ds.fn = fn;
+
+	find_short_object_filename(len, hex_pfx, &ds);
+	find_short_packed_object(len, bin_pfx, &ds);
+	return ds.ambiguous;
+}
+
 const char *find_unique_abbrev(const unsigned char *sha1, int len)
 {
 	int status, exists;
@@ -204,7 +379,7 @@ const char *find_unique_abbrev(const unsigned char *sha1, int len)
 		return hex;
 	while (len < 40) {
 		unsigned char sha1_ret[20];
-		status = get_short_sha1(hex, len, sha1_ret, 1);
+		status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
 		if (exists
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {
@@ -255,7 +430,7 @@ static inline int upstream_mark(const char *string, int len)
 	return 0;
 }
 
-static int get_sha1_1(const char *name, int len, unsigned char *sha1);
+static int get_sha1_1(const char *name, int len, unsigned char *sha1, unsigned lookup_flags);
 
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
@@ -292,7 +467,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		ret = interpret_branch_name(str+at, &buf);
 		if (ret > 0) {
 			/* substitute this branch name and restart */
-			return get_sha1_1(buf.buf, buf.len, sha1);
+			return get_sha1_1(buf.buf, buf.len, sha1, 0);
 		} else if (ret == 0) {
 			return -1;
 		}
@@ -362,7 +537,7 @@ static int get_parent(const char *name, int len,
 		      unsigned char *result, int idx)
 {
 	unsigned char sha1[20];
-	int ret = get_sha1_1(name, len, sha1);
+	int ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
 	struct commit *commit;
 	struct commit_list *p;
 
@@ -395,7 +570,7 @@ static int get_nth_ancestor(const char *name, int len,
 	struct commit *commit;
 	int ret;
 
-	ret = get_sha1_1(name, len, sha1);
+	ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
 	if (ret)
 		return ret;
 	commit = lookup_commit_reference(sha1);
@@ -441,6 +616,7 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 	unsigned char outer[20];
 	const char *sp;
 	unsigned int expected_type = 0;
+	unsigned lookup_flags = 0;
 	struct object *o;
 
 	/*
@@ -476,7 +652,10 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 	else
 		return -1;
 
-	if (get_sha1_1(name, sp - name - 2, outer))
+	if (expected_type == OBJ_COMMIT)
+		lookup_flags = GET_SHA1_COMMITTISH;
+
+	if (get_sha1_1(name, sp - name - 2, outer, lookup_flags))
 		return -1;
 
 	o = parse_object(outer);
@@ -525,6 +704,7 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 static int get_describe_name(const char *name, int len, unsigned char *sha1)
 {
 	const char *cp;
+	unsigned flags = GET_SHA1_QUIETLY | GET_SHA1_COMMIT;
 
 	for (cp = name + len - 1; name + 2 <= cp; cp--) {
 		char ch = *cp;
@@ -535,14 +715,14 @@ static int get_describe_name(const char *name, int len, unsigned char *sha1)
 			if (ch == 'g' && cp[-1] == '-') {
 				cp++;
 				len -= cp - name;
-				return get_short_sha1(cp, len, sha1, 1);
+				return get_short_sha1(cp, len, sha1, flags);
 			}
 		}
 	}
 	return -1;
 }
 
-static int get_sha1_1(const char *name, int len, unsigned char *sha1)
+static int get_sha1_1(const char *name, int len, unsigned char *sha1, unsigned lookup_flags)
 {
 	int ret, has_suffix;
 	const char *cp;
@@ -587,7 +767,7 @@ static int get_sha1_1(const char *name, int len, unsigned char *sha1)
 	if (!ret)
 		return 0;
 
-	return get_short_sha1(name, len, sha1, 0);
+	return get_short_sha1(name, len, sha1, lookup_flags);
 }
 
 /*
@@ -769,7 +949,7 @@ int get_sha1_mb(const char *name, unsigned char *sha1)
 		struct strbuf sb;
 		strbuf_init(&sb, dots - name);
 		strbuf_add(&sb, name, dots - name);
-		st = get_sha1(sb.buf, sha1_tmp);
+		st = get_sha1_committish(sb.buf, sha1_tmp);
 		strbuf_release(&sb);
 	}
 	if (st)
@@ -778,7 +958,7 @@ int get_sha1_mb(const char *name, unsigned char *sha1)
 	if (!one)
 		return -1;
 
