Merge branch 'dt/refs-pseudo'

To prepare for allowing a different "ref" backend to be plugged in
to the system, update_ref()/delete_ref() have been taught about
ref-like things like MERGE_HEAD that are per-worktree (they will
always be written to the filesystem inside $GIT_DIR).

* dt/refs-pseudo:
  pseudoref: check return values from read_ref()
  sequencer: replace write_cherry_pick_head with update_ref
  bisect: use update_ref
  pseudorefs: create and use pseudoref update and delete functions
  refs: add ref_type function
  refs: introduce pseudoref and per-worktree ref concepts

refs.c

@@ -2854,12 +2854,117 @@ static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
 	return 0;
 }
 
+static int is_per_worktree_ref(const char *refname)
+{
+	return !strcmp(refname, "HEAD");
+}
+
+static int is_pseudoref_syntax(const char *refname)
+{
+	const char *c;
+
+	for (c = refname; *c; c++) {
+		if (!isupper(*c) && *c != '-' && *c != '_')
+			return 0;
+	}
+
+	return 1;
+}
+
+enum ref_type ref_type(const char *refname)
+{
+	if (is_per_worktree_ref(refname))
+		return REF_TYPE_PER_WORKTREE;
+	if (is_pseudoref_syntax(refname))
+		return REF_TYPE_PSEUDOREF;
+       return REF_TYPE_NORMAL;
+}
+
+static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
+			   const unsigned char *old_sha1, struct strbuf *err)
+{
+	const char *filename;
+	int fd;
+	static struct lock_file lock;
+	struct strbuf buf = STRBUF_INIT;
+	int ret = -1;
+
+	strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));
+
+	filename = git_path("%s", pseudoref);
+	fd = hold_lock_file_for_update(&lock, filename, LOCK_DIE_ON_ERROR);
+	if (fd < 0) {
+		strbuf_addf(err, "Could not open '%s' for writing: %s",
+			    filename, strerror(errno));
+		return -1;
+	}
+
+	if (old_sha1) {
+		unsigned char actual_old_sha1[20];
+
+		if (read_ref(pseudoref, actual_old_sha1))
+			die("could not read ref '%s'", pseudoref);
+		if (hashcmp(actual_old_sha1, old_sha1)) {
+			strbuf_addf(err, "Unexpected sha1 when writing %s", pseudoref);
+			rollback_lock_file(&lock);
+			goto done;
+		}
+	}
+
+	if (write_in_full(fd, buf.buf, buf.len) != buf.len) {
+		strbuf_addf(err, "Could not write to '%s'", filename);
+		rollback_lock_file(&lock);
+		goto done;
+	}
+
+	commit_lock_file(&lock);
+	ret = 0;
+done:
+	strbuf_release(&buf);
+	return ret;
+}
+
+static int delete_pseudoref(const char *pseudoref, const unsigned char *old_sha1)
+{
+	static struct lock_file lock;
+	const char *filename;
+
+	filename = git_path("%s", pseudoref);
+
+	if (old_sha1 && !is_null_sha1(old_sha1)) {
+		int fd;
+		unsigned char actual_old_sha1[20];
+
+		fd = hold_lock_file_for_update(&lock, filename,
+					       LOCK_DIE_ON_ERROR);
+		if (fd < 0)
+			die_errno(_("Could not open '%s' for writing"), filename);
+		if (read_ref(pseudoref, actual_old_sha1))
+			die("could not read ref '%s'", pseudoref);
+		if (hashcmp(actual_old_sha1, old_sha1)) {
+			warning("Unexpected sha1 when deleting %s", pseudoref);
+			rollback_lock_file(&lock);
+			return -1;
+		}
+
+		unlink(filename);
+		rollback_lock_file(&lock);
+	} else {
+		unlink(filename);
+	}
+
+	return 0;
+}
+
 int delete_ref(const char *refname, const unsigned char *old_sha1,
 	       unsigned int flags)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
+	if (ref_type(refname) == REF_TYPE_PSEUDOREF)
+		return delete_pseudoref(refname, old_sha1);
+
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_sha1,
@@ -3961,17 +4066,25 @@ int update_ref(const char *msg, const char *refname,
 	       const unsigned char *new_sha1, const unsigned char *old_sha1,
 	       unsigned int flags, enum action_on_err onerr)
 {
-	struct ref_transaction *t;
+	struct ref_transaction *t = NULL;
 	struct strbuf err = STRBUF_INIT;
+	int ret = 0;
 
-	t = ref_transaction_begin(&err);
-	if (!t ||
-	    ref_transaction_update(t, refname, new_sha1, old_sha1,
-				   flags, msg, &err) ||
-	    ref_transaction_commit(t, &err)) {
+	if (ref_type(refname) == REF_TYPE_PSEUDOREF) {
+		ret = write_pseudoref(refname, new_sha1, old_sha1, &err);
+	} else {
+		t = ref_transaction_begin(&err);
+		if (!t ||
+		    ref_transaction_update(t, refname, new_sha1, old_sha1,
+					   flags, msg, &err) ||
+		    ref_transaction_commit(t, &err)) {
+			ret = 1;
+			ref_transaction_free(t);
+		}
+	}
+	if (ret) {
 		const char *str = "update_ref failed for ref '%s': %s";
 
-		ref_transaction_free(t);
 		switch (onerr) {
 		case UPDATE_REFS_MSG_ON_ERR:
 			error(str, refname, err.buf);
@@ -3986,7 +4099,8 @@ int update_ref(const char *msg, const char *refname,
 		return 1;
 	}
 	strbuf_release(&err);
-	ref_transaction_free(t);
+	if (t)
+		ref_transaction_free(t);
 	return 0;
 }