Add some appengine build constraints. (#3787)

Support for Go1.9 and appengine was removed in https://github.com/grpc/grpc-go/pull/3767. But it looks like we still need some way to do things differently for certain appengine builds.

channelz/service/func_nonlinux.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux appengine
 
 /*
  *

internal/channelz/types_nonlinux.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux appengine
 
 /*
  *

internal/channelz/util_nonlinux.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux appengine
 
 /*
  *

internal/credentials/spiffe_appengine.go

@@ -0,0 +1,31 @@
+// +build appengine
+
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package credentials
+
+import (
+	"crypto/tls"
+	"net/url"
+)
+
+// SPIFFEIDFromState is a no-op for appengine builds.
+func SPIFFEIDFromState(state tls.ConnectionState) *url.URL {
+	return nil
+}

internal/syscall/syscall_nonlinux.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux appengine
 
 /*
  *

security/advancedtls/sni_appengine.go

@@ -0,0 +1,30 @@
+// +build appengine
+
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package advancedtls
+
+import (
+	"crypto/tls"
+)
+
+// buildGetCertificates is a no-op for appengine builds.
+func buildGetCertificates(clientHello *tls.ClientHelloInfo, o *ServerOptions) (*tls.Certificate, error) {
+	return nil, nil
+}