ccl/sqlproxyccl: replace errPanicWriter with a writer that returns an…

… error

Previously, we had an errPanicWriter struct that panics whenever a Write
call was made. This is used because Receive on the pgproto3 backend and
frontend instances must not call Write. However, panics are not ideal, so
this commit replaces that with a regular writer that just returns an error
when Write is called.

Release note: None

pkg/ccl/sqlproxyccl/interceptor/backend_interceptor.go

@@ -46,8 +46,8 @@ func (bi *BackendInterceptor) ReadMsg() (msg pgproto3.FrontendMessage, err error
 	if err != nil {
 		return nil, err
 	}
-	// errPanicWriter is used here because Receive must not Write.
-	return pgproto3.NewBackend(newChunkReader(msgBytes), &errPanicWriter{}).Receive()
+	// errWriter is used here because Receive must not Write.
+	return pgproto3.NewBackend(newChunkReader(msgBytes), &errWriter{}).Receive()
 }
 
 // WriteMsg writes the given bytes to the writer dst. This is just a helper

pkg/ccl/sqlproxyccl/interceptor/base.go

@@ -243,12 +243,14 @@ func (p *pgInterceptor) ensureNextNBytes(n int) error {
 	return err
 }
 
-var _ io.Writer = &errPanicWriter{}
+var _ io.Writer = &errWriter{}
 
-// errPanicWriter is an io.Writer that panics whenever a Write call is made.
-type errPanicWriter struct{}
+// errWriter is an io.Writer that fails whenever a Write call is made. This is
+// used within ReadMsg for both BackendInterceptor and FrontendInterceptor.
+// Since it's just a Read, Write calls should not be made.
+type errWriter struct{}
 
 // Write implements the io.Writer interface.
-func (w *errPanicWriter) Write(p []byte) (int, error) {
-	panic("unexpected Write call")
+func (w *errWriter) Write(p []byte) (int, error) {
+	return 0, errors.AssertionFailedf("unexpected Write call")
 }

pkg/ccl/sqlproxyccl/interceptor/frontend_interceptor.go

@@ -47,8 +47,8 @@ func (fi *FrontendInterceptor) ReadMsg() (msg pgproto3.BackendMessage, err error
 	if err != nil {
 		return nil, err
 	}
-	// errPanicWriter is used here because Receive must not Write.
-	return pgproto3.NewFrontend(newChunkReader(msgBytes), &errPanicWriter{}).Receive()
+	// errWriter is used here because Receive must not Write.
+	return pgproto3.NewFrontend(newChunkReader(msgBytes), &errWriter{}).Receive()
 }
 
 // WriteMsg writes the given bytes to the writer dst. This is just a helper