Merge branch 'ps/pseudo-refs' into next

Assorted changes around pseudoref handling.

* ps/pseudo-refs:
  bisect: consistently write BISECT_EXPECTED_REV via the refdb
  refs: complete list of special refs
  refs: propagate errno when reading special refs fails
  wt-status: read HEAD and ORIG_HEAD via the refdb

bisect.c

@@ -471,7 +471,6 @@ static int read_bisect_refs(void)
 }
 
 static GIT_PATH_FUNC(git_path_bisect_names, "BISECT_NAMES")
-static GIT_PATH_FUNC(git_path_bisect_expected_rev, "BISECT_EXPECTED_REV")
 static GIT_PATH_FUNC(git_path_bisect_ancestors_ok, "BISECT_ANCESTORS_OK")
 static GIT_PATH_FUNC(git_path_bisect_run, "BISECT_RUN")
 static GIT_PATH_FUNC(git_path_bisect_start, "BISECT_START")
@@ -707,26 +706,10 @@ static enum bisect_error error_if_skipped_commits(struct commit_list *tried,
 
 static int is_expected_rev(const struct object_id *oid)
 {
-	const char *filename = git_path_bisect_expected_rev();
-	struct stat st;
-	struct strbuf str = STRBUF_INIT;
-	FILE *fp;
-	int res = 0;
-
-	if (stat(filename, &st) || !S_ISREG(st.st_mode))
+	struct object_id expected_oid;
+	if (read_ref("BISECT_EXPECTED_REV", &expected_oid))
 		return 0;
-
-	fp = fopen_or_warn(filename, "r");
-	if (!fp)
-		return 0;
-
-	if (strbuf_getline_lf(&str, fp) != EOF)
-		res = !strcmp(str.buf, oid_to_hex(oid));
-
-	strbuf_release(&str);
-	fclose(fp);
-
-	return res;
+	return oideq(oid, &expected_oid);
 }
 
 enum bisect_error bisect_checkout(const struct object_id *bisect_rev,
@@ -1185,10 +1168,10 @@ int bisect_clean_state(void)
 	struct string_list refs_for_removal = STRING_LIST_INIT_NODUP;
 	for_each_ref_in("refs/bisect", mark_for_removal, (void *) &refs_for_removal);
 	string_list_append(&refs_for_removal, xstrdup("BISECT_HEAD"));
+	string_list_append(&refs_for_removal, xstrdup("BISECT_EXPECTED_REV"));
 	result = delete_refs("bisect: remove", &refs_for_removal, REF_NO_DEREF);
 	refs_for_removal.strdup_strings = 1;
 	string_list_clear(&refs_for_removal, 0);
-	unlink_or_warn(git_path_bisect_expected_rev());
 	unlink_or_warn(git_path_bisect_ancestors_ok());
 	unlink_or_warn(git_path_bisect_log());
 	unlink_or_warn(git_path_bisect_names());

builtin/bisect.c

@@ -17,7 +17,6 @@
 #include "revision.h"
 
 static GIT_PATH_FUNC(git_path_bisect_terms, "BISECT_TERMS")
-static GIT_PATH_FUNC(git_path_bisect_expected_rev, "BISECT_EXPECTED_REV")
 static GIT_PATH_FUNC(git_path_bisect_ancestors_ok, "BISECT_ANCESTORS_OK")
 static GIT_PATH_FUNC(git_path_bisect_start, "BISECT_START")
 static GIT_PATH_FUNC(git_path_bisect_log, "BISECT_LOG")
@@ -920,7 +919,6 @@ static enum bisect_error bisect_state(struct bisect_terms *terms, int argc,
 	const char *state;
 	int i, verify_expected = 1;
 	struct object_id oid, expected;
-	struct strbuf buf = STRBUF_INIT;
 	struct oid_array revs = OID_ARRAY_INIT;
 
 	if (!argc)
@@ -975,10 +973,8 @@ static enum bisect_error bisect_state(struct bisect_terms *terms, int argc,
 		oid_array_append(&revs, &commit->object.oid);
 	}
 
-	if (strbuf_read_file(&buf, git_path_bisect_expected_rev(), 0) < the_hash_algo->hexsz ||
-	    get_oid_hex(buf.buf, &expected) < 0)
+	if (read_ref("BISECT_EXPECTED_REV", &expected))
 		verify_expected = 0; /* Ignore invalid file contents */
-	strbuf_release(&buf);
 
 	for (i = 0; i < revs.nr; i++) {
 		if (bisect_write(state, oid_to_hex(&revs.oid[i]), terms, 0)) {
@@ -987,7 +983,7 @@ static enum bisect_error bisect_state(struct bisect_terms *terms, int argc,
 		}
 		if (verify_expected && !oideq(&revs.oid[i], &expected)) {
 			unlink_or_warn(git_path_bisect_ancestors_ok());
-			unlink_or_warn(git_path_bisect_expected_rev());
+			delete_ref(NULL, "BISECT_EXPECTED_REV", NULL, REF_NO_DEREF);
 			verify_expected = 0;
 		}
 	}

refs.c

@@ -1806,8 +1806,10 @@ static int refs_read_special_head(struct ref_store *ref_store,
 	int result = -1;
 	strbuf_addf(&full_path, "%s/%s", ref_store->gitdir, refname);
 
-	if (strbuf_read_file(&content, full_path.buf, 0) < 0)
+	if (strbuf_read_file(&content, full_path.buf, 0) < 0) {
+		*failure_errno = errno;
 		goto done;
+	}
 
 	result = parse_loose_ref_contents(content.buf, oid, referent, type,
 					  failure_errno);
@@ -1818,15 +1820,66 @@ static int refs_read_special_head(struct ref_store *ref_store,
 	return result;
 }
 
