cmd/go: allow the user to specify ar via an environment variable

This allows one to customize which ar to use by fetching its path
from the environment. This way one can swap it out for a
different implementation.

Change-Id: I40d8cbd8a69e97b5254e66081d9bf0b726c10366
GitHub-Last-Rev: 4aa1d631eaca58be6c3a4be40f7404fa75a0db25
GitHub-Pull-Request: golang/go#28746
Reviewed-on: https://go-review.googlesource.com/c/149117
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -1500,6 +1500,10 @@
 // 		The command to use to compile C++ code.
 // 	PKG_CONFIG
 // 		Path to pkg-config tool.
+// 	AR
+// 		The command to use to manipulate library archives when
+// 		building with the gccgo compiler.
+// 		The default is 'ar'.
 //
 // Architecture-specific environment variables:
 //

src/cmd/go/internal/help/helpdoc.go

@@ -546,6 +546,10 @@ Environment variables for use with cgo:
 		The command to use to compile C++ code.
 	PKG_CONFIG
 		Path to pkg-config tool.
+	AR
+		The command to use to manipulate library archives when
+		building with the gccgo compiler.
+		The default is 'ar'.
 
 Architecture-specific environment variables:
 

src/cmd/go/internal/work/gccgo.go

@@ -43,6 +43,14 @@ func (gccgoToolchain) linker() string {
 	return GccgoBin
 }
 
+func (gccgoToolchain) ar() string {
+	ar := os.Getenv("AR")
+	if ar == "" {
+		ar = "ar"
+	}
+	return ar
+}
+
 func checkGccgoBin() {
 	if gccgoErr == nil {
 		return
@@ -183,7 +191,7 @@ func gccgoArchive(basedir, imp string) string {
 	return filepath.Join(filepath.Dir(afile), "lib"+filepath.Base(afile))
 }
 
-func (gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+func (tools gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
 	p := a.Package
 	objdir := a.Objdir
 	var absOfiles []string
@@ -198,7 +206,7 @@ func (gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []string)
 		arArgs = []string{"-X64"}
 	}
 
-	return b.run(a, p.Dir, p.ImportPath, nil, "ar", arArgs, "rc", mkAbs(objdir, afile), absOfiles)
+	return b.run(a, p.Dir, p.ImportPath, nil, tools.ar(), arArgs, "rc", mkAbs(objdir, afile), absOfiles)
 }
 
 func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string, allactions []*Action, buildmode, desc string) error {
@@ -257,11 +265,11 @@ func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string
 				return "", nil
 			}
 		}
-		err := b.run(root, root.Objdir, desc, nil, "ar", "x", newArchive, "_cgo_flags")
+		err := b.run(root, root.Objdir, desc, nil, tools.ar(), "x", newArchive, "_cgo_flags")
 		if err != nil {
 			return "", err
 		}
-		err = b.run(root, ".", desc, nil, "ar", "d", newArchive, "_cgo_flags")
+		err = b.run(root, ".", desc, nil, tools.ar(), "d", newArchive, "_cgo_flags")
 		if err != nil {
 			return "", err
 		}
@@ -473,7 +481,7 @@ func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string
 
 	switch buildmode {
 	case "c-archive":
-		if err := b.run(root, ".", desc, nil, "ar", "rc", realOut, out); err != nil {
+		if err := b.run(root, ".", desc, nil, tools.ar(), "rc", realOut, out); err != nil {
 			return err
 		}
 	}