Do basic error checking in ref_transaction_create() and make it return
non-zero on error. Update all callers to check the result of
ref_transaction_create(). There are currently no conditions in _create that
will return error but there will be in the future. Add an err argument that
will be updated on failure.

Signed-off-by: Ronnie Sahlberg <sahlb...@google.com>
---
 builtin/update-ref.c |  4 +++-
 refs.c               | 18 ++++++++++++------
 refs.h               | 25 ++++++++++++++++++-------
 3 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 3067b11..41121fa 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -226,7 +226,9 @@ static const char *parse_cmd_create(struct strbuf *input, 
const char *next)
        if (*next != line_termination)
                die("create %s: extra input: %s", refname, next);
 
-       ref_transaction_create(transaction, refname, new_sha1, update_flags);
+       if (ref_transaction_create(transaction, refname, new_sha1,
+                                  update_flags, &err))
+               die("%s", err.buf);
 
        update_flags = 0;
        free(refname);
diff --git a/refs.c b/refs.c
index 79a4477..709758d 100644
--- a/refs.c
+++ b/refs.c
@@ -3397,18 +3397,24 @@ int ref_transaction_update(struct ref_transaction 
*transaction,
        return 0;
 }
 
-void ref_transaction_create(struct ref_transaction *transaction,
-                           const char *refname,
-                           const unsigned char *new_sha1,
-                           int flags)
+int ref_transaction_create(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          int flags,
+                          struct strbuf *err)
 {
-       struct ref_update *update = add_update(transaction, refname);
+       struct ref_update *update;
+
+       if (!new_sha1 || is_null_sha1(new_sha1))
+               die("BUG: create ref with null new_sha1");
+
+       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;
+       return 0;
 }
 
 void ref_transaction_delete(struct ref_transaction *transaction,
diff --git a/refs.h b/refs.h
index 32edf3f..22b8cc3 100644
--- a/refs.h
+++ b/refs.h
@@ -215,6 +215,15 @@ enum action_on_err {
 };
 
 /*
+ * Transaction functions that take an err argument will append an error
+ * string to this buffer if there was a failure.
+ * This string is not cleared on each call and may contain an aggregate of
+ * errors from several previous calls.
+ * If the caller needs a guarantee that the buffer will only contain the
+ * current or most recent error it must call strbuf_reset before calling
+ * the transaction function.
+ */
+/*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
  */
@@ -236,7 +245,7 @@ struct ref_transaction *ref_transaction_begin(void);
  * 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. On failure the err buffer will be updated.
+ * rolled back.
  */
 int ref_transaction_update(struct ref_transaction *transaction,
                           const char *refname,
@@ -250,11 +259,15 @@ int ref_transaction_update(struct ref_transaction 
*transaction,
  * 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.
+ * Function returns 0 on success and non-zero on failure. A failure to create
+ * means that the transaction as a whole has failed and will need to be
+ * rolled back.
  */
-void ref_transaction_create(struct ref_transaction *transaction,
-                           const char *refname,
-                           const unsigned char *new_sha1,
-                           int flags);
+int ref_transaction_create(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          int flags,
+                          struct strbuf *err);
 
 /*
  * Add a reference deletion to transaction.  If have_old is true, then
@@ -270,8 +283,6 @@ void ref_transaction_delete(struct ref_transaction 
*transaction,
  * Commit all of the changes that have been queued in transaction, as
  * atomically as possible.  Return a nonzero value if there is a
  * problem.
- * If err is non-NULL we will add an error string to it to explain why
- * the transaction failed. The string does not end in newline.
  */
 int ref_transaction_commit(struct ref_transaction *transaction,
                           const char *msg, struct strbuf *err);
-- 
2.0.0.rc3.474.g0203784

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