transport: see if cert/cred callbacks exist before calling them

Custom transports may want to ask libgit2 to invoke a configured
credential or certificate callback; however they likely do not know if a
callback was actually configured.  Return a sentinal value
(GIT_PASSTHROUGH) if there is no callback configured instead of crashing.

1 files changed, 8 insertions, 2 deletions

diff --git a/include/git2/sys/transport.h b/include/git2/sys/transport.h
index a395de5ed..aac6f9f36 100644
--- a/include/git2/sys/transport.h
+++ b/include/git2/sys/transport.h
@@ -226,7 +226,10 @@ GIT_EXTERN(int) git_transport_smart(
  * @param cert the certificate to pass to the caller
  * @param valid whether we believe the certificate is valid
  * @param hostname the hostname we connected to
- * @return the return value of the callback
+ * @return the return value of the callback: 0 for no error, GIT_PASSTHROUGH
+ *         to indicate that there is no callback registered (or the callback
+ *         refused to validate the certificate and callers should behave as
+ *         if no callback was set), or < 0 for an error
  */
 GIT_EXTERN(int) git_transport_smart_certificate_check(git_transport *transport, git_cert *cert, int valid, const char *hostname);
 
@@ -237,7 +240,10 @@ GIT_EXTERN(int) git_transport_smart_certificate_check(git_transport *transport,
  * @param transport a smart transport
  * @param user the user we saw on the url (if any)
  * @param methods available methods for authentication
- * @return the return value of the callback
+ * @return the return value of the callback: 0 for no error, GIT_PASSTHROUGH
+ *         to indicate that there is no callback registered (or the callback
+ *         refused to provide credentials and callers should behave as if no
+ *         callback was set), or < 0 for an error
  */
 GIT_EXTERN(int) git_transport_smart_credentials(git_cred **out, git_transport *transport, const char *user, int methods);