reftable: don't second-guess errors from flock interface

The `flock` interface is implemented as part of "reftable/system.c" and
thus needs to be implemented by the integrator between the reftable
library and its parent code base. As such, we cannot rely on any
specific implementation thereof.

Regardless of that, users of the `flock` subsystem rely on `errno` being
set to specific values. This is fragile and not documented anywhere and
doesn't really make for a good interface.

Refactor the code so that the implementations themselves are expected to
return reftable-specific error codes. Our implementation of the `flock`
subsystem already knows to do this for all error paths except one.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/stack.c

@@ -698,14 +698,9 @@ static int reftable_stack_init_addition(struct reftable_addition *add,
 
 	err = flock_acquire(&add->tables_list_lock, st->list_file,
 			    st->opts.lock_timeout_ms);
-	if (err < 0) {
+	if (err < 0)
-		if (errno == EEXIST) {
-			err = REFTABLE_LOCK_ERROR;
-		} else {
-			err = REFTABLE_IO_ERROR;
-		}
 		goto done;
-	}
+
 	if (st->opts.default_permissions) {
 		if (chmod(add->tables_list_lock.path,
 			  st->opts.default_permissions) < 0) {
@@ -1212,13 +1207,8 @@ static int stack_compact_range(struct reftable_stack *st,
 	 * which are part of the user-specified range.
 	 */
 	err = flock_acquire(&tables_list_lock, st->list_file, st->opts.lock_timeout_ms);
-	if (err < 0) {
+	if (err < 0)
-		if (errno == EEXIST)
-			err = REFTABLE_LOCK_ERROR;
-		else
-			err = REFTABLE_IO_ERROR;
 		goto done;
-	}
 
 	/*
 	 * Check whether the stack is up-to-date. We unfortunately cannot
@@ -1272,7 +1262,7 @@ static int stack_compact_range(struct reftable_stack *st,
 			 * tables, otherwise there would be nothing to compact.
 			 * In that case, we return a lock error to our caller.
 			 */
-			if (errno == EEXIST && last - (i - 1) >= 2 &&
+			if (err == REFTABLE_LOCK_ERROR && last - (i - 1) >= 2 &&
 			    flags & STACK_COMPACT_RANGE_BEST_EFFORT) {
 				err = 0;
 				/*
@@ -1284,13 +1274,9 @@ static int stack_compact_range(struct reftable_stack *st,
 				 */
 				first = (i - 1) + 1;
 				break;
-			} else if (errno == EEXIST) {
-				err = REFTABLE_LOCK_ERROR;
-				goto done;
-			} else {
-				err = REFTABLE_IO_ERROR;
-				goto done;
 			}
+
+			goto done;
 		}
 
 		/*
@@ -1299,11 +1285,9 @@ static int stack_compact_range(struct reftable_stack *st,
 		 * of tables.
 		 */
 		err = flock_close(&table_locks[nlocks++]);
-		if (err < 0) {
+		if (err < 0)
-			err = REFTABLE_IO_ERROR;
 			goto done;
 	}
-	}
 
 	/*
 	 * We have locked all tables in our range and can thus release the
@@ -1334,13 +1318,8 @@ static int stack_compact_range(struct reftable_stack *st,
 	 * the new table.
 	 */
 	err = flock_acquire(&tables_list_lock, st->list_file, st->opts.lock_timeout_ms);
-	if (err < 0) {
+	if (err < 0)
-		if (errno == EEXIST)
-			err = REFTABLE_LOCK_ERROR;
-		else
-			err = REFTABLE_IO_ERROR;
 		goto done;
-	}
 
 	if (st->opts.default_permissions) {
 		if (chmod(tables_list_lock.path,

reftable/system.c

@@ -72,7 +72,7 @@ int flock_acquire(struct reftable_flock *l, const char *target_path,
 		reftable_free(lockfile);
 		if (errno == EEXIST)
 			return REFTABLE_LOCK_ERROR;
-		return -1;
+		return REFTABLE_IO_ERROR;
 	}
 
 	l->fd = get_lock_file_fd(lockfile);

reftable/system.h

@@ -81,7 +81,9 @@ struct reftable_flock {
  * to acquire the lock. If `timeout_ms` is 0 we don't wait, if it is negative
  * we block indefinitely.
  *
- * Retrun 0 on success, a reftable error code on error.
+ * Retrun 0 on success, a reftable error code on error. Specifically,
+ * `REFTABLE_LOCK_ERROR` should be returned in case the target path is already
+ * locked.
  */
 int flock_acquire(struct reftable_flock *l, const char *target_path,
 		  long timeout_ms);