internal/pkgbits: add Version type

Adds a new Version type to pkgbits to represent the version of the
bitstream. Versions let readers and writers know when different data is
expected to be present or not in the bitstream. These different pieces
of data are called Fields, as an analogy with fields of a struct.
Fields can be added, removed or changed in a Version. Extends Encoder
and Decoder to report which version they are.

Updates #68778

Change-Id: Iaffa1828544fb4cbc47a905de853449bc8e5b91f
Reviewed-on: https://go-review.googlesource.com/c/go/+/605655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/internal/pkgbits/decoder.go

@@ -21,7 +21,7 @@ import (
 // export data.
 type PkgDecoder struct {
 	// version is the file format version.
-	version uint32
+	version Version
 
 	// sync indicates whether the file uses sync markers.
 	sync bool
@@ -68,8 +68,6 @@ func (pr *PkgDecoder) SyncMarkers() bool { return pr.sync }
 // NewPkgDecoder returns a PkgDecoder initialized to read the Unified
 // IR export data from input. pkgPath is the package path for the
 // compilation unit that produced the export data.
-//
-// TODO(mdempsky): Remove pkgPath parameter; unneeded since CL 391014.
 func NewPkgDecoder(pkgPath, input string) PkgDecoder {
 	pr := PkgDecoder{
 		pkgPath: pkgPath,
@@ -80,14 +78,15 @@ func NewPkgDecoder(pkgPath, input string) PkgDecoder {
 
 	r := strings.NewReader(input)
 
-	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
+	var ver uint32
+	assert(binary.Read(r, binary.LittleEndian, &ver) == nil)
+	pr.version = Version(ver)
 
-	switch pr.version {
+	if pr.version >= V2 { // TODO(taking): Switch to numVersions.
-	default:
+		panic(fmt.Errorf("cannot decode %q, export data version %d is too new", pkgPath, pr.version))
-		panicf("unsupported version: %v", pr.version)
+	}
-	case 0:
+
-		// no flags
+	if pr.version.Has(Flags) {
-	case 1:
 		var flags uint32
 		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
 		pr.sync = flags&flagSyncMarkers != 0
@@ -513,3 +512,6 @@ func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
 
 	return path, name, tag
 }
+
+// Version reports the version of the bitstream.
+func (w *Decoder) Version() Version { return w.common.version }

src/internal/pkgbits/encoder.go

@@ -15,20 +15,15 @@ import (
 	"strings"
 )
 
-// currentVersion is the current version number.
+// currentVersion is the current version number written.
-//
+const currentVersion = V1
-//   - v0: initial prototype
-//
-//   - v1: adds the flags uint32 word
-//
-// TODO(mdempsky): For the next version bump:
-//   - remove the legacy "has init" bool from the public root
-//   - remove obj's "derived func instance" bool
-const currentVersion uint32 = 1
 
 // A PkgEncoder provides methods for encoding a package's Unified IR
 // export data.
 type PkgEncoder struct {
+	// version of the bitstream.
+	version Version
+
 	// elems holds the bitstream for previously encoded elements.
 	elems [numRelocs][]string
 
@@ -54,6 +49,8 @@ func (pw *PkgEncoder) SyncMarkers() bool { return pw.syncFrames >= 0 }
 // negative, then sync markers are omitted entirely.
 func NewPkgEncoder(syncFrames int) PkgEncoder {
 	return PkgEncoder{
+		// TODO(taking): Change NewPkgEncoder to take a version as an argument, and remove currentVersion.
+		version:    currentVersion,
 		stringsIdx: make(map[string]Index),
 		syncFrames: syncFrames,
 	}
@@ -69,7 +66,7 @@ func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
 		assert(binary.Write(out, binary.LittleEndian, x) == nil)
 	}
 
-	writeUint32(currentVersion)
+	writeUint32(uint32(pw.version))
 
 	var flags uint32
 	if pw.SyncMarkers() {
@@ -392,3 +389,6 @@ func (w *Encoder) bigFloat(v *big.Float) {
 	b := v.Append(nil, 'p', -1)
 	w.String(string(b)) // TODO: More efficient encoding.
 }
+
+// Version reports the version of the bitstream.
+func (w *Encoder) Version() Version { return w.p.version }

