cmd/go: use -importcfg to invoke compiler, linker

This is a step toward using cached build artifacts: the importcfg
will direct the compiler and linker to read them right from the cache
if necessary. However, this CL does not have a cache yet, so it still
reads them from the usual install location or build location.
Even so, this fixes a long-standing issue that -I and -L (no longer used)
are not expressive enough to describe complex GOPATH setups.

Shared libraries are handled enough that all.bash passes, but
there may still be more work to do here. If so, tests and fixes
can be added in follow-up CLs.

Gccgo will need updating to support -importcfg as well.

Fixes #14271.

Change-Id: I5c52a0a5df0ffbf7436e1130c74e9e24fceff80f
Reviewed-on: https://go-review.googlesource.com/56279
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testshared/shared_test.go

@@ -47,7 +47,7 @@ func run(t *testing.T, msg string, args ...string) {
 func goCmd(t *testing.T, args ...string) {
 	newargs := []string{args[0], "-installsuffix=" + suffix}
 	if testing.Verbose() {
-		newargs = append(newargs, "-v")
+		newargs = append(newargs, "-x")
 	}
 	newargs = append(newargs, args[1:]...)
 	c := exec.Command("go", newargs...)
@@ -58,6 +58,7 @@ func goCmd(t *testing.T, args ...string) {
 		c.Stdout = os.Stdout
 		c.Stderr = os.Stderr
 		err = c.Run()
+		output = []byte("(output above)")
 	} else {
 		output, err = c.CombinedOutput()
 	}

src/cmd/go/internal/cfg/cfg.go

@@ -128,3 +128,28 @@ func isGOROOT(path string) bool {
 	}
 	return stat.IsDir()
 }
+
+// ExternalLinkingForced reports whether external linking is being
+// forced even for programs that do not use cgo.
+func ExternalLinkingForced() bool {
+	if !BuildContext.CgoEnabled {
+		return false
+	}
+	// Currently build modes c-shared, pie (on systems that do not
+	// support PIE with internal linking mode (currently all
+	// systems: issue #18968)), plugin, and -linkshared force
+	// external linking mode, as of course does
+	// -ldflags=-linkmode=external. External linking mode forces
+	// an import of runtime/cgo.
+	pieCgo := BuildBuildmode == "pie"
+	linkmodeExternal := false
+	for i, a := range BuildLdflags {
+		if a == "-linkmode=external" {
+			linkmodeExternal = true
+		}
+		if a == "-linkmode" && i+1 < len(BuildLdflags) && BuildLdflags[i+1] == "external" {
+			linkmodeExternal = true
+		}
+	}
+	return BuildBuildmode == "c-shared" || BuildBuildmode == "plugin" || pieCgo || BuildLinkshared || linkmodeExternal
+}

src/cmd/go/internal/load/pkg.go

@@ -99,6 +99,7 @@ type PackageInternal struct {
 	SFiles       []string
 	AllGoFiles   []string             // gofiles + IgnoredGoFiles, absolute paths
 	Target       string               // installed file for this package (may be executable)
+	Pkgfile      string               // where package will be (or is already) built or installed
 	Fake         bool                 // synthesized package
 	External     bool                 // synthesized external test package
 	ForceLibrary bool                 // this package is a library (even if named "main")
@@ -951,27 +952,9 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		importPaths = append(importPaths, "syscall")
 	}
 
-	if cfg.BuildContext.CgoEnabled && p.Name == "main" && !p.Goroot {
-		// Currently build modes c-shared, pie (on systems that do not
-		// support PIE with internal linking mode (currently all
-		// systems: issue #18968)), plugin, and -linkshared force
-		// external linking mode, as of course does
-		// -ldflags=-linkmode=external. External linking mode forces
-		// an import of runtime/cgo.
-		pieCgo := cfg.BuildBuildmode == "pie"
-		linkmodeExternal := false
-		for i, a := range cfg.BuildLdflags {
-			if a == "-linkmode=external" {
-				linkmodeExternal = true
-			}
-			if a == "-linkmode" && i+1 < len(cfg.BuildLdflags) && cfg.BuildLdflags[i+1] == "external" {
-				linkmodeExternal = true
-			}
-		}
-		if cfg.BuildBuildmode == "c-shared" || cfg.BuildBuildmode == "plugin" || pieCgo || cfg.BuildLinkshared || linkmodeExternal {
+	if p.Name == "main" && !p.Goroot && cfg.ExternalLinkingForced() {
 		importPaths = append(importPaths, "runtime/cgo")
 	}
-	}
 
