.github/workflows: work around golang/go#51629

Incidentally, simplify the go generate CI workflow, by
marking the dnsfallback update non-hermetic (so CI will
skip it) rather than manually filter it out of `go list`.

Updates #4194

Signed-off-by: David Anderson <danderson@tailscale.com>

.github/workflows/go-generate-without-stringer.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env sh
+#
+# This is a temporary hack to work around
+# https://github.com/golang/go/issues/51629 , wherein the stringer
+# generator doesn't work with generics.
+#
+# This script is the equivalent of `go generate ./...`, except that it
+# only runs generate on packages that don't try to use stringer.
+
+set -e
+
+find . -name '*.go' | xargs grep -l go:generate | xargs -n1 dirname | sort -u | while read dir; do
+	if ! egrep "cmd/(stringer|cloner)" $dir/*.go; then
+		set -x
+		go generate -tags=hermetic $dir
+		set +x
+	fi
+done

.github/workflows/go_generate.yml

@@ -25,14 +25,13 @@ jobs:
           fetch-depth: 0
 
       - name: check 'go generate' is clean
+        # The shell script invocation below is a temporary hack for
+        # https://github.com/tailscale/tailscale/issues/4194. When
+        # that issue is fixed, replace its invocation with:
+        # go generate --tags=hermetic ./...
         run: |
-          if [[ "${{github.ref}}" == release-branch/* ]]
-          then
-            pkgs=$(go list ./... | grep -v dnsfallback)
-          else
-            pkgs=$(go list ./... | grep -v dnsfallback)
-          fi
-          go generate $pkgs
+          set -e
+          ./.github/workflows/go-generate-without-stringer.sh
           echo
           echo
           git diff --name-only --exit-code || (echo "The files above need updating. Please run 'go generate'."; exit 1)

net/dnsfallback/dnsfallback.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:generate go run update-dns-fallbacks.go
-
 // Package dnsfallback contains a DNS fallback mechanism
 // for starting up Tailscale when the system DNS is broken or otherwise unavailable.
 package dnsfallback

net/dnsfallback/generate.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !hermetic
+// +build !hermetic
+
+package dnsfallback
+
+//go:generate go run update-dns-fallbacks.go