cmd/go: detect Go assembly before assembling with gcc

Avoids confusing errors from the GNU assembler
processing Go assembly source code.

Fixes #19448.

Change-Id: Ic2c68b2521847cca5a3d078a092e5c60ec340840
Reviewed-on: https://go-review.googlesource.com/46423
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -2310,6 +2310,19 @@ func TestCoverageWithCgo(t *testing.T) {
 	}
 }
 
+func TestCgoAsmError(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+
+	tg := testgo(t)
+	tg.parallel()
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.runFail("build", "cgoasm")
+	tg.grepBoth("package using cgo has Go assembly file", "did not detect Go assembly file")
+}
+
 func TestCgoDependsOnSyscall(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping test that removes $GOROOT/pkg/*_race in short mode")

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

@@ -1333,6 +1333,16 @@ func (b *Builder) build(a *Action) (err error) {
 			}
 			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
 		} else {
+			for _, sfile := range sfiles {
+				data, err := ioutil.ReadFile(filepath.Join(a.Package.Dir, sfile))
+				if err == nil {
+					if bytes.HasPrefix(data, []byte("TEXT")) || bytes.Contains(data, []byte("\nTEXT")) ||
+						bytes.HasPrefix(data, []byte("DATA")) || bytes.Contains(data, []byte("\nDATA")) ||
+						bytes.HasPrefix(data, []byte("GLOBL")) || bytes.Contains(data, []byte("\nGLOBL")) {
+						return fmt.Errorf("package using cgo has Go assembly file %s", sfile)
+					}
+				}
+			}
 			gccfiles = append(gccfiles, sfiles...)
 			sfiles = nil
 		}

src/cmd/go/testdata/src/cgoasm/p.go

+package p
+
+/*
+// hi
+*/
+import "C"
+
+func F() {}

src/cmd/go/testdata/src/cgoasm/p.s

+TEXT asm(SB),$0
+	RET