 	// Everything depends on runtime, except runtime, its internal
 	// subpackages, and unsafe.

src/cmd/go/internal/test/test.go

@@ -887,7 +887,11 @@ func builderTest(b *work.Builder, p *load.Package) (buildAction, runAction, prin
 
 	// The generated main also imports testing, regexp, and os.
 	stk.Push("testmain")
-	for _, dep := range testMainDeps {
+	deps := testMainDeps
+	if cfg.ExternalLinkingForced() {
+		deps = str.StringList(deps, "runtime/cgo")
+	}
+	for _, dep := range deps {
 		if dep == ptest.ImportPath {
 			pmain.Internal.Imports = append(pmain.Internal.Imports, ptest)
 		} else {

src/cmd/go/internal/work/build.go

@@ -877,6 +877,7 @@ func (b *Builder) action1(mode BuildMode, depMode BuildMode, p *load.Package, lo
 		// p.Stale==false implies that p.Internal.Target is up-to-date.
 		// Record target name for use by actions depending on this one.
 		a.Target = p.Internal.Target
+		p.Internal.Pkgfile = a.Target
 		return a
 	}
 
@@ -897,6 +898,7 @@ func (b *Builder) action1(mode BuildMode, depMode BuildMode, p *load.Package, lo
 		a.Func = BuildInstallFunc
 		a.Deps = []*Action{b.action1(ModeBuild, depMode, p, lookshared, forShlib)}
 		a.Target = a.Package.Internal.Target
+		a.Package.Internal.Pkgfile = a.Target
 
 		// Install header for cgo in c-archive and c-shared modes.
 		if p.UsesCgo() && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
@@ -923,6 +925,7 @@ func (b *Builder) action1(mode BuildMode, depMode BuildMode, p *load.Package, lo
 	case ModeBuild:
 		a.Func = (*Builder).build
 		a.Target = a.Objpkg
+		a.Package.Internal.Pkgfile = a.Target
 		if a.Link {
 			// An executable file. (This is the name of a temporary file.)
 			// Because we run the temporary file in 'go run' and 'go test',
@@ -1088,31 +1091,6 @@ func ActionList(root *Action) []*Action {
 	return all
 }
 
-// allArchiveActions returns a list of the archive dependencies of root.
-// This is needed because if package p depends on package q that is in libr.so, the
-// action graph looks like p->libr.so->q and so just scanning through p's
-// dependencies does not find the import dir for q.
-func allArchiveActions(root *Action) []*Action {
-	seen := map[*Action]bool{}
-	r := []*Action{}
-	var walk func(*Action)
-	walk = func(a *Action) {
-		if seen[a] {
-			return
-		}
-		seen[a] = true
-		if strings.HasSuffix(a.Target, ".so") || a == root {
-			for _, a1 := range a.Deps {
-				walk(a1)
-			}
-		} else if strings.HasSuffix(a.Target, ".a") {
-			r = append(r, a)
-		}
-	}
-	walk(root)
-	return r
-}
-
 // do runs the action graph rooted at root.
 func (b *Builder) Do(root *Action) {
 	// Build list of all actions, assigning depth-first post-order priority.
@@ -1391,11 +1369,46 @@ func (b *Builder) build(a *Action) (err error) {
 		}
 	}
 
