go/cmd/go: always link external test packages first

When linking complex packages that use both internal and external tests as well as many dependencies it is critical that the link order be external test package, internal test package, everything else.

This change is a back (forward?) port of the same change that canonical have been maintaining on their fork of the go tool for gccgo. Now that gccgo uses the go tool from upstream, this patch should be applied both to the gofrontend and golang/go repos.

From-SVN: r221800

libgo/go/cmd/go/build.go

@@ -1921,6 +1921,7 @@ func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []
 	// and all LDFLAGS from cgo dependencies.
 	apackagesSeen := make(map[*Package]bool)
 	afiles := []string{}
+	xfiles := []string{}
 	ldflags := b.gccArchArgs()
 	cgoldflags := []string{}
 	usesCgo := false
@@ -1936,7 +1937,12 @@ func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []
 		if !a.p.Standard {
 			if a.p != nil && !apackagesSeen[a.p] {
 				apackagesSeen[a.p] = true
-				if a.p.fake {
+				if a.p.fake && a.p.external {
+					// external _tests, if present must come before
+					// internal _tests. Store these on a seperate list
+					// and place them at the head after this loop.
+					xfiles = append(xfiles, a.target)
+				} else if a.p.fake {
 					// move _test files to the top of the link order
 					afiles = append([]string{a.target}, afiles...)
 				} else {
@@ -1945,6 +1951,7 @@ func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []
 			}
 		}
 	}
+	afiles = append(xfiles, afiles...)
 
 	for _, a := range allactions {
 		if a.p != nil {

libgo/go/cmd/go/pkg.go

@@ -83,6 +83,7 @@ type Package struct {
 	allgofiles   []string             // gofiles + IgnoredGoFiles, absolute paths
 	target       string               // installed file for this package (may be executable)
 	fake         bool                 // synthesized package
+	external     bool                 // synthesized external test package
 	forceBuild   bool                 // this package must be rebuilt
 	forceLibrary bool                 // this package is a library (even if named "main")
 	cmdline      bool                 // defined by files listed on command line

libgo/go/cmd/go/test.go

@@ -692,10 +692,11 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 			build: &build.Package{
 				ImportPos: p.build.XTestImportPos,
 			},
-			imports: ximports,
-			pkgdir:  testDir,
-			fake:    true,
-			Stale:   true,
+			imports:  ximports,
+			pkgdir:   testDir,
+			fake:     true,
+			external: true,
+			Stale:    true,
 		}
 		if pxtestNeedsPtest {
 			pxtest.imports = append(pxtest.imports, ptest)