[PATCH/RFC v2 3/3] refs: always handle non-normal refs in files backend

[David Turner <dturner@twopensource.com>]

Always handle non-normal (per-worktree or pseudo) refs in the files
backend instead of alternate backends.

Sometimes a ref transaction will update both a per-worktree ref and a
normal ref.  For instance, an ordinary commit might update
refs/heads/master and HEAD (or at least HEAD's reflog).

Updates to normal refs continue to go through the chosen backend.

Updates to non-normal refs are moved to a separate files backend
transaction.

Signed-off-by: David Turner <dturner@twopensource.com>
---
 refs.c               | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 refs/refs-internal.h |  2 ++
 2 files changed, 82 insertions(+), 2 deletions(-)

diff --git a/refs.c b/refs.c
index 2496b91..67b0280 100644
--- a/refs.c
+++ b/refs.c
@@ -9,6 +9,11 @@
 #include "object.h"
 #include "tag.h"
 
+static const char split_transaction_fail_warning[] =
+	"A ref transaction was split across two refs backends.  Part of the "
+	"transaction succeeded, but then the update to the per-worktree refs "
+	"failed.  Your repository may be in an inconsistent state.";
+
 /*
  * We always have a files backend and it is the default.
  */
@@ -781,6 +786,13 @@ void ref_transaction_free(struct ref_transaction *transaction)
 	free(transaction);
 }
 
+static void add_update_obj(struct ref_transaction *transaction,
+			   struct ref_update *update)
+{
+	ALLOC_GROW(transaction->updates, transaction->nr + 1, transaction->alloc);
+	transaction->updates[transaction->nr++] = update;
+}
+
 static struct ref_update *add_update(struct ref_transaction *transaction,
 				     const char *refname)
 {
@@ -788,8 +800,7 @@ static struct ref_update *add_update(struct ref_transaction *transaction,
 	struct ref_update *update = xcalloc(1, sizeof(*update) + len);
 
 	memcpy((char *)update->refname, refname, len); /* includes NUL */
-	ALLOC_GROW(transaction->updates, transaction->nr + 1, transaction->alloc);
-	transaction->updates[transaction->nr++] = update;
+	add_update_obj(transaction, update);
 	return update;
 }
 
@@ -1192,11 +1203,39 @@ static int dereference_symrefs(struct ref_transaction *transaction,
 	return 0;
 }
 
+/*
+ * Move all non-normal ref updates into a specially-created
+ * files-backend transaction
+ */
+static int move_abnormal_ref_updates(struct ref_transaction *transaction,
+				     struct ref_transaction *files_transaction,
+				     struct strbuf *err)
+{
+	int i;
+
+	for (i = 0; i < transaction->nr; i++) {
+		int last;
+		struct ref_update *update = transaction->updates[i];
+
+		if (ref_type(update->refname) == REF_TYPE_NORMAL)
+			continue;
+
+		last = --transaction->nr;
+		transaction->updates[i] = transaction->updates[last];
+		files_transaction->updates[files_transaction->nr++] = update;
+	}
+
+	return 0;
+}
+
 int ref_transaction_commit(struct ref_transaction *transaction,
 			   struct strbuf *err)
 {
 	int ret = -1;
 	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
+	struct string_list files_affected_refnames = STRING_LIST_INIT_NODUP;
+	struct string_list_item *item;
+	struct ref_transaction *files_transaction = NULL;
 
 	assert(err);
 
@@ -1212,6 +1251,26 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	if (ret)
 		goto done;
 
+	if (the_refs_backend != &refs_be_files) {
+		files_transaction = ref_transaction_begin(err);
+		if (!files_transaction)
+			goto done;
+
+		ret = move_abnormal_ref_updates(transaction, files_transaction,
+						err);
+		if (ret)
+			goto done;
+
+		/* files backend commit */
+		if (get_affected_refnames(files_transaction,
+						 &files_affected_refnames,
+						 err)) {
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto done;
+		}
+	}
+
+	/* main backend commit */
 	if (get_affected_refnames(transaction, &affected_refnames, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto done;
@@ -1219,8 +1278,24 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 
 	ret = the_refs_backend->transaction_commit(transaction,
 						   &affected_refnames, err);
+	if (ret)
+		goto done;
+
+	if (files_transaction) {
+		ret = refs_be_files.transaction_commit(files_transaction,
+						       &files_affected_refnames,
+						       err);
+		if (ret) {
+			warning(split_transaction_fail_warning);
+			goto done;
+		}
+	}
+
 done:
 	string_list_clear(&affected_refnames, 0);
+	string_list_clear(&files_affected_refnames, 0);
+	if (files_transaction)
+		ref_transaction_free(files_transaction);
 	return ret;
 }
 
@@ -1276,6 +1351,9 @@ int peel_ref(const char *refname, unsigned char *sha1)
 int create_symref(const char *ref_target, const char *refs_heads_master,
 		  const char *logmsg)
 {
+	if (ref_type(ref_target) != REF_TYPE_NORMAL)
+		return refs_be_files.create_symref(ref_target, refs_heads_master,
+						   logmsg);
 	return the_refs_backend->create_symref(ref_target, refs_heads_master,
 					       logmsg);
 }
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 971f450..dd035f5 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -323,4 +323,6 @@ struct ref_storage_be {
 	for_each_replace_ref_fn *for_each_replace_ref;
 };
 
+extern struct ref_storage_be refs_be_files;
+
 #endif /* REFS_REFS_INTERNAL_H */
-- 
2.4.2.749.g730654d-twtrsrc