[PATCH 2/8] refs/files-backend: stop setting errno from lock_ref_oid_basic

From: Han-Wen Nienhuys via GitGitGadget <gitgitgadget@gmail.com>
Date: Apr 29, 2021, 15:32 UTC
Message-ID: <b2c72097e5e8985e7fdd8e3eee66cdf43d1b65c0.1619710329.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.1012.git.git.1619710329.gitgitgadget@gmail.com>

From: Han-Wen Nienhuys <hanwen@google.com>

Errno is a global variable written by almost all system calls, and therefore it is hard to reason about its state. It's also useless for user-visible errors, as it leaves no place to report the offending file and/or syscall.

For the copy/rename support, calls to lock_ref_oid_basic() in this file are followed by:

* lock_ref_oid_basic (copy/rename rollback error path)

* write_ref_to_lockfile (both in the rollback path and the success path of
  copy/rename)

These calls do not inspect the incoming errno. As they perform I/O, they can clobber errno. For this reason, callers cannot reliably observe the errno that lock_ref_oid_basic() generated, so it is unsound for programmatic use.

For files_create_symref() and files_reflog_expire(), grepping over callers showed no callers inspecting errno.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
---
 refs/files-backend.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 119972ee16f8..c9511da1d387 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))
@@ -962,7 +959,7 @@ 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))
@@ -981,20 +978,17 @@ 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 +999,6 @@ 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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