cmd/go: start support for -linkshared

This will fruitlessly rebuild stale packages that are in a shared
library.

Change-Id: I66a6e1adf7818558e7d1351ab215a5021b4a8a6b
Reviewed-on: https://go-review.googlesource.com/8333
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -77,6 +77,9 @@ and test commands:
 
 	-buildmode mode
 		build mode to use. See 'go help buildmodes' for more.
+	-linkshared
+		link against shared libraries previously created with
+		-buildmode=shared
 	-compiler name
 		name of compiler to use, as in runtime.Compiler (gccgo or gc).
 	-gccgoflags 'arg list'
@@ -149,6 +152,7 @@ var buildGccgoflags []string // -gccgoflags flag
 var buildRace bool           // -race flag
 var buildToolExec []string   // -toolexec flag
 var buildBuildmode string    // -buildmode flag
+var buildLinkshared bool     // -linkshared flag
 
 var buildContext = build.Default
 var buildToolchain toolchain = noToolchain{}
@@ -204,6 +208,7 @@ func addBuildFlags(cmd *Command) {
 	cmd.Flag.BoolVar(&buildRace, "race", false, "")
 	cmd.Flag.Var((*stringsFlag)(&buildToolExec), "toolexec", "")
 	cmd.Flag.StringVar(&buildBuildmode, "buildmode", "default", "")
+	cmd.Flag.BoolVar(&buildLinkshared, "linkshared", false, "")
 }
 
 func addBuildFlagsNX(cmd *Command) {
@@ -307,6 +312,15 @@ func buildModeInit() {
 	default:
 		fatalf("buildmode=%s not supported", buildBuildmode)
 	}
+	if buildLinkshared {
+		if goarch != "amd64" || goos != "linux" {
+			fmt.Fprintf(os.Stderr, "go %s: -linkshared is only supported on linux/amd64\n", flag.Args()[0])
+			os.Exit(2)
+		}
+		codegenArg = "-dynlink"
+		// TODO(mwhudson): remove -w when that gets fixed in linker.
+		buildLdflags = append(buildLdflags, "-linkshared", "-w")
+	}
 	if ldBuildmode != "" {
 		buildLdflags = append(buildLdflags, "-buildmode="+ldBuildmode)
 	}

src/cmd/go/doc.go

@@ -97,6 +97,9 @@ and test commands:
 
 	-buildmode mode
 		build mode to use. See 'go help buildmodes' for more.
+	-linkshared
+		link against shared libraries previously created with
+		-buildmode=shared
 	-compiler name
 		name of compiler to use, as in runtime.Compiler (gccgo or gc).
 	-gccgoflags 'arg list'

src/cmd/go/test.go

@@ -314,6 +314,7 @@ func runTest(cmd *Command, args []string) {
 	findExecCmd() // initialize cached result
 
 	raceInit()
+	buildModeInit()
 	pkgs := packagesForBuild(pkgArgs)
 	if len(pkgs) == 0 {
 		fatalf("no packages to test")

src/cmd/go/testflag.go

@@ -48,6 +48,7 @@ var testFlagDefn = []*testFlagSpec{
 	{name: "tags"},
 	{name: "compiler"},
 	{name: "race", boolVar: &buildRace},
+	{name: "linkshared", boolVar: &buildLinkshared},
 	{name: "installsuffix"},
 
 	// passed to 6.out, adding a "test." prefix to the name if necessary: -v becomes -test.v.
@@ -115,7 +116,7 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 		var err error
 		switch f.name {
 		// bool flags.
-		case "a", "c", "i", "n", "x", "v", "race", "cover", "work":
+		case "a", "c", "i", "n", "x", "v", "race", "cover", "work", "linkshared":
 			setBoolFlag(f.boolVar, value)
 		case "o":
 			testO = value