cmd/go: support -buildmode=plugin on linux

Change-Id: I0c8a04457db28c55c35c9a186b63c40f40730e39
Reviewed-on: https://go-review.googlesource.com/27824Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -866,6 +866,10 @@
 // 		position independent executables (PIE). Packages not named
 // 		main are ignored.
 //
+// 	-buildmode=plugin
+// 		Build the listed main packages, plus all packages that they
+// 		import, into a Go plugin. Packages not named main are ignored.
+//
 //
 // File types
 //

src/cmd/go/build.go

@@ -406,6 +406,21 @@ func buildModeInit() {
 			fatalf("-buildmode=shared and -o not supported together")
 		}
 		ldBuildmode = "shared"
+	case "plugin":
+		pkgsFilter = pkgsMain
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			switch platform {
+			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
+				"android/amd64", "android/arm", "android/arm64", "android/386":
+			default:
+				fatalf("-buildmode=plugin not supported on %s\n", platform)
+			}
+			codegenArg = "-dynlink"
+		}
+		exeSuffix = ".so"
+		ldBuildmode = "plugin"
 	default:
 		fatalf("buildmode=%s not supported", buildBuildmode)
 	}
@@ -1665,7 +1680,7 @@ func (b *builder) install(a *action) (err error) {
 	perm := os.FileMode(0666)
 	if a1.link {
 		switch buildBuildmode {
-		case "c-archive", "c-shared":
+		case "c-archive", "c-shared", "plugin":
 		default:
 			perm = 0777
 		}
@@ -2959,7 +2974,7 @@ func (tools gccgoToolchain) cc(b *builder, p *Package, objdir, ofile, cfile stri
 // maybePIC adds -fPIC to the list of arguments if needed.
 func (tools gccgoToolchain) maybePIC(args []string) []string {
 	switch buildBuildmode {
-	case "c-shared", "shared":
+	case "c-shared", "shared", "plugin":
 		args = append(args, "-fPIC")
 	}
 	return args

src/cmd/go/help.go

@@ -577,5 +577,9 @@ are:
 		Build the listed main packages and everything they import into
 		position independent executables (PIE). Packages not named
 		main are ignored.
+
+	-buildmode=plugin
+		Build the listed main packages, plus all packages that they
+		import, into a Go plugin. Packages not named main are ignored.
 `,
 }

src/cmd/go/pkg.go

@@ -775,7 +775,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	useBindir := p.Name == "main"
 	if !p.Standard {
 		switch buildBuildmode {
-		case "c-archive", "c-shared":
+		case "c-archive", "c-shared", "plugin":
 			useBindir = false
 		}
 	}
@@ -846,11 +846,11 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		importPaths = append(importPaths, "syscall")
 	}
 
-	// Currently build modes c-shared, pie, and -linkshared force
+	// Currently build modes c-shared, pie, plugin, and -linkshared force
 	// external linking mode, and external linking mode forces an
 	// import of runtime/cgo.
 	pieCgo := buildBuildmode == "pie" && (buildContext.GOOS != "linux" || buildContext.GOARCH != "amd64")
-	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || pieCgo || buildLinkshared) {
+	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildBuildmode == "plugin" || pieCgo || buildLinkshared) {
 		importPaths = append(importPaths, "runtime/cgo")
 	}