cmd/go: always link external test packages first when using gccgo

This CL is an amagamation of several fixes Canonical have made on their
fork of the cmd/go tool (packaged as gccgo-go.deb on Ubuntu 14.04+).

Additionally this CL brings gccgoToolchain.ldi() up to date with the version
that will ship in gccgo-5.0. As gccgo is most likely to be used with its
own version of the go tool that it supples it makes good sense that the libgo
version should dictate the contents of gccgotoolchain.ld()

Please see https://codereview.appspot.com/222890043/ for more details on the
issues fixed.

Change-Id: Icf7deb43f8e80b424757f1673e6bca7a0aa2a1ac
Reviewed-on: https://go-review.googlesource.com/8250Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -1968,35 +1968,39 @@ func (gccgoToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles
 func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []*action, mainpkg string, ofiles []string) error {
 	// gccgo needs explicit linking with all package dependencies,
 	// and all LDFLAGS from cgo dependencies.
+	apackagesSeen := make(map[*Package]bool)
 	afiles := []string{}
+	xfiles := []string{}
 	ldflags := b.gccArchArgs()
 	cgoldflags := []string{}
 	usesCgo := false
 	cxx := len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0
 	objc := len(p.MFiles) > 0
 
-	// For a given package import path:
-	//   1) prefer a test package (created by (*builder).test) to a non-test package
-	//   2) prefer the output of an install action to the output of a build action
-	//      because the install action will delete the output of the build
-	//      action
-	// Iterating over the list backwards (reverse dependency order) ensures that we
-	// always see an install before a build.
-	importPathsSeen := make(map[string]bool)
+	// Prefer the output of an install action to the output of a build action,
+	// because the install action will delete the output of the build action.
+	// Iterate over the list backward (reverse dependency order) so that we
+	// always see the install before the build.
 	for i := len(allactions) - 1; i >= 0; i-- {
 		a := allactions[i]
-		if a.p.fake && !importPathsSeen[a.p.ImportPath] {
-			importPathsSeen[a.p.ImportPath] = true
+		if !a.p.Standard {
+			if a.p != nil && !apackagesSeen[a.p] {
+				apackagesSeen[a.p] = true
+				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 {
 					afiles = append(afiles, a.target)
 				}
 			}
-	for i := len(allactions) - 1; i >= 0; i-- {
-		a := allactions[i]
-		if !a.p.Standard && !importPathsSeen[a.p.ImportPath] {
-			importPathsSeen[a.p.ImportPath] = true
-			afiles = append(afiles, a.target)
 		}
 	}
+	afiles = append(xfiles, afiles...)
 
 	for _, a := range allactions {
 		cgoldflags = append(cgoldflags, a.p.CgoLDFLAGS...)

src/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

src/cmd/go/test.go

@@ -690,6 +690,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 			imports:  ximports,
 			pkgdir:   testDir,
 			fake:     true,
+			external: true,
 			Stale:    true,
 		}
 		if pxtestNeedsPtest {