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>

1 files changed, 65 insertions, 0 deletions

diff --git a/refs.h b/refs.h
index 08e60ac07f..0518dd5203 100644
--- a/refs.h
+++ b/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,