-	// Prepare Go import path list.
-	inc := b.includeArgs("-I", allArchiveActions(a))
+	// NOTE: We used to call allArchiveActions(a) here and use it for -I.
+	// The comment on allArchiveActions(a) said:
+	//
+	//     allArchiveActions returns a list of the archive dependencies of root.
+	//     This is needed because if package p depends on package q that is in libr.so, the
+	//     action graph looks like p->libr.so->q and so just scanning through p's
+	//     dependencies does not find the import dir for q.
+	//
+	// If that's true, then the action graph is wrong, and q should be listed
+	// as a direct dependency of p as well as indirectly through libr.so.
+
+	// Prepare Go import config.
+	var icfg bytes.Buffer
+	for _, path := range a.Package.Imports {
+		i := strings.LastIndex(path, "/vendor/")
+		if i >= 0 {
+			i += len("/vendor/")
+		} else if strings.HasPrefix(path, "vendor/") {
+			i = len("vendor/")
+		} else {
+			continue
+		}
+		fmt.Fprintf(&icfg, "importmap %s=%s\n", path[i:], path)
+	}
+	for _, p1 := range a.Package.Internal.Imports {
+		if p1.ImportPath == "unsafe" {
+			continue
+		}
+		// TODO(rsc): runtime/internal/sys appears twice sometimes,
+		// because of the blind append in ../load/pkg.go that
+		// claims to fix issue 13655. That's probably not the right fix.
+		// Look into that.
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, p1.Internal.Pkgfile)
+	}
+	if err := b.writeFile(objdir+"importcfg", icfg.Bytes()); err != nil {
+		return err
+	}
 
 	// Compile Go.
-	ofile, out, err := BuildToolchain.gc(b, a.Package, a.Objpkg, objdir, len(sfiles) > 0, inc, gofiles)
+	ofile, out, err := BuildToolchain.gc(b, a.Package, a.Objpkg, objdir, objdir+"importcfg", len(sfiles) > 0, gofiles)
 	if len(out) > 0 {
 		b.showOutput(a.Package.Dir, a.Package.ImportPath, b.processOutput(out))
 		if err != nil {
@@ -1477,11 +1490,16 @@ func (b *Builder) build(a *Action) (err error) {
 
 	// Link if needed.
 	if a.Link {
+		importcfg := a.Objdir + "importcfg.link"
+		if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+			return err
+		}
+
 		// The compiler only cares about direct imports, but the
 		// linker needs the whole dependency tree.
 		all := ActionList(a)
 		all = all[:len(all)-1] // drop a
-		if err := BuildToolchain.ld(b, a, a.Target, all, a.Objpkg, objects); err != nil {
+		if err := BuildToolchain.ld(b, a, a.Target, importcfg, all, a.Objpkg, objects); err != nil {
 			return err
 		}
 	}
@@ -1489,6 +1507,32 @@ func (b *Builder) build(a *Action) (err error) {
 	return nil
 }
 
+func (b *Builder) writeLinkImportcfg(a *Action, file string) error {
+	// Prepare Go import cfg.
+	var icfg bytes.Buffer
+	p := a.Package
+	if p == nil {
+		// For linkShared, build fake package to serve as root
+		// for InternalDeps call.
+		p = new(load.Package)
+		for _, a1 := range a.Deps {
+			if a1.Package != nil {
+				p.Internal.Imports = append(p.Internal.Imports, a1.Package)
+			}
+		}
+	}
+	for _, p1 := range p.InternalDeps() {
+		if p1.ImportPath == "unsafe" {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, p1.Internal.Pkgfile)
+		if p1.Shlib != "" {
+			fmt.Fprintf(&icfg, "packageshlib %s=%s\n", p1.ImportPath, p1.Shlib)
+		}
+	}
+	return b.writeFile(file, icfg.Bytes())
+}
+
 // PkgconfigCmd returns a pkg-config binary name
 // defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
 func (b *Builder) PkgconfigCmd() string {
@@ -1569,9 +1613,14 @@ func (b *Builder) installShlibname(a *Action) error {
 }
 
 func (b *Builder) linkShared(a *Action) (err error) {
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
 	allactions := ActionList(a)
 	allactions = allactions[:len(allactions)-1]
-	return BuildToolchain.ldShared(b, a.Deps, a.Target, allactions)
+	return BuildToolchain.ldShared(b, a.Deps, a.Target, importcfg, allactions)
 }
 
