cmd/link: use path from "cc --print-prog-name ar" for c-archive buildmode

When external linking with -buildmode=c-archive, the Go linker
eventually invokes the "ar" tool to create the final archive library.
Prior to this patch, if the '-extar' flag was not in use, we would
just run "ar". This works well in most cases but breaks down if we're
doing cross-compilation targeting Windows (macos system "ar"
apparently doesn't create the windows symdef section correctly). To
fix the problem, capture the output of "cc --print-prog-name ar" and
invoke "ar" using the path returned by that command.

Fixes #59221.

Change-Id: I9de66e98947c42633b16fde7208c2958d62fe7cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/479775
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: thanm

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

@@ -1316,7 +1316,12 @@ func (ctxt *Link) archive() {
 	exitIfErrors()
 
 	if *flagExtar == "" {
+		const printProgName = "--print-prog-name=ar"
+		cc := ctxt.extld()
 		*flagExtar = "ar"
+		if linkerFlagSupported(ctxt.Arch, cc[0], "", printProgName) {
+			*flagExtar = ctxt.findExtLinkTool("ar")
+		}
 	}
 
 	mayberemoveoutfile()
@@ -1875,22 +1880,8 @@ func (ctxt *Link) hostlink() {
 
 	if combineDwarf {
 		// Find "dsymutils" and "strip" tools using CC --print-prog-name.
-		var cc []string
-		cc = append(cc, ctxt.extld()...)
-		cc = append(cc, hostlinkArchArgs(ctxt.Arch)...)
-		cc = append(cc, "--print-prog-name", "dsymutil")
-		out, err := exec.Command(cc[0], cc[1:]...).CombinedOutput()
-		if err != nil {
-			Exitf("%s: finding dsymutil failed: %v\n%s", os.Args[0], err, out)
-		}
-		dsymutilCmd := strings.TrimSuffix(string(out), "\n")
-
-		cc[len(cc)-1] = "strip"
-		out, err = exec.Command(cc[0], cc[1:]...).CombinedOutput()
-		if err != nil {
-			Exitf("%s: finding strip failed: %v\n%s", os.Args[0], err, out)
-		}
-		stripCmd := strings.TrimSuffix(string(out), "\n")
+		dsymutilCmd := ctxt.findExtLinkTool("dsymutil")
+		stripCmd := ctxt.findExtLinkTool("dsymutil")
 
 		dsym := filepath.Join(*flagTmpdir, "go.dwarf")
 		if out, err := exec.Command(dsymutilCmd, "-f", *flagOutfile, "-o", dsym).CombinedOutput(); err != nil {
@@ -2763,3 +2754,19 @@ func captureHostObj(h *Hostobj) {
 	fmt.Fprintf(os.Stderr, "link: info: captured host object %s to %s\n",
 		h.file, opath)
 }
+
+// findExtLinkTool invokes the external linker CC with --print-prog-name
+// passing the name of the tool we're interested in, such as "strip",
+// "ar", or "dsymutil", and returns the path passed back from the command.
+func (ctxt *Link) findExtLinkTool(toolname string) string {
+	var cc []string
+	cc = append(cc, ctxt.extld()...)
+	cc = append(cc, hostlinkArchArgs(ctxt.Arch)...)
+	cc = append(cc, "--print-prog-name", toolname)
+	out, err := exec.Command(cc[0], cc[1:]...).CombinedOutput()
+	if err != nil {
+		Exitf("%s: finding %s failed: %v\n%s", os.Args[0], toolname, err, out)
+	}
+	cmdpath := strings.TrimSuffix(string(out), "\n")
+	return cmdpath
+}