commit: expose the different kinds of errors

We should be checking whether the object we're looking up is a commit,
and we should let the caller know whether the not-found return code
comes from a bad object type or just a missing signature.

1 files changed, 6 insertions, 0 deletions

diff --git a/include/git2/commit.h b/include/git2/commit.h
index a92277417..3488c7440 100644
--- a/include/git2/commit.h
+++ b/include/git2/commit.h
@@ -266,12 +266,18 @@ GIT_EXTERN(int) git_commit_header_field(git_buf *out, const git_commit *commit,
 /**
  * Extract the signature from a commit
  *
+ * If the id is not for a commit, the error class will be
+ * `GITERR_INVALID`. If the commit does not have a signature, the
+ * error class will be `GITERR_OBJECT`.
+ *
  * @param signature the signature block
  * @param signed_data signed data; this is the commit contents minus the signature block
  * @param repo the repository in which the commit exists
  * @param commit_id the commit from which to extract the data
  * @param field the name of the header field containing the signature
  * block; pass `NULL` to extract the default 'gpgsig'
+ * @return 0 on success, GIT_ENOTFOUND if the id is not for a commit
+ * or the commit does not have a signature.
  */
 GIT_EXTERN(int) git_commit_extract_signature(git_buf *signature, git_buf *signed_data, git_repository *repo, git_oid *commit_id, const char *field);