mtproto,rpc: support Android legacy startup

(cherry picked from commit 06b6ae2bb2f90bd7cc1d6a8404a82aff507e6821)

internal/rpc/compat.go

@@ -0,0 +1,44 @@
+package rpc
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.uber.org/zap"
+
+	"github.com/gotd/td/bin"
+)
+
+// dispatchCompat handles explicitly allowlisted legacy TL constructors that are
+// still emitted by supported clients but are absent from gotd's pinned schema.
+func (r *Router) dispatchCompat(ctx context.Context, b *bin.Buffer, id uint32) (bin.Encoder, bool, error) {
+	start := time.Now()
+	var (
+		enc  bin.Encoder
+		name string
+		err  error
+	)
+
+	switch id {
+	case legacyLangpackGetLanguagesTypeID:
+		name = "langpack.getLanguages#800fd57d"
+		enc, err = r.handleLegacyLangpackGetLanguages(ctx, b)
+	default:
+		return nil, false, nil
+	}
+
+	fields := append([]zap.Field{
+		zap.String("method", name),
+		zap.String("type_id", fmt.Sprintf("%#x", id)),
+		zap.Bool("compat", true),
+		zap.Duration("dur", time.Since(start)),
+	}, r.contextLogFields(ctx)...)
+	if err != nil {
+		fields = append(fields, zap.Error(err))
+		r.log.Info("RPC compat handled", fields...)
+	} else {
+		r.log.Debug("RPC compat handled", fields...)
+	}
+	return enc, true, err
+}

internal/rpc/langpack.go

@@ -2,12 +2,19 @@ package rpc
 
 import (
 	"context"
+	"strings"
 
+	"github.com/gotd/td/bin"
 	"github.com/gotd/td/tg"
 )
 
+const legacyLangpackGetLanguagesTypeID uint32 = 0x800fd57d
+
 // registerLangpack 注册 langpack.* RPC handler。
 func (r *Router) registerLangpack(d *tg.ServerDispatcher) {
+	d.OnLangpackGetLanguages(func(ctx context.Context, langPack string) ([]tg.LangPackLanguage, error) {
+		return r.langpackLanguages(ctx, langPack), nil
+	})
 	d.OnLangpackGetLangPack(func(ctx context.Context, req *tg.LangpackGetLangPackRequest) (*tg.LangPackDifference, error) {
 		if r.deps.LangPack == nil {
 			return &tg.LangPackDifference{LangCode: req.LangCode}, nil
@@ -39,3 +46,54 @@ func (r *Router) registerLangpack(d *tg.ServerDispatcher) {
 		return tgLangPackStrings(pack.Strings), nil
 	})
 }
+
+func (r *Router) handleLegacyLangpackGetLanguages(ctx context.Context, b *bin.Buffer) (bin.Encoder, error) {
+	if err := b.ConsumeID(legacyLangpackGetLanguagesTypeID); err != nil {
+		return nil, err
+	}
+	return &tg.LangPackLanguageVector{Elems: r.langpackLanguages(ctx, "")}, nil
+}
+
+func (r *Router) langpackLanguages(ctx context.Context, langPack string) []tg.LangPackLanguage {
+	if langPack == "" {
+		langPack = langPackFromClient(ctx)
+	}
+	_ = langPack
+	return []tg.LangPackLanguage{
+		{
+			Official:        true,
+			Name:            "English",
+			NativeName:      "English",
+			LangCode:        "en",
+			PluralCode:      "en",
+			StringsCount:    0,
+			TranslatedCount: 0,
+			TranslationsURL: "",
+		},
+		{
+			Official:        true,
+			Name:            "Chinese (Simplified)",
+			NativeName:      "Chinese (Simplified)",
+			LangCode:        "zh-hans",
+			PluralCode:      "zh",
+			StringsCount:    0,
+			TranslatedCount: 0,
+			TranslationsURL: "",
+		},
+	}
+}
+
+func langPackFromClient(ctx context.Context) string {
+	info, ok := ClientInfoFrom(ctx)
+	if !ok {
+		return "tdesktop"
+	}
+	if info.LangPack != "" {
+		return info.LangPack
+	}
+	client := strings.ToLower(info.DeviceModel + " " + info.SystemVersion + " " + info.AppVersion)
+	if strings.Contains(client, "android") {
+		return "android"
+	}
+	return "tdesktop"
+}

internal/rpc/langpack_compat_test.go

@@ -0,0 +1,54 @@
+package rpc
+
+import (
+	"context"
+	"testing"
+
+	"go.uber.org/zap/zaptest"
+
+	"github.com/gotd/td/bin"
+	"github.com/gotd/td/clock"
+	"github.com/gotd/td/tg"
+)
+
+func TestLangpackGetLanguagesCurrentAndLegacy(t *testing.T) {
+	r := New(Config{DC: 2, IP: "127.0.0.1", Port: 2398}, Deps{}, zaptest.NewLogger(t), clock.System)
+
+	t.Run("current layer", func(t *testing.T) {
+		var in bin.Buffer
+		if err := (&tg.LangpackGetLanguagesRequest{LangPack: "tdesktop"}).Encode(&in); err != nil {
+			t.Fatalf("encode request: %v", err)
+		}
+		assertLangpackLanguages(t, r, context.Background(), &in)
+	})
+
+	t.Run("legacy android no args", func(t *testing.T) {
+		var in bin.Buffer
+		in.PutID(legacyLangpackGetLanguagesTypeID)
+		ctx := WithClientInfo(context.Background(), ClientInfo{
+			DeviceModel: "Android",
+			AppVersion:  "12.7.3",
+			LangCode:    "en",
+		})
+		assertLangpackLanguages(t, r, ctx, &in)
+	})
+}
+
+func assertLangpackLanguages(t *testing.T, r *Router, ctx context.Context, in *bin.Buffer) {
+	t.Helper()
+	enc, err := r.Dispatch(ctx, [8]byte{}, 0, in)
+	if err != nil {
+		t.Fatalf("dispatch langpack.getLanguages: %v", err)
+	}
+	var out bin.Buffer
+	if err := enc.Encode(&out); err != nil {
+		t.Fatalf("encode response: %v", err)
+	}
+	var langs tg.LangPackLanguageVector
+	if err := langs.Decode(&out); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+	if len(langs.Elems) == 0 || langs.Elems[0].LangCode != "en" {
+		t.Fatalf("languages = %+v, want English entry", langs.Elems)
+	}
+}

internal/rpc/router.go

@@ -306,6 +306,9 @@ func (r *Router) dispatch(ctx context.Context, b *bin.Buffer, depth int) (bin.En
 		return r.dispatch(ctx, &bin.Buffer{Buf: inner.data}, depth+1)
 
 	default:
+		if enc, ok, err := r.dispatchCompat(ctx, b, id); ok {
+			return enc, err
+		}
 		start := time.Now()
 		enc, err := r.dispatcher.Handle(ctx, b)
 		dur := time.Since(start)