Go: Keep errors in Go.

Helps with #9931

- graph.go: Return a nil Operation if the operation creation failed.
  This ensures that accidental usage of the nil operation results in
  a panic/stacktrace purely in Go (nil pointer dereference) instead
  of a SIGSEGV in the underlying C API.

- operation.go: Attempt to help in the developer find the root cause
  of the nil pointer dereference with a chattier panic message.

PiperOrigin-RevId: 156220056

1 files changed, 5 insertions, 0 deletions

diff --git a/tensorflow/go/operation.go b/tensorflow/go/operation.go
index e8f67c4f73..8fcad61f4c 100644
--- a/tensorflow/go/operation.go
+++ b/tensorflow/go/operation.go
@@ -113,6 +113,11 @@ func (p Output) Shape() Shape {
 }
 
 func (p Output) c() C.TF_Output {
+	if p.Op == nil {
+		// Attempt to provide a more useful panic message than "nil
+		// pointer dereference".
+		panic("nil-Operation. If the Output was created with a Scope object, see Scope.Err() for details.")
+	}
 	return C.TF_Output{oper: p.Op.c, index: C.int(p.Index)}
 }