src/internal/pkgbits/pkgbits_test.go

@@ -0,0 +1,67 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits_test
+
+import (
+	"internal/pkgbits"
+	"strings"
+	"testing"
+)
+
+func TestRoundTrip(t *testing.T) {
+	pw := pkgbits.NewPkgEncoder(-1)
+	w := pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPublic)
+	w.Flush()
+
+	var b strings.Builder
+	_ = pw.DumpTo(&b)
+	input := b.String()
+
+	pr := pkgbits.NewPkgDecoder("package_id", input)
+	r := pr.NewDecoder(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
+
+	if r.Version() != w.Version() {
+		t.Errorf("Expected reader version %q to be the writer version %q", r.Version(), w.Version())
+	}
+}
+
+// Type checker to enforce that know V* have the constant values they must have.
+var _ [0]bool = [pkgbits.V0]bool{}
+var _ [1]bool = [pkgbits.V1]bool{}
+
+func TestVersions(t *testing.T) {
+	type vfpair struct {
+		v pkgbits.Version
+		f pkgbits.Field
+	}
+
+	// has field tests
+	for _, c := range []vfpair{
+		{pkgbits.V1, pkgbits.Flags},
+		{pkgbits.V2, pkgbits.Flags},
+		{pkgbits.V0, pkgbits.HasInit},
+		{pkgbits.V1, pkgbits.HasInit},
+		{pkgbits.V0, pkgbits.DerivedFuncInstance},
+		{pkgbits.V1, pkgbits.DerivedFuncInstance},
+		{pkgbits.V2, pkgbits.AliasTypeParamNames},
+	} {
+		if !c.v.Has(c.f) {
+			t.Errorf("Expected version %v to have field %v", c.v, c.f)
+		}
+	}
+
+	// does not have field tests
+	for _, c := range []vfpair{
+		{pkgbits.V0, pkgbits.Flags},
+		{pkgbits.V2, pkgbits.HasInit},
+		{pkgbits.V2, pkgbits.DerivedFuncInstance},
+		{pkgbits.V0, pkgbits.AliasTypeParamNames},
+		{pkgbits.V1, pkgbits.AliasTypeParamNames},
+	} {
+		if c.v.Has(c.f) {
+			t.Errorf("Expected version %v to not have field %v", c.v, c.f)
+		}
+	}
+}

src/internal/pkgbits/version.go

@@ -0,0 +1,79 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+// Version indicates a version of a unified IR bitstream.
+// Each Version indicates the addition, removal, or change of
+// new data in the bitstream.
+//
+// These are serialized to disk and the interpretation remains fixed.
+type Version uint32
+
+const (
+	// V0: initial prototype.
+	//
+	// All data that is not assigned a Field is in version V0
+	// and has not been deprecated.
+	V0 Version = iota
+
+	// V1: adds the Flags uint32 word
+	V1
+
+	// V2: removes unused legacy fields and supports type parameters for aliases.
+	// - remove the legacy "has init" bool from the public root
+	// - remove obj's "derived func instance" bool
+	// - add a TypeParamNames field to ObjAlias
+	V2
+
+	numVersions = iota
+)
+
+// Field denotes a unit of data in the serialized unified IR bitstream.
+// It is conceptually a like field in a structure.
+//
+// We only really need Fields when the data may or may not be present
+// in a stream based on the Version of the bitstream.
+//
+// Unlike much of pkgbits, Fields are not serialized and
+// can change values as needed.
+type Field int
+
+const (
+	// Flags in a uint32 in the header of a bitstream
+	// that is used to indicate whether optional features are enabled.
+	Flags Field = iota
+
+	// Deprecated: HasInit was a bool indicating whether a package
+	// has any init functions.
+	HasInit
+
+	// Deprecated: DerivedFuncInstance was a bool indicating
+	// whether an object was a function instance.
+	DerivedFuncInstance
+
+	// ObjAlias has a list of TypeParamNames.
+	AliasTypeParamNames
+
+	numFields = iota
+)
+
+// introduced is the version a field was added.
+var introduced = [numFields]Version{
+	Flags:               V1,
+	AliasTypeParamNames: V2,
+}
+
+// removed is the version a field was removed in or 0 for fields
+// that have not yet been deprecated.
+// (So removed[f]-1 is the last version it is included in.)
+var removed = [numFields]Version{
+	HasInit:             V2,
+	DerivedFuncInstance: V2,
+}
+
+// Has reports whether field f is present in a bitstream at version v.
+func (v Version) Has(f Field) bool {
+	return introduced[f] <= v && (v < removed[f] || removed[f] == V0)
+}