go/parser: simplify code to read from an io.Reader (cleanup)

ioutil.ReadAll didn't exist when we wrote that parser code
originally (in 2009). Now it does, so use it. This may also
make that code path slightly more efficient.

Also, now that we are guaranteed to have a fast path for reading
from an io.Reader (and thus an io.ReadCloser), simplify setup
code for parser.ParseFile calls in srcimporter.Importer.ParseFiles.

Remove the associated TODO since we cannot reproduce any significant
performance differences when running go test -run ImportStdLib for
the case where we used to read directly from a file (even before the
change to the parser).

Fixes #19281.

Change-Id: I816459d092bb9e27fdc85089b8f21d57ec3fd79a
Reviewed-on: https://go-review.googlesource.com/85395Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/build/deps_test.go

@@ -223,7 +223,7 @@ var pkgDeps = map[string][]string{
 	"go/importer":               {"L4", "go/build", "go/internal/gccgoimporter", "go/internal/gcimporter", "go/internal/srcimporter", "go/token", "go/types"},
 	"go/internal/gcimporter":    {"L4", "OS", "go/build", "go/constant", "go/token", "go/types", "text/scanner"},
 	"go/internal/gccgoimporter": {"L4", "OS", "debug/elf", "go/constant", "go/token", "go/types", "text/scanner"},
-	"go/internal/srcimporter":   {"L4", "fmt", "go/ast", "go/build", "go/parser", "go/token", "go/types", "path/filepath"},
+	"go/internal/srcimporter":   {"L4", "OS", "fmt", "go/ast", "go/build", "go/parser", "go/token", "go/types", "path/filepath"},
 	"go/types":                  {"L4", "GOPARSER", "container/heap", "go/constant"},
 
 	// One of a kind.

src/go/internal/srcimporter/srcimporter.go

@@ -13,6 +13,8 @@ import (
 	"go/parser"
 	"go/token"
 	"go/types"
+	"io"
+	"os"
 	"path/filepath"
 	"sync"
 )
@@ -162,7 +164,11 @@ func (p *Importer) ImportFrom(path, srcDir string, mode types.ImportMode) (*type
 }
 
 func (p *Importer) parseFiles(dir string, filenames []string) ([]*ast.File, error) {
-	open := p.ctxt.OpenFile // possibly nil
+	// use build.Context's OpenFile if there is one
+	open := p.ctxt.OpenFile
+	if open == nil {
+		open = func(name string) (io.ReadCloser, error) { return os.Open(name) }
+	}
 
 	files := make([]*ast.File, len(filenames))
 	errors := make([]error, len(filenames))
@@ -172,22 +178,13 @@ func (p *Importer) parseFiles(dir string, filenames []string) ([]*ast.File, erro
 	for i, filename := range filenames {
 		go func(i int, filepath string) {
 			defer wg.Done()
-			if open != nil {
 			src, err := open(filepath)
 			if err != nil {
-					errors[i] = fmt.Errorf("opening package file %s failed (%v)", filepath, err)
+				errors[i] = err // open provides operation and filename in error
 				return
 			}
 			files[i], errors[i] = parser.ParseFile(p.fset, filepath, src, 0)
 			src.Close() // ignore Close error - parsing may have succeeded which is all we need
-			} else {
-				// Special-case when ctxt doesn't provide a custom OpenFile and use the
-				// parser's file reading mechanism directly. This appears to be quite a
-				// bit faster than opening the file and providing an io.ReaderCloser in
-				// both cases.
-				// TODO(gri) investigate performance difference (issue #19281)
-				files[i], errors[i] = parser.ParseFile(p.fset, filepath, nil, 0)
-			}
 		}(i, p.joinPath(dir, filename))
 	}
 	wg.Wait()

src/go/parser/interface.go

@@ -35,11 +35,7 @@ func readSource(filename string, src interface{}) ([]byte, error) {
 				return s.Bytes(), nil
 			}
 		case io.Reader:
-			var buf bytes.Buffer
-			if _, err := io.Copy(&buf, s); err != nil {
-				return nil, err
-			}
-			return buf.Bytes(), nil
+			return ioutil.ReadAll(s)
 		}
 		return nil, errors.New("invalid source")
 	}