Update ref_transaction_update() do some basic error checking and return
non-zero on error. Update all callers to check ref_transaction_update() for
error. There are currently no conditions in _update that will return error but
there will be in the future. Add an err argument that will be updated on
failure. In future patches we will start doing both locking and checking
for name conflicts in _update instead of _commit at which time this function
will start returning errors for these conditions.

Also check for BUGs during update and die(BUG:...) if we are calling
_update with have_old but the old_sha1 pointer is NULL.

Reviewed-by: Jonathan Nieder <jrnie...@gmail.com>
Signed-off-by: Ronnie Sahlberg <sahlb...@google.com>
---
 builtin/update-ref.c | 12 +++++++-----
 refs.c               | 18 ++++++++++++------
 refs.h               | 14 +++++++++-----
 3 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 88ab785..3067b11 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -16,6 +16,7 @@ static struct ref_transaction *transaction;
 
 static char line_termination = '\n';
 static int update_flags;
+static struct strbuf err = STRBUF_INIT;
 
 /*
  * Parse one whitespace- or NUL-terminated, possibly C-quoted argument
@@ -197,8 +198,9 @@ static const char *parse_cmd_update(struct strbuf *input, 
const char *next)
        if (*next != line_termination)
                die("update %s: extra input: %s", refname, next);
 
-       ref_transaction_update(transaction, refname, new_sha1, old_sha1,
-                              update_flags, have_old);
+       if (ref_transaction_update(transaction, refname, new_sha1, old_sha1,
+                                  update_flags, have_old, &err))
+               die("%s", err.buf);
 
        update_flags = 0;
        free(refname);
@@ -286,8 +288,9 @@ static const char *parse_cmd_verify(struct strbuf *input, 
const char *next)
        if (*next != line_termination)
                die("verify %s: extra input: %s", refname, next);
 
-       ref_transaction_update(transaction, refname, new_sha1, old_sha1,
-                              update_flags, have_old);
+       if (ref_transaction_update(transaction, refname, new_sha1, old_sha1,
+                                  update_flags, have_old, &err))
+               die("%s", err.buf);
 
        update_flags = 0;
        free(refname);
@@ -342,7 +345,6 @@ int cmd_update_ref(int argc, const char **argv, const char 
*prefix)
        const char *refname, *oldval, *msg = NULL;
        unsigned char sha1[20], oldsha1[20];
        int delete = 0, no_deref = 0, read_stdin = 0, end_null = 0, flags = 0;
-       struct strbuf err = STRBUF_INIT;
        struct option options[] = {
                OPT_STRING( 'm', NULL, &msg, N_("reason"), N_("reason of the 
update")),
                OPT_BOOL('d', NULL, &delete, N_("delete the reference")),
diff --git a/refs.c b/refs.c
index fdbd107..9c02687 100644
--- a/refs.c
+++ b/refs.c
@@ -3425,19 +3425,25 @@ static struct ref_update *add_update(struct 
ref_transaction *transaction,
        return update;
 }
 
-void ref_transaction_update(struct ref_transaction *transaction,
-                           const char *refname,
-                           const unsigned char *new_sha1,
-                           const unsigned char *old_sha1,
-                           int flags, int have_old)
+int ref_transaction_update(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          const unsigned char *old_sha1,
+                          int flags, int have_old,
+                          struct strbuf *err)
 {
-       struct ref_update *update = add_update(transaction, refname);
+       struct ref_update *update;
+
+       if (have_old && !old_sha1)
+               die("BUG: have_old is true but old_sha1 is NULL");
 
+       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);
+       return 0;
 }
 
 void ref_transaction_create(struct ref_transaction *transaction,
diff --git a/refs.h b/refs.h
index 1eb447d..f7a5e42 100644
--- a/refs.h
+++ b/refs.h
@@ -242,12 +242,16 @@ struct ref_transaction *ref_transaction_begin(void);
  * 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.
+ * 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.
  */
-void ref_transaction_update(struct ref_transaction *transaction,
-                           const char *refname,
-                           const unsigned char *new_sha1,
-                           const unsigned char *old_sha1,
-                           int flags, int have_old);
+int ref_transaction_update(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          const unsigned char *old_sha1,
+                          int flags, int have_old,
+                          struct strbuf *err);
 
 /*
  * Add a reference creation to transaction.  new_sha1 is the value
-- 
2.0.0.282.g3799eda.dirty

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