 // BuildInstallFunc is the action for installing a single package or executable.
@@ -1775,6 +1824,17 @@ func (b *Builder) copyFile(a *Action, dst, src string, perm os.FileMode, force b
 	return nil
 }
 
+// writeFile writes the text to file.
+func (b *Builder) writeFile(file string, text []byte) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	return ioutil.WriteFile(file, text, 0666)
+}
+
 // Install the cgo export header file, if there is one.
 func (b *Builder) installHeader(a *Action) error {
 	src := a.Objdir + "_cgo_install.h"
@@ -2114,7 +2174,7 @@ func mkAbs(dir, f string) string {
 type toolchain interface {
 	// gc runs the compiler in a specific directory on a set of files
 	// and returns the name of the generated output file.
-	gc(b *Builder, p *load.Package, archive, objdir string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, out []byte, err error)
+	gc(b *Builder, p *load.Package, archive, objdir, importcfg string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error)
 	// cc runs the toolchain's C compiler in a directory on a C file
 	// to produce an output file.
 	cc(b *Builder, p *load.Package, objdir, ofile, cfile string) error
@@ -2128,9 +2188,9 @@ type toolchain interface {
 	// typically it is run in the object directory.
 	pack(b *Builder, p *load.Package, objdir, afile string, ofiles []string) error
 	// ld runs the linker to create an executable starting at mainpkg.
-	ld(b *Builder, root *Action, out string, allactions []*Action, mainpkg string, ofiles []string) error
+	ld(b *Builder, root *Action, out, importcfg string, allactions []*Action, mainpkg string, ofiles []string) error
 	// ldShared runs the linker to create a shared library containing the pkgs built by toplevelactions
-	ldShared(b *Builder, toplevelactions []*Action, out string, allactions []*Action) error
+	ldShared(b *Builder, toplevelactions []*Action, out, importcfg string, allactions []*Action) error
 
 	compiler() string
 	linker() string
@@ -2153,7 +2213,7 @@ func (noToolchain) linker() string {
 	return ""
 }
 
-func (noToolchain) gc(b *Builder, p *load.Package, archive, objdir string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, out []byte, err error) {
+func (noToolchain) gc(b *Builder, p *load.Package, archive, objdir, importcfg string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error) {
 	return "", nil, noCompiler()
 }
 
@@ -2170,11 +2230,11 @@ func (noToolchain) pack(b *Builder, p *load.Package, objdir, afile string, ofile
 	return noCompiler()
 }
 
-func (noToolchain) ld(b *Builder, root *Action, out string, allactions []*Action, mainpkg string, ofiles []string) error {
+func (noToolchain) ld(b *Builder, root *Action, out, importcfg string, allactions []*Action, mainpkg string, ofiles []string) error {
 	return noCompiler()
 }
 
-func (noToolchain) ldShared(b *Builder, toplevelactions []*Action, out string, allactions []*Action) error {
+func (noToolchain) ldShared(b *Builder, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
 	return noCompiler()
 }
 
@@ -2193,7 +2253,7 @@ func (gcToolchain) linker() string {
 	return base.Tool("link")
 }
 
-func (gcToolchain) gc(b *Builder, p *load.Package, archive, objdir string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, output []byte, err error) {
+func (gcToolchain) gc(b *Builder, p *load.Package, archive, objdir, importcfg string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
 	if archive != "" {
 		ofile = archive
 	} else {
@@ -2266,7 +2326,10 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, objdir string, asmhd
 			}
 		}
 	}
-	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", b.WorkDir, gcflags, gcargs, "-D", p.Internal.LocalPrefix, importArgs}
+	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", b.WorkDir, gcflags, gcargs, "-D", p.Internal.LocalPrefix}
+	if importcfg != "" {
+		args = append(args, "-importcfg", importcfg)
+	}
 	if ofile == archive {
 		args = append(args, "-pack")
 	}
@@ -2521,8 +2584,7 @@ func setextld(ldflags []string, compiler []string) []string {
 	return ldflags
 }
 
