refs: add a concept of a reference transaction

Build out the API for dealing with a bunch of reference checks and
changes within a transaction.  Define an opaque ref_transaction type
that is managed entirely within refs.c.  Introduce functions for
beginning a transaction, adding updates to a transaction, and
committing/rolling back a transaction.

This API will soon replace update_refs().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -3267,6 +3267,96 @@ static int update_ref_write(const char *action, const char *refname,
 	return 0;
 }
 
+/*
+ * Data structure for holding a reference transaction, which can
+ * consist of checks and updates to multiple references, carried out
+ * as atomically as possible.  This structure is opaque to callers.
+ */
+struct ref_transaction {
+	struct ref_update **updates;
+	size_t alloc;
+	size_t nr;
+};
+
+struct ref_transaction *ref_transaction_begin(void)
+{
+	return xcalloc(1, sizeof(struct ref_transaction));
+}
+
+static void ref_transaction_free(struct ref_transaction *transaction)
+{
+	int i;
+
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+
+		free((char *)update->ref_name);
+		free(update);
+	}
+
+	free(transaction->updates);
+	free(transaction);
+}
+
+void ref_transaction_rollback(struct ref_transaction *transaction)
+{
+	ref_transaction_free(transaction);
+}
+
+static struct ref_update *add_update(struct ref_transaction *transaction,
+				     const char *refname)
+{
+	struct ref_update *update = xcalloc(1, sizeof(*update));
+
+	update->ref_name = xstrdup(refname);
+	ALLOC_GROW(transaction->updates, transaction->nr + 1, transaction->alloc);
+	transaction->updates[transaction->nr++] = update;
+	return update;
+}
+
+void ref_transaction_update(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *new_sha1, unsigned char *old_sha1,
+			    int flags, int have_old)
+{
+	struct ref_update *update = add_update(transaction, refname);
+
+	hashcpy(update->new_sha1, new_sha1);
+	update->flags = flags;
+	update->have_old = have_old;
+	if (have_old)
+		hashcpy(update->old_sha1, old_sha1);
+}
+
+void ref_transaction_create(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *new_sha1,
+			    int flags)
+{
+	struct ref_update *update = add_update(transaction, refname);
+
+	assert(!is_null_sha1(new_sha1));
+	hashcpy(update->new_sha1, new_sha1);
+	hashclr(update->old_sha1);
+	update->flags = flags;
+	update->have_old = 1;
+}
+
+void ref_transaction_delete(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *old_sha1,
+			    int flags, int have_old)
+{
+	struct ref_update *update = add_update(transaction, refname);
+
+	update->flags = flags;
+	update->have_old = have_old;
+	if (have_old) {
+		assert(!is_null_sha1(old_sha1));
+		hashcpy(update->old_sha1, old_sha1);
+	}
+}
+
 int update_ref(const char *action, const char *refname,
 	       const unsigned char *sha1, const unsigned char *oldval,
 	       int flags, enum action_on_err onerr)
@@ -3378,6 +3468,15 @@ int update_refs(const char *action, struct ref_update * const *updates_orig,
 	return ret;
 }
 
+int ref_transaction_commit(struct ref_transaction *transaction,
+			   const char *msg, enum action_on_err onerr)
+{
+	int ret = update_refs(msg, transaction->updates, transaction->nr,
+			      onerr);
+	ref_transaction_free(transaction);
+	return ret;
+}
+
 char *shorten_unambiguous_ref(const char *refname, int strict)
 {
 	int i;

refs.h

@@ -24,6 +24,8 @@ struct ref_update {
 	int have_old; /* 1 if old_sha1 is valid, 0 otherwise */
 };
 
+struct ref_transaction;
+
 /*
  * Bit values set in the flags argument passed to each_ref_fn():
  */
@@ -220,6 +222,69 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Begin a reference transaction.  The reference transaction must
+ * eventually be commited using ref_transaction_commit() or rolled
+ * back using ref_transaction_rollback().
+ */
+struct ref_transaction *ref_transaction_begin(void);
+
+/*
+ * Roll back a ref_transaction and free all associated data.
+ */
+void ref_transaction_rollback(struct ref_transaction *transaction);
+
+
+/*
+ * The following functions add a reference check or update to a
+ * ref_transaction.  In all of them, refname is the name of the
+ * reference to be affected.  The functions make internal copies of
+ * refname, so the caller retains ownership of the parameter.  flags
+ * can be REF_NODEREF; it is passed to update_ref_lock().
+ */
+
+
+/*
+ * Add a reference update to transaction.  new_sha1 is the value that
+ * the reference should have after the update, or zeros if it should
+ * be deleted.  If have_old is true, then old_sha1 holds the value
+ * that the reference should have had before the update, or zeros if
+ * it must not have existed beforehand.
+ */
+void ref_transaction_update(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *new_sha1, unsigned char *old_sha1,
+			    int flags, int have_old);
+
+/*
+ * Add a reference creation to transaction.  new_sha1 is the value
+ * that the reference should have after the update; it must not be the
+ * null SHA-1.  It is verified that the reference does not exist
+ * already.
+ */
+void ref_transaction_create(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *new_sha1,
+			    int flags);
+
+/*
+ * Add a reference deletion to transaction.  If have_old is true, then
+ * old_sha1 holds the value that the reference should have had before
+ * the update (which must not be the null SHA-1).
+ */
+void ref_transaction_delete(struct ref_transaction *transaction,
+			    const char *refname,
+			    unsigned char *old_sha1,
+			    int flags, int have_old);
+
+/*
+ * Commit all of the changes that have been queued in transaction, as
+ * atomically as possible.  Return a nonzero value if there is a
+ * problem.  The ref_transaction is freed by this function.
+ */
+int ref_transaction_commit(struct ref_transaction *transaction,
+			   const char *msg, enum action_on_err onerr);
+
 /** Lock a ref and then write its file */
 int update_ref(const char *action, const char *refname,
 		const unsigned char *sha1, const unsigned char *oldval,