cmd/compile/internal/types: unexport PkgMap, remove PkgList

- PkgMap was only needed to test import/export in a "cleanroom"
  environment, with debugFormat set. Provided helper function
  instead.

- PkgList was only used to identify directly imported packages.
  Instead, compute that list explicitly from the package map.
  It happens only once, the list is small, and it's more robust
  than keeping two data structures in sync.

Change-Id: I82dce3c0b5cb816faae58708e877799359c20fcb
Reviewed-on: https://go-review.googlesource.com/41078Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/export.go

@@ -173,13 +173,9 @@ func dumpexport() {
 
 		// verify that we can read the copied export data back in
 		// (use empty package map to avoid collisions)
-		savedPkgMap := types.PkgMap
-		savedPkgs := types.PkgList
-		types.PkgMap = make(map[string]*types.Pkg)
-		types.PkgList = nil
-		Import(types.NewPkg("", ""), bufio.NewReader(&copy)) // must not die
-		types.PkgList = savedPkgs
-		types.PkgMap = savedPkgMap
+		types.CleanroomDo(func() {
+			Import(types.NewPkg("", ""), bufio.NewReader(&copy)) // must not die
+		})
 	} else {
 		size = export(bout.Writer, Debug_export != 0)
 	}

src/cmd/compile/internal/gc/reflect.go

@@ -1501,10 +1501,8 @@ func dumptypestructs() {
 	}
 
 	// generate import strings for imported packages
-	for _, p := range types.PkgList {
-		if p.Direct {
-			dimportpath(p)
-		}
+	for _, p := range types.ImportedPkgList() {
+		dimportpath(p)
 	}
 
 	// do basic types if compiling package runtime.

src/cmd/compile/internal/types/pkg.go

@@ -8,8 +8,12 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"fmt"
+	"sort"
 )
 
+// pkgMap maps a package path to a package.
+var pkgMap = make(map[string]*Pkg)
+
 type Pkg struct {
 	Path     string // string literal used in import statement, e.g. "runtime/internal/sys"
 	Name     string // package name, e.g. "sys"
@@ -20,14 +24,11 @@ type Pkg struct {
 	Syms     map[string]*Sym
 }
 
-var PkgMap = make(map[string]*Pkg)
-var PkgList []*Pkg
-
 // NewPkg returns a new Pkg for the given package path and name.
 // Unless name is the empty string, if the package exists already,
 // the existing package name and the provided name must match.
 func NewPkg(path, name string) *Pkg {
-	if p := PkgMap[path]; p != nil {
+	if p := pkgMap[path]; p != nil {
 		if name != "" && p.Name != name {
 			panic(fmt.Sprintf("conflicting package names %s and %s for path %q", p.Name, name, path))
 		}
@@ -39,12 +40,30 @@ func NewPkg(path, name string) *Pkg {
 	p.Name = name
 	p.Prefix = objabi.PathToPrefix(path)
 	p.Syms = make(map[string]*Sym)
-	PkgMap[path] = p
-	PkgList = append(PkgList, p)
+	pkgMap[path] = p
 
 	return p
 }
 
+// ImportedPkgList returns the list of directly imported packages.
+// The list is sorted by package path.
+func ImportedPkgList() []*Pkg {
+	var list []*Pkg
+	for _, p := range pkgMap {
+		if p.Direct {
+			list = append(list, p)
+		}
+	}
+	sort.Sort(byPath(list))
+	return list
+}
+
+type byPath []*Pkg
+
+func (a byPath) Len() int           { return len(a) }
+func (a byPath) Less(i, j int) bool { return a[i].Path < a[j].Path }
+func (a byPath) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
 var nopkg = &Pkg{
 	Syms: make(map[string]*Sym),
 }
@@ -99,3 +118,12 @@ func InternString(b []byte) string {
 	}
 	return s
 }
+
+// CleanroomDo invokes f in an environment with with no preexisting packages.
+// For testing of import/export only.
+func CleanroomDo(f func()) {
+	saved := pkgMap
+	pkgMap = make(map[string]*Pkg)
+	f()
+	pkgMap = saved
+}