-func (gcToolchain) ld(b *Builder, root *Action, out string, allactions []*Action, mainpkg string, ofiles []string) error {
-	importArgs := b.includeArgs("-L", allactions)
+func (gcToolchain) ld(b *Builder, root *Action, out, importcfg string, allactions []*Action, mainpkg string, ofiles []string) error {
 	cxx := len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
 	for _, a := range allactions {
 		if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
@@ -2572,11 +2634,10 @@ func (gcToolchain) ld(b *Builder, root *Action, out string, allactions []*Action
 		dir, out = filepath.Split(out)
 	}
 
-	return b.run(dir, root.Package.ImportPath, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, importArgs, ldflags, mainpkg)
+	return b.run(dir, root.Package.ImportPath, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags, mainpkg)
 }
 
-func (gcToolchain) ldShared(b *Builder, toplevelactions []*Action, out string, allactions []*Action) error {
-	importArgs := b.includeArgs("-L", allactions)
+func (gcToolchain) ldShared(b *Builder, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
 	ldflags := []string{"-installsuffix", cfg.BuildContext.InstallSuffix}
 	ldflags = append(ldflags, "-buildmode=shared")
 	ldflags = append(ldflags, cfg.BuildLdflags...)
@@ -2603,7 +2664,7 @@ func (gcToolchain) ldShared(b *Builder, toplevelactions []*Action, out string, a
 		}
 		ldflags = append(ldflags, d.Package.ImportPath+"="+d.Target)
 	}
-	return b.run(".", out, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, importArgs, ldflags)
+	return b.run(".", out, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags)
 }
 
 func (gcToolchain) cc(b *Builder, p *load.Package, objdir, ofile, cfile string) error {
@@ -2642,7 +2703,7 @@ func checkGccgoBin() {
 	os.Exit(2)
 }
 
-func (tools gccgoToolchain) gc(b *Builder, p *load.Package, archive, objdir string, asmhdr bool, importArgs []string, gofiles []string) (ofile string, output []byte, err error) {
+func (tools gccgoToolchain) gc(b *Builder, p *load.Package, archive, objdir, importcfg string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
 	out := "_go_.o"
 	ofile = objdir + out
 	gcargs := []string{"-g"}
@@ -2654,57 +2715,11 @@ func (tools gccgoToolchain) gc(b *Builder, p *load.Package, archive, objdir stri
 		gcargs = append(gcargs, "-fgo-relative-import-path="+p.Internal.LocalPrefix)
 	}
 
-	// Handle vendor directories
-	savedirs := []string{}
-	for _, incdir := range importArgs {
-		if incdir != "-I" {
-			savedirs = append(savedirs, incdir)
-		}
-	}
-
-	for _, path := range p.Imports {
-		// If this is a new vendor path, add it to the list of importArgs
-		if i := strings.LastIndex(path, "/vendor"); i >= 0 {
-			for _, dir := range savedirs {
-				// Check if the vendor path is already included in dir
-				if strings.HasSuffix(dir, path[:i+len("/vendor")]) {
-					continue
-				}
-				// Make sure this vendor path is not already in the list for importArgs
-				vendorPath := dir + "/" + path[:i+len("/vendor")]
-				for _, imp := range importArgs {
-					if imp == "-I" {
-						continue
-					}
-					// This vendorPath is already in the list
-					if imp == vendorPath {
-						goto nextSuffixPath
-					}
-				}
-				// New vendorPath not yet in the importArgs list, so add it
-				importArgs = append(importArgs, "-I", vendorPath)
-			nextSuffixPath:
+	args := str.StringList(tools.compiler(), "-c", gcargs, "-o", ofile)
+	if importcfg != "" {
+		args = append(args, "-importcfg", importcfg)
 	}
-		} else if strings.HasPrefix(path, "vendor/") {
-			for _, dir := range savedirs {
-				// Make sure this vendor path is not already in the list for importArgs
-				vendorPath := dir + "/" + path[len("/vendor"):]
-				for _, imp := range importArgs {
-					if imp == "-I" {
-						continue
-					}
-					if imp == vendorPath {
-						goto nextPrefixPath
-					}
-				}
-				// This vendor path is needed and not already in the list, so add it
-				importArgs = append(importArgs, "-I", vendorPath)
-			nextPrefixPath:
-			}
-		}
-	}
-
-	args := str.StringList(tools.compiler(), importArgs, "-c", gcargs, "-o", ofile, buildGccgoflags)
+	args = append(args, buildGccgoflags...)
 	for _, f := range gofiles {
 		args = append(args, mkAbs(p.Dir, f))
 	}
@@ -2748,7 +2763,7 @@ func (gccgoToolchain) pack(b *Builder, p *load.Package, objdir, afile string, of
 	return b.run(p.Dir, p.ImportPath, nil, "ar", "rc", mkAbs(objdir, afile), absOfiles)
 }
 
