ref_transaction_verify(): new function to check a reference's value

If NULL is passed to ref_transaction_update()'s new_sha1 parameter,
then just verify old_sha1 (under lock) without trying to change the
new value of the reference.

Use this functionality to add a new function ref_transaction_verify(),
which checks the current value of the reference under lock but doesn't
change it.

Use ref_transaction_verify() in the implementation of "git update-ref
--stdin"'s "verify" command to avoid the awkward need to "update" the
reference to its existing value.

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

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 226995f0290a3323d86b7c6649fa688f32aaa76b..3d79a46b036fa7bea29bbc0941442b5dd92f648c 100644 (file)
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -282,7 +282,6 @@ static const char *parse_cmd_verify(struct ref_transaction *transaction,
 {
        struct strbuf err = STRBUF_INIT;
        char *refname;
-       unsigned char new_sha1[20];
        unsigned char old_sha1[20];
 
        refname = parse_refname(input, &next);
@@ -293,13 +292,11 @@ static const char *parse_cmd_verify(struct ref_transaction *transaction,
                            PARSE_SHA1_OLD))
                hashclr(old_sha1);
 
-       hashcpy(new_sha1, old_sha1);
-
        if (*next != line_termination)
                die("verify %s: extra input: %s", refname, next);
 
-       if (ref_transaction_update(transaction, refname, new_sha1, old_sha1,
-                                  update_flags, msg, &err))
+       if (ref_transaction_verify(transaction, refname, old_sha1,
+                                  update_flags, &err))
                die("%s", err.buf);
 
        update_flags = 0;

diff --git a/refs.c b/refs.c
index d5bfd1179044b9a6aeec1492a31d2a907b951d36..1aaba3f7f7eda4910bc9c159487ff47ecc60fbb6 100644 (file)
--- a/refs.c
+++ b/refs.c
@@ -46,11 +46,17 @@ static unsigned char refname_disposition[256] = {
  */
 #define REF_ISPRUNING  0x04
 
+/*
+ * Used as a flag in ref_update::flags when the reference should be
+ * updated to new_sha1.
+ */
+#define REF_HAVE_NEW   0x08
+
 /*
  * Used as a flag in ref_update::flags when old_sha1 should be
  * checked.
  */
-#define REF_HAVE_OLD   0x08
+#define REF_HAVE_OLD   0x10
 
 /*
  * Try to read one refname component from the front of refname.
@@ -3577,10 +3583,17 @@ int for_each_reflog(each_ref_fn fn, void *cb_data)
  * not exist before update.
  */
 struct ref_update {
+       /*
+        * If (flags & REF_HAVE_NEW), set the reference to this value:
+        */
        unsigned char new_sha1[20];
+       /*
+        * If (flags & REF_HAVE_OLD), check that the reference
+        * previously had this value:
+        */
        unsigned char old_sha1[20];
        /*
-        * One or more of REF_HAVE_OLD, REF_NODEREF,
+        * One or more of REF_HAVE_NEW, REF_HAVE_OLD, REF_NODEREF,
         * REF_DELETING, and REF_ISPRUNING:
         */
        unsigned int flags;
@@ -3665,7 +3678,7 @@ int ref_transaction_update(struct ref_transaction *transaction,
        if (transaction->state != REF_TRANSACTION_OPEN)
                die("BUG: update called for transaction that is not open");
 
-       if (!is_null_sha1(new_sha1) &&
+       if (new_sha1 && !is_null_sha1(new_sha1) &&
            check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                strbuf_addf(err, "refusing to update ref with bad name %s",
                            refname);
@@ -3673,7 +3686,10 @@ int ref_transaction_update(struct ref_transaction *transaction,
        }
 
        update = add_update(transaction, refname);
-       hashcpy(update->new_sha1, new_sha1);
+       if (new_sha1) {
+               hashcpy(update->new_sha1, new_sha1);
+               flags |= REF_HAVE_NEW;
+       }
        if (old_sha1) {
                hashcpy(update->old_sha1, old_sha1);
                flags |= REF_HAVE_OLD;
@@ -3709,6 +3725,19 @@ int ref_transaction_delete(struct ref_transaction *transaction,
                                      flags, msg, err);
 }
 
