Revert "Fail when expected files are not produced by protoc (#4287)" (#4324)

**What type of PR is this?**
Bug fix


**What does this PR do? Why is it needed?**
It's common for a `go_proto_library` to include proto files with and
without service definitions at the same time. In this case, the gRPC
plugins are needed, but some gRPC plugins don't generate the grpc.pb.go
files if there is no service definition.

Partially reverting #4287 to restore the previous behavior of creating
an empty file

**Which issues(s) does this PR fix?**

Fixes #4317

**Other notes for review**

---------

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>

Author: linzhp

go/tools/builders/protoc.go

@@ -20,8 +20,6 @@ import (
 	"errors"
 	"flag"
 	"fmt"
-	"go/parser"
-	"go/token"
 	"io/ioutil"
 	"log"
 	"os"
@@ -185,9 +183,15 @@ func run(args []string) error {
 	for _, f := range files {
 		switch {
 		case f.expected && !f.created:
-			return fmt.Errorf("file %q expected from plugin %q but not created", f.path, *plugin)
+			// Some plugins only create output files if the proto source files
+			// have relevant definitions (e.g., services for grpc_gateway). Create
+			// trivial files that the compiler will ignore for missing outputs.
+			data := []byte("// +build ignore\n\npackage ignore")
+			if err := ioutil.WriteFile(abs(f.path), data, 0644); err != nil {
+				return err
+			}
 		case f.expected && f.ambiguous:
-			fmt.Fprintf(buf, "Ambiguous output %v.\n", f.path)
+			fmt.Fprintf(buf, "ambiguous output %v.\n", f.path)
 		case f.from != nil:
 			data, err := ioutil.ReadFile(f.from.path)
 			if err != nil {
@@ -203,15 +207,6 @@ func run(args []string) error {
 			fmt.Fprintf(buf, "Check that the go_package option is %q.", *importpath)
 			return errors.New(buf.String())
 		}
-
-		if filepath.Ext(f.path) != ".go" {
-			continue
-		}
-		// Additionally check that created files are valid, because invalid Go file causes cache poisoning.
-		_, err := parser.ParseFile(token.NewFileSet(), f.path, nil, parser.PackageClauseOnly)
-		if err != nil {
-			return fmt.Errorf("plugin %q created an invalid Go file %q: %v", *plugin, f.path, err)
-		}
 	}
 
 	return nil