cmd/link: add -extar option to set ar program for c-archive

People who want to use -buildmode=c-archive in unusual cross-compilation
setups will need something like this.  It could also be done via (yet
another) environment variable but I use -extar by analogy with the
existing -extld.

Change-Id: I354cfabc4c470603affd13cd946997b3a24c0e6c
Reviewed-on: https://go-review.googlesource.com/18913
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testcarchive/test.bash

@@ -85,4 +85,22 @@ if ! $bin; then
 fi
 rm -rf libgo4.a libgo4.h testp pkg
 
+rm -f testar
+cat >testar <<EOF
+#!/usr/bin/env bash
+while expr \$1 : '[-]' >/dev/null; do
+  shift
+done
+echo "testar" > \$1
+echo "testar" > $(pwd)/testar.ran
+EOF
+chmod +x testar
+rm -f testar.ran
+GOPATH=$(pwd) go build -buildmode=c-archive -ldflags=-extar=$(pwd)/testar -o libgo4.a libgo4
+if ! test -f testar.ran; then
+    echo "FAIL test5"
+    status=1
+fi
+rm -rf libgo4.a libgo4.h testar testar.ran pkg
+
 exit $status

src/cmd/link/doc.go

@@ -52,6 +52,9 @@ Flags:
 		The dynamic header is on by default, even without any
 		references to dynamic libraries, because many common
 		system tools now assume the presence of the header.
+	-extar ar
+		Set the external archive program (default "ar").
+		Used only for -buildmode=c-archive.
 	-extld linker
 		Set the external linker (default "clang" or "gcc").
 	-extldflags flags

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

@@ -207,6 +207,7 @@ var (
 	tmpdir             string
 	extld              string
 	extldflags         string
+	extar              string
 	libgccfile         string
 	debug_s            int // backup old value of debug['s']
 	Ctxt               *Link
@@ -1015,8 +1016,12 @@ func archive() {
 		return
 	}
 
+	if extar == "" {
+		extar = "ar"
+	}
+
 	mayberemoveoutfile()
-	argv := []string{"ar", "-q", "-c", "-s", outfile}
+	argv := []string{extar, "-q", "-c", "-s", outfile}
 	argv = append(argv, fmt.Sprintf("%s/go.o", tmpdir))
 	argv = append(argv, hostobjCopy()...)
 

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

@@ -89,6 +89,7 @@ func Ldmain() {
 	flag.Var(&Buildmode, "buildmode", "set build `mode`")
 	obj.Flagcount("c", "dump call graph", &Debug['c'])
 	obj.Flagcount("d", "disable dynamic executable", &Debug['d'])
+	obj.Flagstr("extar", "archive program for buildmode=c-archive", &extar)
 	obj.Flagstr("extld", "use `linker` when linking in external mode", &extld)
 	obj.Flagstr("extldflags", "pass `flags` to external linker", &extldflags)
 	obj.Flagcount("f", "ignore version mismatch", &Debug['f'])