cmd/go/internal/work: make toolchain builds reproducible when buildmode=pie

When buildmode=pie, external linking is forced, and our toolchain build id
will be included in the external build id, resulting in the building of
a toolchain tool will never reach a fixed point id.

More importantly, this change will make make.bash converge on self-hosted
Android builds (Android refuses to run non-PIE executables).

Fixes #31320
Updates #18968

Change-Id: Icb5db9f4b1b688afe37f4dafe261ffda580fa4e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/170942
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/gc.go

@@ -21,6 +21,7 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/str"
 	"cmd/internal/objabi"
+	"cmd/internal/sys"
 	"crypto/sha1"
 )
 
@@ -525,8 +526,14 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 	// Store BuildID inside toolchain binaries as a unique identifier of the
 	// tool being run, for use by content-based staleness determination.
 	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
+		// When buildmode=pie, external linking will include our build
+		// id in the external linker's build id, which will cause our
+		// build id to not match the next time the tool is built.
+		// Rely on the external build id instead.
+		if ldBuildmode != "pie" || !sys.PIEDefaultsToExternalLink(cfg.Goos, cfg.Goarch) {
 			ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
 		}
+	}
 
 	// If the user has not specified the -extld option, then specify the
 	// appropriate linker. In case of C++ code, use the compiler named

src/cmd/internal/sys/supported.go

@@ -27,3 +27,11 @@ func MSanSupported(goos, goarch string) bool {
 		return false
 	}
 }
+
+// PIEDefaultsToExternalLink reports whether goos/goarch defaults
+// to external linking for buildmode=pie.
+func PIEDefaultsToExternalLink(goos, goarch string) bool {
+	// Currently all systems external link PIE binaries.
+	// See https://golang.org/issue/18968.
+	return true
+}

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

@@ -256,8 +256,8 @@ func determineLinkMode(ctxt *Link) {
 				ctxt.LinkMode = LinkExternal
 			} else if iscgo && externalobj {
 				ctxt.LinkMode = LinkExternal
-			} else if ctxt.BuildMode == BuildModePIE {
-				ctxt.LinkMode = LinkExternal // https://golang.org/issue/18968
+			} else if ctxt.BuildMode == BuildModePIE && sys.PIEDefaultsToExternalLink(objabi.GOOS, objabi.GOARCH) {
+				ctxt.LinkMode = LinkExternal
 			} else {
 				ctxt.LinkMode = LinkInternal
 			}