-	if (get_sha1(dots[3] ? (dots + 3) : "HEAD", sha1_tmp))
+	if (get_sha1_committish(dots[3] ? (dots + 3) : "HEAD", sha1_tmp))
 		return -1;
 	two = lookup_commit_reference_gently(sha1_tmp, 0);
 	if (!two)
@@ -905,7 +1085,52 @@ int strbuf_check_branch_ref(struct strbuf *sb, const char *name)
 int get_sha1(const char *name, unsigned char *sha1)
 {
 	struct object_context unused;
-	return get_sha1_with_context(name, sha1, &unused);
+	return get_sha1_with_context(name, 0, sha1, &unused);
+}
+
+/*
+ * Many callers know that the user meant to name a committish by
+ * syntactical positions where the object name appears.  Calling this
+ * function allows the machinery to disambiguate shorter-than-unique
+ * abbreviated object names between committish and others.
+ *
+ * Note that this does NOT error out when the named object is not a
+ * committish. It is merely to give a hint to the disambiguation
+ * machinery.
+ */
+int get_sha1_committish(const char *name, unsigned char *sha1)
+{
+	struct object_context unused;
+	return get_sha1_with_context(name, GET_SHA1_COMMITTISH,
+				     sha1, &unused);
+}
+
+int get_sha1_treeish(const char *name, unsigned char *sha1)
+{
+	struct object_context unused;
+	return get_sha1_with_context(name, GET_SHA1_TREEISH,
+				     sha1, &unused);
+}
+
+int get_sha1_commit(const char *name, unsigned char *sha1)
+{
+	struct object_context unused;
+	return get_sha1_with_context(name, GET_SHA1_COMMIT,
+				     sha1, &unused);
+}
+
+int get_sha1_tree(const char *name, unsigned char *sha1)
+{
+	struct object_context unused;
+	return get_sha1_with_context(name, GET_SHA1_TREE,
+				     sha1, &unused);
+}
+
+int get_sha1_blob(const char *name, unsigned char *sha1)
+{
+	struct object_context unused;
+	return get_sha1_with_context(name, GET_SHA1_BLOB,
+				     sha1, &unused);
 }
 
 /* Must be called only when object_name:filename doesn't exist. */
@@ -1004,16 +1229,6 @@ static void diagnose_invalid_index_path(int stage,
 }
 
 
-int get_sha1_with_mode_1(const char *name, unsigned char *sha1, unsigned *mode,
-			 int only_to_die, const char *prefix)
-{
-	struct object_context oc;
-	int ret;
-	ret = get_sha1_with_context_1(name, sha1, &oc, only_to_die, prefix);
-	*mode = oc.mode;
-	return ret;
-}
-
 static char *resolve_relative_path(const char *rel)
 {
 	if (prefixcmp(rel, "./") && prefixcmp(rel, "../"))
@@ -1031,20 +1246,24 @@ static char *resolve_relative_path(const char *rel)
 			   rel);
 }
 
-int get_sha1_with_context_1(const char *name, unsigned char *sha1,
-			    struct object_context *oc,
-			    int only_to_die, const char *prefix)
+static int get_sha1_with_context_1(const char *name,
+				   unsigned flags,
+				   const char *prefix,
+				   unsigned char *sha1,
+				   struct object_context *oc)
 {
 	int ret, bracket_depth;
 	int namelen = strlen(name);
 	const char *cp;
+	int only_to_die = flags & GET_SHA1_ONLY_TO_DIE;
 
 	memset(oc, 0, sizeof(*oc));
 	oc->mode = S_IFINVALID;
-	ret = get_sha1_1(name, namelen, sha1);
+	ret = get_sha1_1(name, namelen, sha1, flags);
 	if (!ret)
 		return ret;
-	/* sha1:path --> object name of path in ent sha1
+	/*
+	 * sha1:path --> object name of path in ent sha1
 	 * :path -> object name of absolute path in index
 	 * :./path -> object name of path relative to cwd in index
 	 * :[0-3]:path -> object name of path in index at stage
@@ -1119,7 +1338,7 @@ int get_sha1_with_context_1(const char *name, unsigned char *sha1,
 			strncpy(object_name, name, cp-name);
 			object_name[cp-name] = '\0';
 		}
-		if (!get_sha1_1(name, cp-name, tree_sha1)) {
+		if (!get_sha1_1(name, cp-name, tree_sha1, GET_SHA1_TREEISH)) {
 			const char *filename = cp+1;
 			char *new_filename = NULL;
 
