[PATCH v4 2/6] refs/files-backend: stop setting errno from lock_ref_oid_basic

["Han-Wen Nienhuys via GitGitGadget" <gitgitgadget@xxxxxxxxx>]

From: Han-Wen Nienhuys <hanwen@xxxxxxxxxx>

refs/files-backend.c::lock_ref_oid_basic() tries to signal how it failed
to its callers using errno.

It is safe to stop setting errno here, because the callers of this
file-scope static function are

* files_copy_or_rename_ref()
* files_create_symref()
* files_reflog_expire()

None of them looks at errno after seeing a negative return from
lock_ref_oid_basic() to make any decision, and no caller of these three
functions looks at errno after they signal a failure by returning a
negative value. In particular,

* files_copy_or_rename_ref() - here, calls are followed by error()
(which performs I/O) or write_ref_to_lockfile() (which calls
parse_object() which may perform I/O)

* files_create_symref() - here, calls are followed by error() or
create_symref_locked() (which performs I/O and does not inspect
errno)

* files_reflog_expire() - here, calls are followed by error() or
refs_reflog_exists() (which calls a function in a vtable that is not
documented to use and/or preserve errno)

Signed-off-by: Han-Wen Nienhuys <hanwen@xxxxxxxxxx>
Reviewed-By: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
---
 refs/files-backend.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 677b7e4cdd2..83ddfb3b627 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -910,7 +910,6 @@ static int create_reflock(const char *path, void *cb)
 
 /*
  * Locks a ref returning the lock on success and NULL on failure.
- * On failure errno is set to something meaningful.
  */
 static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 					   const char *refname,
@@ -922,7 +921,6 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 {
 	struct strbuf ref_file = STRBUF_INIT;
 	struct ref_lock *lock;
-	int last_errno = 0;
 	int mustexist = (old_oid && !is_null_oid(old_oid));
 	int resolve_flags = RESOLVE_REF_NO_RECURSE;
 	int resolved;
@@ -949,7 +947,6 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 		 * to remain.
 		 */
 		if (remove_empty_directories(&ref_file)) {
-			last_errno = errno;
 			if (!refs_verify_refname_available(
 					    &refs->base,
 					    refname, extras, skip, err))
@@ -962,10 +959,13 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 						     &lock->old_oid, type);
 	}
 	if (!resolved) {
-		last_errno = errno;
+		int last_errno = errno;
 		if (last_errno != ENOTDIR ||
-		    !refs_verify_refname_available(&refs->base, refname,
-						   extras, skip, err))
+		    /* in case of D/F conflict, try to generate a better error
+		     * message. If that fails, fall back to strerror(ENOTDIR).
+		     */
+		    !refs_verify_refname_available(&refs->base, refname, extras,
+						   skip, err))
 			strbuf_addf(err, "unable to resolve reference '%s': %s",
 				    refname, strerror(last_errno));
 
@@ -981,20 +981,17 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 	if (is_null_oid(&lock->old_oid) &&
 	    refs_verify_refname_available(refs->packed_ref_store, refname,
 					  extras, skip, err)) {
-		last_errno = ENOTDIR;
 		goto error_return;
 	}
 
 	lock->ref_name = xstrdup(refname);
 
 	if (raceproof_create_file(ref_file.buf, create_reflock, &lock->lk)) {
-		last_errno = errno;
 		unable_to_lock_message(ref_file.buf, errno, err);
 		goto error_return;
 	}
 
 	if (verify_lock(&refs->base, lock, old_oid, mustexist, err)) {
-		last_errno = errno;
 		goto error_return;
 	}
 	goto out;
@@ -1005,7 +1002,6 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 
  out:
 	strbuf_release(&ref_file);
-	errno = last_errno;
 	return lock;
 }
 
-- 
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