stash: use a transaction to modify the reflog

The stash is implemented as the refs/stash reference and its reflog. In
order to modify the reflog, we need avoid races by making sure we're the
only ones allowed to modify the reflog.

We achieve this via the transactions API. Locking the reference gives us
exclusive write access, letting us modify and write it without races.

src/stash.c

@@ -15,6 +15,7 @@
 #include "git2/status.h"
 #include "git2/checkout.h"
 #include "git2/index.h"
+#include "git2/transaction.h"
 #include "signature.h"
 
 static int create_error(int error, const char *msg)
@@ -601,14 +602,21 @@ int git_stash_drop(
 	git_repository *repo,
 	size_t index)
 {
-	git_reference *stash;
+	git_transaction *tx;
+	git_reference *stash = NULL;
 	git_reflog *reflog = NULL;
 	size_t max;
 	int error;
 
-	if ((error = git_reference_lookup(&stash, repo, GIT_REFS_STASH_FILE)) < 0)
+	if ((error = git_transaction_new(&tx, repo)) < 0)
 		return error;
 
+	if ((error = git_transaction_lock(tx, GIT_REFS_STASH_FILE)) < 0)
+		goto cleanup;
+
+	if ((error = git_reference_lookup(&stash, repo, GIT_REFS_STASH_FILE)) < 0)
+		goto cleanup;
+
 	if ((error = git_reflog_read(&reflog, repo, GIT_REFS_STASH_FILE)) < 0)
 		goto cleanup;
 
@@ -623,29 +631,25 @@ int git_stash_drop(
 	if ((error = git_reflog_drop(reflog, index, true)) < 0)
 		goto cleanup;
 
-	if ((error = git_reflog_write(reflog)) < 0)
+	if ((error = git_transaction_set_reflog(tx, GIT_REFS_STASH_FILE, reflog)) < 0)
 		goto cleanup;
 
 	if (max == 1) {
-		error = git_reference_delete(stash);
-		git_reference_free(stash);
-		stash = NULL;
+		if ((error = git_transaction_remove(tx, GIT_REFS_STASH_FILE)) < 0)
+			goto cleanup;
 	} else if (index == 0) {
 		const git_reflog_entry *entry;
 
 		entry = git_reflog_entry_byindex(reflog, 0);
-
-		git_reference_free(stash);
-		error = git_reference_create(&stash, repo, GIT_REFS_STASH_FILE, &entry->oid_cur, 1, NULL, NULL);
-		if (error < 0)
+		if ((error = git_transaction_set_target(tx, GIT_REFS_STASH_FILE, &entry->oid_cur, NULL, NULL)) < 0)
 			goto cleanup;
-
-		/* We need to undo the writing that we just did */
-		error = git_reflog_write(reflog);
 	}
 
+	error = git_transaction_commit(tx);
+
 cleanup:
 	git_reference_free(stash);
+	git_transaction_free(tx);
 	git_reflog_free(reflog);
 	return error;
 }