@@ -1146,3 +1365,22 @@ int get_sha1_with_context_1(const char *name, unsigned char *sha1,
 	}
 	return ret;
 }
+
+/*
+ * Call this function when you know "name" given by the end user must
+ * name an object but it doesn't; the function _may_ die with a better
+ * diagnostic message than "no such object 'name'", e.g. "Path 'doc' does not
+ * exist in 'HEAD'" when given "HEAD:doc", or it may return in which case
+ * you have a chance to diagnose the error further.
+ */
+void maybe_die_on_misspelt_object_name(const char *name, const char *prefix)
+{
+	struct object_context oc;
+	unsigned char sha1[20];
+	get_sha1_with_context_1(name, GET_SHA1_ONLY_TO_DIE, prefix, sha1, &oc);
+}
+
+int get_sha1_with_context(const char *str, unsigned flags, unsigned char *sha1, struct object_context *orc)
+{
+	return get_sha1_with_context_1(str, flags, NULL, sha1, orc);
+}
diff --git a/t/t1512-rev-parse-disambiguation.sh b/t/t1512-rev-parse-disambiguation.sh
new file mode 100755
index 000000000..6b3d797ce
--- /dev/null
+++ b/t/t1512-rev-parse-disambiguation.sh
@@ -0,0 +1,264 @@
+#!/bin/sh
+
+test_description='object name disambiguation
+
+Create blobs, trees, commits and a tag that all share the same
+prefix, and make sure "git rev-parse" can take advantage of
+type information to disambiguate short object names that are
+not necessarily unique.
+
+The final history used in the test has five commits, with the bottom
+one tagged as v1.0.0.  They all have one regular file each.
+
+  +-------------------------------------------+
+  |                                           |
+  |           .-------b3wettvi---- ad2uee     |
+  |          /                   /            |
+  |  a2onsxbvj---czy8f73t--ioiley5o           |
+  |                                           |
+  +-------------------------------------------+
+
+'
+
+. ./test-lib.sh
+
+test_expect_success 'blob and tree' '
+	test_tick &&
+	(
+		for i in 0 1 2 3 4 5 6 7 8 9
+		do
+			echo $i
+		done
+		echo
+		echo b1rwzyc3
+	) >a0blgqsjc &&
+
+	# create one blob 0000000000b36
+	git add a0blgqsjc &&
+
+	# create one tree 0000000000cdc
+	git write-tree
+'
+
+test_expect_success 'warn ambiguity when no candidate matches type hint' '
+	test_must_fail git rev-parse --verify 000000000^{commit} 2>actual &&
+	grep "short SHA1 000000000 is ambiguous" actual
+'
+
+test_expect_success 'disambiguate tree-ish' '
+	# feed tree-ish in an unambiguous way
+	git rev-parse --verify 0000000000cdc:a0blgqsjc &&
+
+	# ambiguous at the object name level, but there is only one
+	# such tree-ish (the other is a blob)
+	git rev-parse --verify 000000000:a0blgqsjc
+'
+
+test_expect_success 'disambiguate blob' '
+	sed -e "s/|$//" >patch <<-EOF &&
+	diff --git a/frotz b/frotz
+	index 000000000..ffffff 100644
+	--- a/frotz
+	+++ b/frotz
+	@@ -10,3 +10,4 @@
+	 9
+	 |
+	 b1rwzyc3
+	+irwry
+	EOF
+	(
+		GIT_INDEX_FILE=frotz &&
+		export GIT_INDEX_FILE &&
+		git apply --build-fake-ancestor frotz patch &&
+		git cat-file blob :frotz >actual
+	) &&
+	test_cmp a0blgqsjc actual
+'
+
+test_expect_success 'disambiguate tree' '
+	commit=$(echo "d7xm" | git commit-tree 000000000) &&
+	test $(git rev-parse $commit^{tree}) = $(git rev-parse 0000000000cdc)
+'
+
+test_expect_success 'first commit' '
+	# create one commit 0000000000e4f
+	git commit -m a2onsxbvj
+'
+
+test_expect_success 'disambiguate commit-ish' '
+	# feed commit-ish in an unambiguous way
+	git rev-parse --verify 0000000000e4f^{commit} &&
+
+	# ambiguous at the object name level, but there is only one
+	# such commit (the others are tree and blob)
+	git rev-parse --verify 000000000^{commit} &&
+
+	# likewise
+	git rev-parse --verify 000000000^0
+'
+
+test_expect_success 'disambiguate commit' '
+	commit=$(echo "hoaxj" | git commit-tree 0000000000cdc -p 000000000) &&
+	test $(git rev-parse $commit^) = $(git rev-parse 0000000000e4f)
+'
+
+test_expect_success 'log name1..name2 takes only commit-ishes on both ends' '
+	git log 000000000..000000000 &&
+	git log ..000000000 &&
+	git log 000000000.. &&
+	git log 000000000...000000000 &&
+	git log ...000000000 &&
+	git log 000000000...
+'
+
+test_expect_success 'rev-parse name1..name2 takes only commit-ishes on both ends' '
+	git rev-parse 000000000..000000000 &&
+	git rev-parse ..000000000 &&
+	git rev-parse 000000000..
+'
+
+test_expect_success 'git log takes only commit-ish' '
