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

cmd/weaver/main.go

@@ -74,11 +74,25 @@ func main() {
 	switch flag.Arg(0) {
 	case "generate":
 		generateFlags := flag.NewFlagSet("generate", flag.ExitOnError)
+		tags := generateFlags.String("tags", "not specified", "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 := "--tags=ignoreWeaverGen"
+		if *tags != "not specified" { // tags flag was specified
+			if *tags == "" || *tags == "." {
+				// User specified the tags flag but didn't provide any tags.
+				fmt.Fprintln(os.Stderr, "No tags provided.")
+				os.Exit(1)
+			}
+			// 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
+		}
+		idx := len(flag.Args()) - len(generateFlags.Args())
+		if err := generate.Generate(".", flag.Args()[idx:], 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] [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 +89,22 @@ 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 "// +build good" line at the top of
+  the file.
+  weaver generate -tags good
+
+  # Generate code for all files that have a "// +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)
+	BuildTags string
 }
 
 // Generate generates Service Weaver code for the specified packages.
@@ -104,11 +116,13 @@ 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{opt.BuildTags}
 	}
 	pkgList, err := packages.Load(cfg, pkgs...)
 	if err != nil {

internal/tool/generate/generator_test.go

@@ -102,7 +102,8 @@ 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: "--tags=ignoreWeaverGen",
 	}
 	if err := Generate(tmp, []string{tmp}, opt); err != nil {
 		return "", err
@@ -237,6 +238,80 @@ func TestGenerator(t *testing.T) {
 	}
 }
 
+// TestGeneratorBuildTags runs "weaver generate" on all the files in
+// testdata/tags and checks if the command succeeds. Each file should have some build tags.
+func TestGeneratorBuildTags(t *testing.T) {
+	const dir = "testdata/tags"
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("cannot list files in %q", dir)
+	}
+
+	tmp := t.TempDir()
+	save := func(f, data string) {
+		if err := os.WriteFile(filepath.Join(tmp, f), []byte(data), 0644); err != nil {
+			t.Fatalf("error writing %s: %v", f, err)
+		}
+	}
+
+	// Copy the files from dir to the temp directory.
+	for _, file := range files {
+		filename := file.Name()
+		if !strings.HasSuffix(filename, ".go") || strings.HasSuffix(filename, generatedCodeFile) {
+			continue
+		}
+
+		// 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)
+		save(filename, contents)
+	}
+	save("go.mod", goModFile)
+
+	// Run "go mod tidy"
+	tidy := exec.Command("go", "mod", "tidy")
+	tidy.Dir = tmp
+	tidy.Stdout = os.Stdout
+	tidy.Stderr = os.Stderr
+	if err := tidy.Run(); err != nil {
+		t.Fatalf("go mod tidy: %v", err)
+	}
+
+	// Run the "weaver generate" command with no build tags. Verify that the command
+	// doesn't succeed because bad.go does not compile.
+	err = Generate(tmp, []string{tmp}, Options{Warn: func(err error) { t.Log(err) }})
+	if err == nil {
+		t.Fatal("expected generator to return an error; got nil error")
+	}
+	// Verify that no weaver_gen.go file was generated.
+	_, err = os.ReadFile(filepath.Join(tmp, generatedCodeFile))
+	if err == nil {
+		t.Fatal("expected no generated file")
+	}
+
+	// Run the "weaver generate" command with the build tag "good". Verify that the
+	// command succeeds because the bad.go file is ignored, and the weaver_gen.go
+	// contains only generated code for the good.go.
+	err = Generate(tmp, []string{tmp}, Options{Warn: func(err error) { t.Log(err) }, BuildTags: "--tags=good"})
+	if err != nil {
+		t.Fatalf("unexpected generator error: %v", err)
+	}
+	// Verify that the weaver_gen.go file doesn't contain generated code for the bad service.
+	output, err := os.ReadFile(filepath.Join(tmp, generatedCodeFile))
+	if err != nil {
+		t.Fatalf("unable to read the ")
+	}
+	if strings.Contains(string(output), "bad") {
+		t.Fatalf("unexpected generated code for the bad service")
+	}
+	if !strings.Contains(string(output), "good") {
+		t.Fatalf("expected 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

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

@@ -0,0 +1,22 @@
+//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 +1,23 @@
+//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
+}