imports: bake in the standard library again

Fast path for the common case. Avoids scanning GOPATH usually.

LGTM=r, crawshaw
R=david.crawshaw, adg, r, crawshaw
CC=golang-codereviews
https://golang.org/cl/56820043

imports/fix.go

@@ -271,6 +271,12 @@ func loadExportsGoPath(dir string) map[string]bool {
 var findImport = findImportGoPath
 
 func findImportGoPath(pkgName string, symbols map[string]bool) (string, error) {
+	// Fast path for the standard library.
+	// In the common case we hopefully never have to scan the GOPATH, which can
+	// be slow with moving disks.
+	if pkg, ok := findImportStdlib(pkgName, symbols); ok {
+		return pkg, nil
+	}
 
 	pkgIndexOnce.Do(loadPkgIndex)
 
@@ -325,3 +331,18 @@ type visitFn func(node ast.Node) ast.Visitor
 func (fn visitFn) Visit(node ast.Node) ast.Visitor {
 	return fn(node)
 }
+
+func findImportStdlib(shortPkg string, symbols map[string]bool) (importPath string, ok bool) {
+	for symbol := range symbols {
+		path := stdlib[shortPkg+"."+symbol]
+		if path == "" {
+			return "", false
+		}
+		if importPath != "" && importPath != path {
+			// Ambiguous. Symbols pointed to different things.
+			return "", false
+		}
+		importPath = path
+	}
+	return importPath, importPath != ""
+}

imports/fix_test.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sync"
 	"testing"
 )
 
@@ -508,6 +509,15 @@ func TestFindImportGoPath(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(goroot)
+
+	pkgIndexOnce = sync.Once{}
+
+	origStdlib := stdlib
+	defer func() {
+		stdlib = origStdlib
+	}()
+	stdlib = nil
+
 	// Test against imaginary bits/bytes package in std lib
 	bytesDir := filepath.Join(goroot, "src", "pkg", "bits", "bytes")
 	if err := os.MkdirAll(bytesDir, 0755); err != nil {
@@ -547,3 +557,34 @@ type Buffer2 struct {}
 		t.Errorf(`findImportGoPath("bytes", Missing ...)=%q, want ""`, got)
 	}
 }
+
+func TestFindImportStdlib(t *testing.T) {
+	tests := []struct {
+		pkg     string
+		symbols []string
+		want    string
+	}{
+		{"http", []string{"Get"}, "net/http"},
+		{"http", []string{"Get", "Post"}, "net/http"},
+		{"http", []string{"Get", "Foo"}, ""},
+		{"bytes", []string{"Buffer"}, "bytes"},
+		{"ioutil", []string{"Discard"}, "io/ioutil"},
+	}
+	for _, tt := range tests {
+		got, ok := findImportStdlib(tt.pkg, strSet(tt.symbols))
+		if (got != "") != ok {
+			t.Error("findImportStdlib return value inconsistent")
+		}
+		if got != tt.want {
+			t.Errorf("findImportStdlib(%q, %q) = %q; want %q", tt.pkg, tt.symbols, got, tt.want)
+		}
+	}
+}
+
+func strSet(ss []string) map[string]bool {
+	m := make(map[string]bool)
+	for _, s := range ss {
+		m[s] = true
+	}
+	return m
+}

imports/mkstdlib.go

@@ -0,0 +1,90 @@
+// +build ignore
+
+// mkstdlib generates the zstdlib.go file, containing the Go standard
+// library API symbols. It's baked into the binary to avoid scanning
+// GOPATH in the common case.
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"go/format"
+	"io"
+	"log"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+func mustOpen(name string) io.Reader {
+	f, err := os.Open(name)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return f
+}
+
+func api(base string) string {
+	return filepath.Join(os.Getenv("GOROOT"), "api", base)
+}
+
+var sym = regexp.MustCompile(`^pkg (\S+).*?, (?:var|func|type|const) ([A-Z]\w*)`)
+
+func main() {
+	var buf bytes.Buffer
+	outf := func(format string, args ...interface{}) {
+		fmt.Fprintf(&buf, format, args...)
+	}
+	outf("// AUTO-GENERATED BY mkstdlib.go\n\n")
+	outf("package imports\n")
+	outf("var stdlib = map[string]string{\n")
+	f := io.MultiReader(
+		mustOpen(api("go1.txt")),
+		mustOpen(api("go1.1.txt")),
+		mustOpen(api("go1.2.txt")),
+	)
+	sc := bufio.NewScanner(f)
+	fullImport := map[string]string{} // "zip.NewReader" => "archive/zip"
+	ambiguous := map[string]bool{}
+	var keys []string
+	for sc.Scan() {
+		l := sc.Text()
+		has := func(v string) bool { return strings.Contains(l, v) }
+		if has("struct, ") || has("interface, ") || has(", method (") {
+			continue
+		}
+		if m := sym.FindStringSubmatch(l); m != nil {
+			full := m[1]
+			key := path.Base(full) + "." + m[2]
+			if exist, ok := fullImport[key]; ok {
+				if exist != full {
+					ambiguous[key] = true
+				}
+			} else {
+				fullImport[key] = full
+				keys = append(keys, key)
+			}
+		}
+	}
+	if err := sc.Err(); err != nil {
+		log.Fatal(err)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		if ambiguous[key] {
+			outf("\t// %q is ambiguous\n", key)
+		} else {
+			outf("\t%q: %q,\n", key, fullImport[key])
+		}
+	}
+	outf("}\n")
+	fmtbuf, err := format.Source(buf.Bytes())
+	if err != nil {
+		log.Fatal(err)
+	}
+	os.Stdout.Write(fmtbuf)
+}