+	git log 000000000
+'
+
+test_expect_success 'git reset takes only commit-ish' '
+	git reset 000000000
+'
+
+test_expect_success 'first tag' '
+	# create one tag 0000000000f8f
+	git tag -a -m j7cp83um v1.0.0
+'
+
+test_expect_failure 'two semi-ambiguous commit-ish' '
+	# Once the parser becomes ultra-smart, it could notice that
+	# 110282 before ^{commit} name many different objects, but
+	# that only two (HEAD and v1.0.0 tag) can be peeled to commit,
+	# and that peeling them down to commit yield the same commit
+	# without ambiguity.
+	git rev-parse --verify 110282^{commit} &&
+
+	# likewise
+	git log 000000000..000000000 &&
+	git log ..000000000 &&
+	git log 000000000.. &&
+	git log 000000000...000000000 &&
+	git log ...000000000 &&
+	git log 000000000...
+'
+
+test_expect_failure 'three semi-ambiguous tree-ish' '
+	# Likewise for tree-ish.  HEAD, v1.0.0 and HEAD^{tree} share
+	# the prefix but peeling them to tree yields the same thing
+	git rev-parse --verify 000000000^{tree}
+'
+
+test_expect_success 'parse describe name' '
+	# feed an unambiguous describe name
+	git rev-parse --verify v1.0.0-0-g0000000000e4f &&
+
+	# ambiguous at the object name level, but there is only one
+	# such commit (others are blob, tree and tag)
+	git rev-parse --verify v1.0.0-0-g000000000
+'
+
+test_expect_success 'more history' '
+	# commit 0000000000043
+	git mv a0blgqsjc d12cr3h8t &&
+	echo h62xsjeu >>d12cr3h8t &&
+	git add d12cr3h8t &&
+
+	test_tick &&
+	git commit -m czy8f73t &&
+
+	# commit 00000000008ec
+	git mv d12cr3h8t j000jmpzn &&
+	echo j08bekfvt >>j000jmpzn &&
+	git add j000jmpzn &&
+
+	test_tick &&
+	git commit -m ioiley5o &&
+
+	# commit 0000000005b0
+	git checkout v1.0.0^0 &&
+	git mv a0blgqsjc f5518nwu &&
+
+	for i in h62xsjeu j08bekfvt kg7xflhm
+	do
+		echo $i
+	done >>f5518nwu &&
+	git add f5518nwu &&
+
+	test_tick &&
+	git commit -m b3wettvi &&
+	side=$(git rev-parse HEAD) &&
+
+	# commit 000000000066
+	git checkout master &&
+
+	# If you use recursive, merge will fail and you will need to
+	# clean up a0blgqsjc as well.  If you use resolve, merge will
+	# succeed.
+	test_might_fail git merge --no-commit -s recursive $side &&
+	git rm -f f5518nwu j000jmpzn &&
+
+	test_might_fail git rm -f a0blgqsjc &&
+	(
+		git cat-file blob $side:f5518nwu
+		echo j3l0i9s6
+	) >ab2gs879 &&
+	git add ab2gs879 &&
+
+	test_tick &&
+	git commit -m ad2uee
+
+'
+
+test_expect_failure 'parse describe name taking advantage of generation' '
+	# ambiguous at the object name level, but there is only one
+	# such commit at generation 0
+	git rev-parse --verify v1.0.0-0-g000000000 &&
+
+	# likewise for generation 2 and 4
+	git rev-parse --verify v1.0.0-2-g000000000 &&
+	git rev-parse --verify v1.0.0-4-g000000000
+'
+
+# Note: because rev-parse does not even try to disambiguate based on
+# the generation number, this test currently succeeds for a wrong
+# reason.  When it learns to use the generation number, the previous
+# test should succeed, and also this test should fail because the
+# describe name used in the test with generation number can name two
+# commits.  Make sure that such a future enhancement does not randomly
+# pick one.
+test_expect_success 'parse describe name not ignoring ambiguity' '
+	# ambiguous at the object name level, and there are two such
+	# commits at generation 1
+	test_must_fail git rev-parse --verify v1.0.0-1-g000000000
+'
+
+test_expect_success 'ambiguous commit-ish' '
+	# Now there are many commits that begin with the
+	# common prefix, none of these should pick one at
+	# random.  They all should result in ambiguity errors.
+	test_must_fail git rev-parse --verify 110282^{commit} &&
+
+	# likewise
+	test_must_fail git log 000000000..000000000 &&
+	test_must_fail git log ..000000000 &&
+	test_must_fail git log 000000000.. &&
+	test_must_fail git log 000000000...000000000 &&
+	test_must_fail git log ...000000000 &&
+	test_must_fail git log 000000000...
+'
+
+test_expect_success 'rev-parse --disambiguate' '
+	# The test creates 16 objects that share the prefix and two
+	# commits created by commit-tree in earlier tests share a
+	# different prefix.
+	git rev-parse --disambiguate=000000000 >actual &&
+	test $(wc -l <actual) = 16 &&
+	test "$(sed -e "s/^\(.........\).*/\1/" actual | sort -u)" = 000000000
+'
+
+test_done