-func (tools gccgoToolchain) link(b *Builder, root *Action, out string, allactions []*Action, mainpkg string, ofiles []string, buildmode, desc string) error {
+func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string, allactions []*Action, mainpkg string, ofiles []string, buildmode, desc string) error {
 	// gccgo needs explicit linking with all package dependencies,
 	// and all LDFLAGS from cgo dependencies.
 	apackagePathsSeen := make(map[string]bool)
@@ -3014,14 +3029,14 @@ func (tools gccgoToolchain) link(b *Builder, root *Action, out string, allaction
 	return nil
 }
 
-func (tools gccgoToolchain) ld(b *Builder, root *Action, out string, allactions []*Action, mainpkg string, ofiles []string) error {
-	return tools.link(b, root, out, allactions, mainpkg, ofiles, ldBuildmode, root.Package.ImportPath)
+func (tools gccgoToolchain) ld(b *Builder, root *Action, out, importcfg string, allactions []*Action, mainpkg string, ofiles []string) error {
+	return tools.link(b, root, out, importcfg, allactions, mainpkg, ofiles, ldBuildmode, root.Package.ImportPath)
 }
 
-func (tools gccgoToolchain) ldShared(b *Builder, toplevelactions []*Action, out string, allactions []*Action) error {
+func (tools gccgoToolchain) ldShared(b *Builder, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
 	fakeRoot := &Action{}
 	fakeRoot.Deps = toplevelactions
-	return tools.link(b, fakeRoot, out, allactions, "", nil, "shared", out)
+	return tools.link(b, fakeRoot, out, importcfg, allactions, "", nil, "shared", out)
 }
 
 func (tools gccgoToolchain) cc(b *Builder, p *load.Package, objdir, ofile, cfile string) error {
@@ -3638,7 +3653,7 @@ func (b *Builder) swigDoIntSize(objdir string) (intsize string, err error) {
 
 	p := load.GoFilesPackage(srcs)
 
-	if _, _, e := BuildToolchain.gc(b, p, "", objdir, false, nil, srcs); e != nil {
+	if _, _, e := BuildToolchain.gc(b, p, "", objdir, "", false, srcs); e != nil {
 		return "32", nil
 	}
 	return "64", nil

