go.tools/go/importer: record low-level encoding format

This avoids confusion when trying to read correctly
encoded export data that happens to be encoded in
a different format (debug vs product).

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/82090043

go/importer/export.go

@@ -21,12 +21,21 @@ const (
 	trace = false // print emitted data
 )
 
+// format returns a byte indicating the low-level encoding/decoding format
+// (debug vs product).
+func format() byte {
+	if debug {
+		return 'd'
+	}
+	return 'p'
+}
+
 // ExportData serializes the interface (exported package objects)
 // of package pkg and returns the corresponding data. The export
 // format is described elsewhere (TODO).
 func ExportData(pkg *types.Package) []byte {
 	p := exporter{
-		data:     []byte(magic),
+		data:     append([]byte(magic), format()),
 		pkgIndex: make(map[*types.Package]int),
 		typIndex: make(map[types.Type]int),
 	}
@@ -394,8 +403,8 @@ func (p *exporter) bytes(b []byte) {
 }
 
 // marker emits a marker byte and position information which makes
-// it easy for a reader to detect if it is "out of sync". Used in
+// it easy for a reader to detect if it is "out of sync". Used for
-// debug mode only.
+// debug format only.
 func (p *exporter) marker(m byte) {
 	if debug {
 		p.data = append(p.data, m)

go/importer/import.go

@@ -24,14 +24,29 @@ import (
 // data.
 func ImportData(imports map[string]*types.Package, data []byte) (*types.Package, error) {
 	// check magic string
-	if n := len(magic); len(data) < n || string(data[:n]) != magic {
+	var s string
-		return nil, fmt.Errorf("incorrect magic string: got %q; want %q", data[:n], magic)
+	if len(data) >= len(magic) {
+		s = string(data[:len(magic)])
+		data = data[len(magic):]
+	}
+	if s != magic {
+		return nil, fmt.Errorf("incorrect magic string: got %q; want %q", s, magic)
+	}
+
+	// check low-level encoding format
+	var m byte = 'm' // missing format
+	if len(data) > 0 {
+		m = data[0]
+		data = data[1:]
+	}
+	if m != format() {
+		return nil, fmt.Errorf("incorrect low-level encoding format: got %c; want %c", m, format())
 	}
 
 	p := importer{
-		data:     data[len(magic):],
+		data:     data,
 		imports:  imports,
-		consumed: len(magic), // for debugging only
+		consumed: len(magic) + 1, // for debugging only
 	}
 
 	// populate typList with predeclared types