cmd/go: ensure pkgsFilter is run before build

Return an error when a user passes -o and -buildmode=exe to build a package
without a main.

Fixes #20017.

Change-Id: I07d49c75e7088a96f00afe18c9faa842c5d71afb
Reviewed-on: https://go-review.googlesource.com/49371
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -2045,6 +2045,16 @@ func TestGoTestMutexprofileDashOControlsBinaryLocation(t *testing.T) {
 	tg.wantExecutable("myerrors.test"+exeSuffix, "go test -mutexprofile -o myerrors.test did not create myerrors.test")
 }
 
+func TestGoBuildNonMain(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	// TODO: tg.parallel()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.runFail("build", "-buildmode=exe", "-o", "not_main"+exeSuffix, "not_main")
+	tg.grepStderr("-buildmode=exe requires exactly one main package", "go build with -o and -buildmode=exe should on a non-main package should throw an error")
+	tg.mustNotExist("not_main" + exeSuffix)
+}
+
 func TestGoTestDashCDashOControlsBinaryLocation(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()

src/cmd/go/internal/work/build.go

@@ -310,6 +310,8 @@ func runBuild(cmd *base.Command, args []string) {
 		depMode = ModeInstall
 	}
 
+	pkgs = pkgsFilter(load.Packages(args))
+
 	if cfg.BuildO != "" {
 		if len(pkgs) > 1 {
 			base.Fatalf("go build: cannot use -o with multiple packages")
@@ -325,8 +327,6 @@ func runBuild(cmd *base.Command, args []string) {
 		return
 	}
 
-	pkgs = pkgsFilter(load.Packages(args))
-
 	a := &Action{Mode: "go build"}
 	for _, p := range pkgs {
 		a.Deps = append(a.Deps, b.AutoAction(ModeBuild, depMode, p))

src/cmd/go/internal/work/init.go

@@ -123,6 +123,12 @@ func buildModeInit() {
 	case "exe":
 		pkgsFilter = pkgsMain
 		ldBuildmode = "exe"
+		// Set the pkgsFilter to oneMainPkg if the user passed a specific binary output
+		// and is using buildmode=exe for a better error message.
+		// See issue #20017.
+		if cfg.BuildO != "" {
+			pkgsFilter = oneMainPkg
+		}
 	case "pie":
 		if cfg.BuildRace {
 			base.Fatalf("-buildmode=pie not supported when -race is enabled")

src/cmd/go/testdata/src/not_main/not_main.go

@@ -0,0 +1,3 @@
+package not_main
+
+func F() {}