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

@@ -116,6 +116,16 @@ func findlib(ctxt *Link, lib string) (string, bool) {
 			pname = name
 		} else {
 			pkg := pkgname(lib)
+			// Add .a if needed; the new -importcfg modes
+			// do not put .a into the package name anymore.
+			// This only matters when people try to mix
+			// compiles using -importcfg with links not using -importcfg,
+			// such as when running quick things like
+			// 'go tool compile x.go && go tool link x.o'
+			// by hand against a standard library built using -importcfg.
+			if !strings.HasSuffix(name, ".a") && !strings.HasSuffix(name, ".o") {
+				name += ".a"
+			}
 			// try dot, -L "libdir", and then goroot.
 			for _, dir := range ctxt.Libdir {
 				if *FlagLinkshared {
@@ -163,14 +173,15 @@ func addlib(ctxt *Link, src string, obj string, lib string) *Library {
  *	objref: object file referring to package
  *	file: object file, e.g., /home/rsc/go/pkg/container/vector.a
  *	pkg: package import path, e.g. container/vector
+ *	shlib: path to shared library, or .shlibname file holding path
  */
-func addlibpath(ctxt *Link, srcref string, objref string, file string, pkg string, shlibnamefile string) *Library {
+func addlibpath(ctxt *Link, srcref string, objref string, file string, pkg string, shlib string) *Library {
 	if l := ctxt.LibraryByPkg[pkg]; l != nil {
 		return l
 	}
 
 	if ctxt.Debugvlog > 1 {
-		ctxt.Logf("%5.2f addlibpath: srcref: %s objref: %s file: %s pkg: %s shlibnamefile: %s\n", Cputime(), srcref, objref, file, pkg, shlibnamefile)
+		ctxt.Logf("%5.2f addlibpath: srcref: %s objref: %s file: %s pkg: %s shlib: %s\n", Cputime(), srcref, objref, file, pkg, shlib)
 	}
 
 	l := &Library{}
@@ -180,12 +191,15 @@ func addlibpath(ctxt *Link, srcref string, objref string, file string, pkg strin
 	l.Srcref = srcref
 	l.File = file
 	l.Pkg = pkg
-	if shlibnamefile != "" {
-		shlibbytes, err := ioutil.ReadFile(shlibnamefile)
+	if shlib != "" {
+		if strings.HasSuffix(shlib, ".shlibname") {
+			data, err := ioutil.ReadFile(shlib)
 			if err != nil {
-			Errorf(nil, "cannot read %s: %v", shlibnamefile, err)
+				Errorf(nil, "cannot read %s: %v", shlib, err)
+			}
+			shlib = strings.TrimSpace(string(data))
 		}
-		l.Shlib = strings.TrimSpace(string(shlibbytes))
+		l.Shlib = shlib
 	}
 	return l
 }

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

@@ -338,8 +338,8 @@ func errorexit() {
 
 func loadinternal(ctxt *Link, name string) *Library {
 	if *FlagLinkshared && ctxt.PackageShlib != nil {
-		if shlibname := ctxt.PackageShlib[name]; shlibname != "" {
-			return addlibpath(ctxt, "internal", "internal", "", name, shlibname)
+		if shlib := ctxt.PackageShlib[name]; shlib != "" {
+			return addlibpath(ctxt, "internal", "internal", "", name, shlib)
 		}
 	}
 	if ctxt.PackageFile != nil {
@@ -423,22 +423,23 @@ func (ctxt *Link) loadlib() {
 		loadinternal(ctxt, "runtime/msan")
 	}
 
-	var i int
-	for i = 0; i < len(ctxt.Library); i++ {
-		if ctxt.Library[i].Shlib == "" {
+	// ctxt.Library grows during the loop, so not a range loop.
+	for i := 0; i < len(ctxt.Library); i++ {
+		lib := ctxt.Library[i]
+		if lib.Shlib == "" {
 			if ctxt.Debugvlog > 1 {
-				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), ctxt.Library[i].File, ctxt.Library[i].Objref)
+				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), lib.File, lib.Objref)
 			}
-			objfile(ctxt, ctxt.Library[i])
+			objfile(ctxt, lib)
 		}
 	}
 
-	for i = 0; i < len(ctxt.Library); i++ {
-		if ctxt.Library[i].Shlib != "" {
+	for _, lib := range ctxt.Library {
+		if lib.Shlib != "" {
 			if ctxt.Debugvlog > 1 {
-				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), ctxt.Library[i].Shlib, ctxt.Library[i].Objref)
+				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), lib.Shlib, lib.Objref)
 			}
-			ldshlibsyms(ctxt, ctxt.Library[i].Shlib)
+			ldshlibsyms(ctxt, lib.Shlib)
 		}
 	}
 
