cmd/go, cmd/link: silence bogus Apple Xcode warning

Certain installations of Xcode are affected by a bug that causes
them to print an inconsequential link-time warning that looks like:

	ld: warning: text-based stub file /System/Library/Frameworks//Security.framework/Security.tbd and library file /System/Library/Frameworks//Security.framework/Security are out of sync. Falling back to library file for linking.

This has nothing to do with Go, and we've sent this repro case
to Apple:

	$ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables | grep version
	version: 10.0.0.0.1.1535735448
	$ clang --version
	Apple LLVM version 10.0.0 (clang-1000.10.44.2)
	Target: x86_64-apple-darwin17.7.0
	Thread model: posix
	InstalledDir: /Library/Developer/CommandLineTools/usr/bin
	$ cat > issue.c
	int main() { return 0; }
	^D
	$ clang issue.c -framework CoreFoundation
	ld: warning: text-based stub file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation.tbd and library file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation are out of sync. Falling back to library file for linking.
	$

Even if Apple does release a fixed Xcode, many people are seeing
this useless warning, and we might as well make it go away.

Fixes #26073.

Change-Id: Ifc17ba7da1f6b59e233c11ebdab7241cb6656324
Reviewed-on: https://go-review.googlesource.com/c/144112
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/cmd/go/internal/work/exec.go

@@ -2080,14 +2080,37 @@ func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []s
 }
 
 // gccld runs the gcc linker to create an executable from a set of object files.
-func (b *Builder) gccld(p *load.Package, objdir, out string, flags []string, objs []string) error {
+func (b *Builder) gccld(p *load.Package, objdir, outfile string, flags []string, objs []string) error {
 	var cmd []string
 	if len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0 {
 		cmd = b.GxxCmd(p.Dir, objdir)
 	} else {
 		cmd = b.GccCmd(p.Dir, objdir)
 	}
-	return b.run(nil, p.Dir, p.ImportPath, b.cCompilerEnv(), cmd, "-o", out, objs, flags)
+
+	cmdargs := []interface{}{cmd, "-o", outfile, objs, flags}
+	dir := p.Dir
+	out, err := b.runOut(dir, b.cCompilerEnv(), cmdargs...)
+	if len(out) > 0 {
+		// Filter out useless linker warnings caused by bugs outside Go.
+		// See also cmd/link/internal/ld's hostlink method.
+		var save [][]byte
+		for _, line := range bytes.SplitAfter(out, []byte("\n")) {
+			// golang.org/issue/26073 - Apple Xcode bug
+			if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
+				continue
+			}
+			save = append(save, line)
+		}
+		out = bytes.Join(save, nil)
+		if len(out) > 0 {
+			b.showOutput(nil, dir, p.ImportPath, b.processOutput(out))
+			if err != nil {
+				err = errPrintedOutput
+			}
+		}
+	}
+	return err
 }
 
 // Grab these before main helpfully overwrites them.

src/cmd/link/internal/ld/lib.go

@@ -1338,9 +1338,24 @@ func (ctxt *Link) hostlink() {
 		ctxt.Logf("\n")
 	}
 
-	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
+	out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput()
+	if err != nil {
 		Exitf("running %s failed: %v\n%s", argv[0], err, out)
-	} else if len(out) > 0 {
+	}
+
+	// Filter out useless linker warnings caused by bugs outside Go.
+	// See also cmd/go/internal/work/exec.go's gccld method.
+	var save [][]byte
+	for _, line := range bytes.SplitAfter(out, []byte("\n")) {
+		// golang.org/issue/26073 - Apple Xcode bug
+		if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
+			continue
+		}
+		save = append(save, line)
+	}
+	out = bytes.Join(save, nil)
+
+	if len(out) > 0 {
 		// always print external output even if the command is successful, so that we don't
 		// swallow linker warnings (see https://golang.org/issue/17935).
 		ctxt.Logf("%s", out)