+int ref_transaction_verify(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *old_sha1,
+                          unsigned int flags,
+                          struct strbuf *err)
+{
+       if (!old_sha1)
+               die("BUG: verify called with old_sha1 set to NULL");
+       return ref_transaction_update(transaction, refname,
+                                     NULL, old_sha1,
+                                     flags, NULL, err);
+}
+
 int update_ref(const char *action, const char *refname,
               const unsigned char *sha1, const unsigned char *oldval,
               unsigned int flags, enum action_on_err onerr)
@@ -3797,7 +3826,7 @@ int ref_transaction_commit(struct ref_transaction *transaction,
                struct ref_update *update = updates[i];
                unsigned int flags = update->flags;
 
-               if (is_null_sha1(update->new_sha1))
+               if ((flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1))
                        flags |= REF_DELETING;
                update->lock = lock_ref_sha1_basic(
                                update->refname,
@@ -3819,8 +3848,9 @@ int ref_transaction_commit(struct ref_transaction *transaction,
        /* Perform updates first so live commits remain referenced */
        for (i = 0; i < n; i++) {
                struct ref_update *update = updates[i];
+               int flags = update->flags;
 
-               if (!is_null_sha1(update->new_sha1)) {
+               if ((flags & REF_HAVE_NEW) && !is_null_sha1(update->new_sha1)) {
                        if (write_ref_sha1(update->lock, update->new_sha1,
                                           update->msg)) {
                                update->lock = NULL; /* freed by write_ref_sha1 */
@@ -3836,14 +3866,15 @@ int ref_transaction_commit(struct ref_transaction *transaction,
        /* Perform deletes now that updates are safely completed */
        for (i = 0; i < n; i++) {
                struct ref_update *update = updates[i];
+               int flags = update->flags;
 
-               if (update->lock) {
+               if ((flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1)) {
                        if (delete_ref_loose(update->lock, update->type, err)) {
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        }
 
-                       if (!(update->flags & REF_ISPRUNING))
+                       if (!(flags & REF_ISPRUNING))
                                string_list_append(&refs_to_delete,
                                                   update->lock->ref_name);
                }

diff --git a/refs.h b/refs.h
index 100731d1c87b6130e301f1c7bbc2e765828959e7..5e139d5f8c95f5de4c3fdb9898c65e50d54332cf 100644 (file)
--- a/refs.h
+++ b/refs.h
@@ -263,14 +263,19 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err);
  */
 
 /*
- * Add a reference update to transaction.  new_sha1 is the value that
- * the reference should have after the update, or null_sha1 if it should
- * be deleted.  If old_sha1 is non-NULL, then it is the value
- * that the reference should have had before the update, or null_sha1 if
- * it must not have existed beforehand.
- * Function returns 0 on success and non-zero on failure. A failure to update
- * means that the transaction as a whole has failed and will need to be
- * rolled back.
+ * Add a reference update to transaction. new_sha1 is the value that
+ * the reference should have after the update, or null_sha1 if it
+ * should be deleted. If new_sha1 is NULL, then the reference is not
+ * changed at all. old_sha1 is the value that the reference must have
+ * before the update, or null_sha1 if it must not have existed
+ * beforehand. The old value is checked after the lock is taken to
+ * prevent races. If the old value doesn't agree with old_sha1, the
+ * whole transaction fails. If old_sha1 is NULL, then the previous
+ * value is not checked.
+ *
+ * Return 0 on success and non-zero on failure. Any failure in the
+ * transaction means that the transaction as a whole has failed and
+ * will need to be rolled back.
  */
 int ref_transaction_update(struct ref_transaction *transaction,
                           const char *refname,
@@ -308,6 +313,19 @@ int ref_transaction_delete(struct ref_transaction *transaction,
                           unsigned int flags, const char *msg,
                           struct strbuf *err);
 
+/*
+ * Verify, within a transaction, that refname has the value old_sha1,
+ * or, if old_sha1 is null_sha1, then verify that the reference
+ * doesn't exist. old_sha1 must be non-NULL. Function returns 0 on
+ * success and non-zero on failure. A failure to verify means that the
+ * transaction as a whole has failed and will need to be rolled back.
+ */
+int ref_transaction_verify(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *old_sha1,
+                          unsigned int flags,
+                          struct strbuf *err);
+
 /*
  * Commit all of the changes that have been queued in transaction, as
  * atomically as possible.