@@ -461,21 +462,19 @@ func (ctxt *Link) loadlib() {
 		toc.Type = SDYNIMPORT
 	}
 
-	if Linkmode == LinkExternal && !iscgo {
+	if Linkmode == LinkExternal && !iscgo && ctxt.LibraryByPkg["runtime/cgo"] == nil {
 		// This indicates a user requested -linkmode=external.
 		// The startup code uses an import of runtime/cgo to decide
 		// whether to initialize the TLS.  So give it one. This could
 		// be handled differently but it's an unusual case.
-		loadinternal(ctxt, "runtime/cgo")
-
-		if i < len(ctxt.Library) {
-			if ctxt.Library[i].Shlib != "" {
-				ldshlibsyms(ctxt, ctxt.Library[i].Shlib)
+		if lib := loadinternal(ctxt, "runtime/cgo"); lib != nil {
+			if lib.Shlib != "" {
+				ldshlibsyms(ctxt, lib.Shlib)
 			} else {
 				if Buildmode == BuildmodeShared || *FlagLinkshared {
 					Exitf("cannot implicitly include runtime/cgo in a shared library")
 				}
-				objfile(ctxt, ctxt.Library[i])
+				objfile(ctxt, lib)
 			}
 		}
 	}
@@ -633,9 +632,9 @@ func (ctxt *Link) loadlib() {
 	// If package versioning is required, generate a hash of the
 	// the packages used in the link.
 	if Buildmode == BuildmodeShared || Buildmode == BuildmodePlugin || ctxt.Syms.ROLookup("plugin.Open", 0) != nil {
-		for i = 0; i < len(ctxt.Library); i++ {
-			if ctxt.Library[i].Shlib == "" {
-				genhash(ctxt, ctxt.Library[i])
+		for _, lib := range ctxt.Library {
+			if lib.Shlib == "" {
+				genhash(ctxt, lib)
 			}
 		}
 	}
@@ -1538,6 +1537,9 @@ func readnote(f *elf.File, name []byte, typ int32) ([]byte, error) {
 }
 
 func findshlib(ctxt *Link, shlib string) string {
+	if filepath.IsAbs(shlib) {
+		return shlib
+	}
 	for _, libdir := range ctxt.Libdir {
 		libpath := filepath.Join(libdir, shlib)
 		if _, err := os.Stat(libpath); err == nil {
@@ -1549,10 +1551,16 @@ func findshlib(ctxt *Link, shlib string) string {
 }
 
 func ldshlibsyms(ctxt *Link, shlib string) {
-	libpath := findshlib(ctxt, shlib)
+	var libpath string
+	if filepath.IsAbs(shlib) {
+		libpath = shlib
+		shlib = filepath.Base(shlib)
+	} else {
+		libpath = findshlib(ctxt, shlib)
 		if libpath == "" {
 			return
 		}
+	}
 	for _, processedlib := range ctxt.Shlibs {
 		if processedlib.Path == libpath {
 			return
@@ -1580,7 +1588,22 @@ func ldshlibsyms(ctxt *Link, shlib string) {
 		Errorf(nil, "cannot read dep list from shared library %s: %v", libpath, err)
 		return
 	}
-	deps := strings.Split(string(depsbytes), "\n")
+	var deps []string
+	for _, dep := range strings.Split(string(depsbytes), "\n") {
+		if dep == "" {
+			continue
+		}
+		if !filepath.IsAbs(dep) {
+			// If the dep can be interpreted as a path relative to the shlib
+			// in which it was found, do that. Otherwise, we will leave it
+			// to be resolved by libdir lookup.
+			abs := filepath.Join(filepath.Dir(libpath), dep)
+			if _, err := os.Stat(abs); err == nil {
+				dep = abs
+			}
+		}
+		deps = append(deps, dep)
+	}
 
 	syms, err := f.DynamicSymbols()
 	if err != nil {