+static int is_special_ref(const char *refname)
+{
+	/*
+	 * Special references get written and read directly via the filesystem
+	 * by the subsystems that create them. Thus, they must not go through
+	 * the reference backend but must instead be read directly. It is
+	 * arguable whether this behaviour is sensible, or whether it's simply
+	 * a leaky abstraction enabled by us only having a single reference
+	 * backend implementation. But at least for a subset of references it
+	 * indeed does make sense to treat them specially:
+	 *
+	 * - FETCH_HEAD may contain multiple object IDs, and each one of them
+	 *   carries additional metadata like where it came from.
+	 *
+	 * - MERGE_HEAD may contain multiple object IDs when merging multiple
+	 *   heads.
+	 *
+	 * There are some exceptions that you might expect to see on this list
+	 * but which are handled exclusively via the reference backend:
+	 *
+	 * - BISECT_EXPECTED_REV
+	 *
+	 * - CHERRY_PICK_HEAD
+	 *
+	 * - HEAD
+	 *
+	 * - ORIG_HEAD
+	 *
+	 * - "rebase-apply/" and "rebase-merge/" contain all of the state for
+	 *   rebases, including some reference-like files. These are
+	 *   exclusively read and written via the filesystem and never go
+	 *   through the refdb.
+	 *
+	 * Writing or deleting references must consistently go either through
+	 * the filesystem (special refs) or through the reference backend
+	 * (normal ones).
+	 */
+	static const char * const special_refs[] = {
+		"AUTO_MERGE",
+		"FETCH_HEAD",
+		"MERGE_AUTOSTASH",
+		"MERGE_HEAD",
+	};
+	size_t i;
+
+	for (i = 0; i < ARRAY_SIZE(special_refs); i++)
+		if (!strcmp(refname, special_refs[i]))
+			return 1;
+
+	return 0;
+}
+
 int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 		      struct object_id *oid, struct strbuf *referent,
 		      unsigned int *type, int *failure_errno)
 {
 	assert(failure_errno);
-	if (!strcmp(refname, "FETCH_HEAD") || !strcmp(refname, "MERGE_HEAD")) {
+	if (is_special_ref(refname))
 		return refs_read_special_head(ref_store, refname, oid, referent,
 					      type, failure_errno);
-	}
 
 	return ref_store->be->read_raw_ref(ref_store, refname, oid, referent,
 					   type, failure_errno);

t/t1403-show-ref.sh

@@ -268,4 +268,14 @@ test_expect_success '--exists with directory fails with generic error' '
 	test_cmp expect err
 '
 
+test_expect_success '--exists with non-existent special ref' '
+	test_expect_code 2 git show-ref --exists FETCH_HEAD
+'
+
+test_expect_success '--exists with existing special ref' '
+	test_when_finished "rm .git/FETCH_HEAD" &&
+	git rev-parse HEAD >.git/FETCH_HEAD &&
+	git show-ref --exists FETCH_HEAD
+'
+
 test_done

t/t6030-bisect-porcelain.sh

@@ -1182,7 +1182,7 @@ test_expect_success 'git bisect reset cleans bisection state properly' '
 	git bisect bad $HASH4 &&
 	git bisect reset &&
 	test -z "$(git for-each-ref "refs/bisect/*")" &&
-	test_path_is_missing ".git/BISECT_EXPECTED_REV" &&
+	test_ref_missing BISECT_EXPECTED_REV &&
 	test_path_is_missing ".git/BISECT_ANCESTORS_OK" &&
 	test_path_is_missing ".git/BISECT_LOG" &&
 	test_path_is_missing ".git/BISECT_RUN" &&

wt-status.c

@@ -1296,26 +1296,32 @@ static char *read_line_from_git_path(const char *filename)
 static int split_commit_in_progress(struct wt_status *s)
 {
 	int split_in_progress = 0;
-	char *head, *orig_head, *rebase_amend, *rebase_orig_head;
+	struct object_id head_oid, orig_head_oid;
+	char *rebase_amend, *rebase_orig_head;
+	int head_flags, orig_head_flags;
 
 	if ((!s->amend && !s->nowarn && !s->workdir_dirty) ||
 	    !s->branch || strcmp(s->branch, "HEAD"))
 		return 0;
 
-	head = read_line_from_git_path("HEAD");
-	orig_head = read_line_from_git_path("ORIG_HEAD");
+	if (read_ref_full("HEAD", RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+			  &head_oid, &head_flags) ||
+	    read_ref_full("ORIG_HEAD", RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+			  &orig_head_oid, &orig_head_flags))
+		return 0;
+	if (head_flags & REF_ISSYMREF || orig_head_flags & REF_ISSYMREF)
+		return 0;
+
 	rebase_amend = read_line_from_git_path("rebase-merge/amend");
 	rebase_orig_head = read_line_from_git_path("rebase-merge/orig-head");
 
-	if (!head || !orig_head || !rebase_amend || !rebase_orig_head)
+	if (!rebase_amend || !rebase_orig_head)
 		; /* fall through, no split in progress */
 	else if (!strcmp(rebase_amend, rebase_orig_head))
-		split_in_progress = !!strcmp(head, rebase_amend);
-	else if (strcmp(orig_head, rebase_orig_head))
+		split_in_progress = !!strcmp(oid_to_hex(&head_oid), rebase_amend);
+	else if (strcmp(oid_to_hex(&orig_head_oid), rebase_orig_head))
 		split_in_progress = 1;
 
-	free(head);
-	free(orig_head);
 	free(rebase_amend);
 	free(rebase_orig_head);