Add build tags support (ServiceWeaver#796)

* Add build tags support

`go build` allows the user to specify build tags. These tags enable the
user to discard files based on various tags when they build the
application binary.

`weaver generate` which is a wrapper around `go build` doesn't allow the
user to specify build tags.

This PR adds built tags support for the `weaver generate` command.

The syntax of passing build tags to the `weaver generate` command is similar to how build tags are specified when using `go build`.

E.g.,

weaver generate -tags good,prod
weaver generate --tags=good

* Addressed sanjay's comments.

* Rewrote TestGeneratorWithBuildTags to use runGenerator

* Fixed comments in generator_test

cmd/weaver/main.go

@@ -74,11 +74,19 @@ func main() {
 	switch flag.Arg(0) {
 	case "generate":
 		generateFlags := flag.NewFlagSet("generate", flag.ExitOnError)
+		tags := generateFlags.String("tags", "", "Optional tags for the generate command")
 		generateFlags.Usage = func() {
 			fmt.Fprintln(os.Stderr, generate.Usage)
 		}
 		generateFlags.Parse(flag.Args()[1:])
-		if err := generate.Generate(".", flag.Args()[1:], generate.Options{}); err != nil {
+		buildTags := "ignoreWeaverGen"
+		if *tags != "" { // tags flag was specified
+			// TODO(rgrandl): we assume that the user specify the tags properly. I.e.,
+			// a single tag, or a list of tags separated by comma. We may want to do
+			// extra validation at some point.
+			buildTags = buildTags + "," + *tags
+		}
+		if err := generate.Generate(".", generateFlags.Args(), generate.Options{BuildTags: buildTags}); err != nil {
 			fmt.Fprintln(os.Stderr, err)
 			os.Exit(1)
 		}

internal/tool/generate/generator.go

@@ -54,7 +54,7 @@ const (
 	Usage = `Generate code for a Service Weaver application.
 
 Usage:
-  weaver generate [packages]
+  weaver generate [-tags taglist] [packages]
 
 Description:
   "weaver generate" generates code for the Service Weaver applications in the
@@ -64,6 +64,9 @@ Description:
   file in the package's directory. For example, "weaver generate . ./foo" will
   create ./weaver_gen.go and ./foo/weaver_gen.go.
 
+  You specify build tags for "weaver generate" in the same way you specify build
+  tags for go build. See "go help build" for more information.
+
   You specify packages for "weaver generate" in the same way you specify
   packages for go build, go test, go vet, etc. See "go help packages" for more
   information.
@@ -86,13 +89,21 @@ Examples:
   weaver generate ./foo
 
   # Generate code for all packages in all subdirectories of current directory.
-  weaver generate ./...`
+  weaver generate ./...
+
+  # Generate code for all files that have a "//go:build good" line at the top of
+  the file.
+  weaver generate -tags good
+
+  # Generate code for all files that have a "//go:build good,prod" line at the
+  top of the file.
+  weaver generate -tags good,prod`
 )
 
 // Options controls the operation of Generate.
 type Options struct {
-	// If non-nil, use the specified function to report warnings.
-	Warn func(error)
+	Warn      func(error) // If non-nil, use the specified function to report warnings
+	BuildTags string
 }
 
 // Generate generates Service Weaver code for the specified packages.
@@ -104,11 +115,13 @@ func Generate(dir string, pkgs []string, opt Options) error {
 	}
 	fset := token.NewFileSet()
 	cfg := &packages.Config{
-		Mode:       packages.NeedName | packages.NeedSyntax | packages.NeedImports | packages.NeedTypes | packages.NeedTypesInfo,
-		Dir:        dir,
-		Fset:       fset,
-		ParseFile:  parseNonWeaverGenFile,
-		BuildFlags: []string{"--tags=ignoreWeaverGen"},
+		Mode:      packages.NeedName | packages.NeedSyntax | packages.NeedImports | packages.NeedTypes | packages.NeedTypesInfo,
+		Dir:       dir,
+		Fset:      fset,
+		ParseFile: parseNonWeaverGenFile,
+	}
+	if len(opt.BuildTags) > 0 {
+		cfg.BuildFlags = []string{"-tags", opt.BuildTags}
 	}
 	pkgList, err := packages.Load(cfg, pkgs...)
 	if err != nil {

internal/tool/generate/generator_test.go

@@ -62,7 +62,8 @@ replace github.com/ServiceWeaver/weaver => %s
 //
 // If "weaver generate" succeeds, the produced weaver_gen.go file is written in
 // the provided directory with name ${filename}_weaver_gen.go.
-func runGenerator(t *testing.T, directory, filename, contents string, subdirs []string) (string, error) {
+func runGenerator(t *testing.T, directory, filename, contents string, subdirs []string,
+	buildTags []string) (string, error) {
 	// runGenerator creates a temporary directory, copies the file and all
 	// subdirs into it, writes a go.mod file, runs "go mod tidy", and finally
 	// runs "weaver generate".
@@ -102,7 +103,8 @@ func runGenerator(t *testing.T, directory, filename, contents string, subdirs []
 
