cmd/go: fix LDFLAGS handling, enable misc/cgo/testso on Darwin

Fixes #5479.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/9416047

misc/cgo/testso/cgoso.go

@@ -5,7 +5,15 @@
 package cgosotest
 
 /*
-#cgo LDFLAGS: -L. -lcgosotest
+// intentionally write the same LDFLAGS differently
+// to test correct handling of LDFLAGS.
+#cgo linux LDFLAGS: -L. -lcgosotest
+#cgo freebsd LDFLAGS: -L. -l cgosotest
+#cgo openbsd LDFLAGS: -L. -l cgosotest
+#cgo netbsd LDFLAGS: -L. libcgosotest.so
+#cgo darwin LDFLAGS: -L. libcgosotest.dylib
+#cgo windows LDFLAGS: -L. libcgosotest.dll
+
 void init(void);
 void sofunc(void);
 */

misc/cgo/testso/test.bash

@@ -4,7 +4,19 @@
 # license that can be found in the LICENSE file.
 
 set -e
-$(go env CC) $(go env GOGCCFLAGS) -shared -o libcgosotest.so cgoso_c.c
+
+args=
+dyld_envvar=LD_LIBRARY_PATH
+ext=so
+if [ "$(uname)" == "Darwin" ]; then
+	args="-undefined suppress -flat_namespace"
+	dyld_envvar=DYLD_LIBRARY_PATH
+	ext=dylib
+fi
+
+dylib=libcgosotest.$ext
+$(go env CC) $(go env GOGCCFLAGS) -shared $args -o $dylib cgoso_c.c
 go build main.go
-LD_LIBRARY_PATH=. ./main
-rm -f libcgosotest.so main
+
+eval "$dyld_envvar"=. ./main
+rm -rf $dylib main *.dSYM

src/cmd/go/build.go

@@ -1855,14 +1855,24 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 	var linkobj []string
 
 	var bareLDFLAGS []string
-	// filter out -lsomelib, and -framework X if on Darwin
+	// filter out -lsomelib, -l somelib, *.{so,dll,dylib}, and (on Darwin) -framework X
 	for i := 0; i < len(cgoLDFLAGS); i++ {
 		f := cgoLDFLAGS[i]
-		if !strings.HasPrefix(f, "-l") {
-			if goos == "darwin" && f == "-framework" { // skip the -framework X
-				i += 1
-				continue
+		switch {
+		// skip "-lc" or "-l somelib"
+		case strings.HasPrefix(f, "-l"):
+			if f == "-l" {
+				i++
 			}
+		// skip "-framework X" on Darwin
+		case goos == "darwin" && f == "-framework":
+			i++
+		// skip "*.{dylib,so,dll}"
+		case strings.HasSuffix(f, ".dylib"),
+			strings.HasSuffix(f, ".so"),
+			strings.HasSuffix(f, ".dll"):
+			continue
+		default:
 			bareLDFLAGS = append(bareLDFLAGS, f)
 		}
 	}

src/run.bash

@@ -104,7 +104,6 @@ esac
 
 [ "$CGO_ENABLED" != 1 ] ||
 [ "$GOHOSTOS" == windows ] ||
-[ "$GOHOSTOS" == darwin ] ||
 (xcd ../misc/cgo/testso
 ./test.bash
 ) || exit $?