 	// Run "weaver generate".
 	opt := Options{
-		Warn: func(err error) { t.Log(err) },
+		Warn:      func(err error) { t.Log(err) },
+		BuildTags: "ignoreWeaverGen" + "," + strings.Join(buildTags, ","),
 	}
 	if err := Generate(tmp, []string{tmp}, opt); err != nil {
 		return "", err
@@ -134,7 +136,7 @@ func runGenerator(t *testing.T, directory, filename, contents string, subdirs []
 	if err := tidy.Run(); err != nil {
 		t.Fatalf("go mod tidy: %v", err)
 	}
-	gobuild := exec.Command("go", "build")
+	gobuild := exec.Command("go", "build", "-tags="+opt.BuildTags)
 	gobuild.Dir = tmp
 	gobuild.Stdout = os.Stdout
 	gobuild.Stderr = os.Stderr
@@ -218,7 +220,7 @@ func TestGenerator(t *testing.T) {
 			}
 
 			// Run "weaver generate".
-			output, err := runGenerator(t, dir, filename, contents, []string{"sub1", "sub2"})
+			output, err := runGenerator(t, dir, filename, contents, []string{"sub1", "sub2"}, nil)
 			if err != nil {
 				t.Fatalf("error running generator: %v", err)
 			}
@@ -237,6 +239,47 @@ func TestGenerator(t *testing.T) {
 	}
 }
 
+// TestGeneratorBuildWithTags runs "weaver generate" on all the files in
+// testdata/tags and checks if the command succeeds. Each file should have some build tags.
+func TestGeneratorBuildWithTags(t *testing.T) {
+	const dir = "testdata/tags"
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("cannot list files in %q", dir)
+	}
+
+	for _, file := range files {
+		filename := file.Name()
+		if !strings.HasSuffix(filename, ".go") || strings.HasSuffix(filename, generatedCodeFile) {
+			continue
+		}
+		t.Run(filename, func(t *testing.T) {
+			t.Parallel()
+
+			// Read the test file.
+			bits, err := os.ReadFile(filepath.Join(dir, filename))
+			if err != nil {
+				t.Fatalf("cannot read %q: %v", filename, err)
+			}
+			contents := string(bits)
+			// Run "weaver generate".
+			output, err := runGenerator(t, dir, filename, contents, nil, []string{"good"})
+
+			if filename == "good.go" {
+				// Verify that the error is nil and the weaver_gen.go contains generated code for the good service.
+				if err != nil || !strings.Contains(output, "GoodService") {
+					t.Fatalf("expected generated code for the good service")
+				}
+				return
+			}
+			// For the bad.go verify that the error is not nil and there is no output.
+			if err == nil || len(output) > 0 {
+				t.Fatalf("expected no generated code for the good service")
+			}
+		})
+	}
+}
+
 // TestGeneratorErrors runs "weaver generate" on all of the files in
 // testdata/errors.
 // Every file in testdata/errors must begin with a single line header that looks
@@ -286,7 +329,7 @@ func TestGeneratorErrors(t *testing.T) {
 			}
 
 			// Run "weaver generate".
-			output, err := runGenerator(t, dir, filename, contents, []string{})
+			output, err := runGenerator(t, dir, filename, contents, nil, nil)
 			errfile := strings.TrimSuffix(filename, ".go") + "_error.txt"
 			if err == nil {
 				os.Remove(filepath.Join(dir, errfile))

internal/tool/generate/testdata/tags/bad.go

@@ -0,0 +1,36 @@
+// Copyright 2024 Google LLC
+//
+// 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.
+
+//go:build !good
+
+package tags
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/ServiceWeaver/weaver"
+)
+
+type BadService interface {
+	DoSomething(context.Context) error
+}
+
+type badServiceImpl struct {
+	weaver.Implements[BadService]
+}
+
+func (b *badServiceImpl) DoSomething(context.Context) error {
+	Some code that does not compile
+}

internal/tool/generate/testdata/tags/good.go

@@ -0,0 +1,37 @@
+// Copyright 2024 Google LLC
+//
+// 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.
+
+//go:build good
+
+package tags
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/ServiceWeaver/weaver"
+)
+
+type GoodService interface {
+	DoSomething(context.Context) error
+}
+
+type goodServiceImpl struct {
+	weaver.Implements[GoodService]
+}
+
+func (g *goodServiceImpl) DoSomething(context.Context) error {
+	fmt.Println("Hello world!")
+	return nil
+}