encoding: make TextDecoder handle BOM correctly

Do not accept the BOM if it comes from a different encoding, and
only discard the BOM after it has actually been read (including
when it is spread over multiple chunks in streaming mode).

Fixes: #25315

Author: addaleax

lib/internal/encoding.js

@@ -484,25 +484,22 @@ function makeTextDecoderJS() {
         this[kFlags] |= CONVERTER_FLAGS_FLUSH;
       }
 
-      if (!this[kBOMSeen] && !(this[kFlags] & CONVERTER_FLAGS_IGNORE_BOM)) {
-        if (this[kEncoding] === 'utf-8') {
-          if (input.length >= 3 &&
-              input[0] === 0xEF && input[1] === 0xBB && input[2] === 0xBF) {
-            input = input.slice(3);
-          }
-        } else if (this[kEncoding] === 'utf-16le') {
-          if (input.length >= 2 && input[0] === 0xFF && input[1] === 0xFE) {
-            input = input.slice(2);
-          }
+      let result = this[kFlags] & CONVERTER_FLAGS_FLUSH ?
+        this[kHandle].end(input) :
+        this[kHandle].write(input);
+
+      if (result.length > 0 &&
+          !this[kBOMSeen] &&
+          !(this[kFlags] & CONVERTER_FLAGS_IGNORE_BOM)) {
+        // If the very first result in the stream is a BOM, and we are not
+        // explicitly told to ignore it, then we discard it.
+        if (result[0] === '\ufeff') {
+          result = result.slice(1);
         }
         this[kBOMSeen] = true;
       }
 
-      if (this[kFlags] & CONVERTER_FLAGS_FLUSH) {
-        return this[kHandle].end(input);
-      }
-
-      return this[kHandle].write(input);
+      return result;
     }
   }
 

src/node_buffer.cc

@@ -219,10 +219,10 @@ size_t Length(Local<Object> obj) {
 }
 
 
-inline MaybeLocal<Uint8Array> New(Environment* env,
-                                  Local<ArrayBuffer> ab,
-                                  size_t byte_offset,
-                                  size_t length) {
+MaybeLocal<Uint8Array> New(Environment* env,
+                           Local<ArrayBuffer> ab,
+                           size_t byte_offset,
+                           size_t length) {
   CHECK(!env->buffer_prototype_object().IsEmpty());
   Local<Uint8Array> ui = Uint8Array::New(ab, byte_offset, length);
   Maybe<bool> mb =

src/node_i18n.cc

@@ -95,6 +95,7 @@ using v8::NewStringType;
 using v8::Object;
 using v8::ObjectTemplate;
 using v8::String;
+using v8::Uint8Array;
 using v8::Value;
 
 namespace i18n {
@@ -227,25 +228,41 @@ class ConverterObject : public BaseObject, Converter {
     const char* source = input.data();
     size_t source_length = input.length();
 
-    if (converter->unicode_ && !converter->ignoreBOM_ && !converter->bomSeen_) {
-      int32_t bomOffset = 0;
-      ucnv_detectUnicodeSignature(source, source_length, &bomOffset, &status);
-      source += bomOffset;
-      source_length -= bomOffset;
-      converter->bomSeen_ = true;
-    }
-
     UChar* target = *result;
     ucnv_toUnicode(converter->conv,
                    &target, target + (limit * sizeof(UChar)),
                    &source, source + source_length,
                    nullptr, flush, &status);
 
     if (U_SUCCESS(status)) {
-      if (limit > 0)
+      bool omit_initial_bom = false;
+      if (limit > 0) {
         result.SetLength(target - &result[0]);
+        if (result.length() > 0 &&
+            converter->unicode_ &&
+            !converter->ignoreBOM_ &&
+            !converter->bomSeen_) {
+          // If the very first result in the stream is a BOM, and we are not
+          // explicitly told to ignore it, then we mark it for discarding.
+          if (result[0] == 0xFEFF) {
+            omit_initial_bom = true;
+          }
+          converter->bomSeen_ = true;
+        }
+      }
       ret = ToBufferEndian(env, &result);
-      args.GetReturnValue().Set(ret.ToLocalChecked());
+      if (omit_initial_bom && !ret.IsEmpty()) {
+        // Peform `ret = ret.slice(2)`.
+        CHECK(ret.ToLocalChecked()->IsUint8Array());
+        Local<Uint8Array> orig_ret = ret.ToLocalChecked().As<Uint8Array>();
+        ret = Buffer::New(env,
+                          orig_ret->Buffer(),
+                          orig_ret->ByteOffset() + 2,
+                          orig_ret->ByteLength() - 2)
+                              .FromMaybe(Local<Uint8Array>());
+      }
+      if (!ret.IsEmpty())
+        args.GetReturnValue().Set(ret.ToLocalChecked());
       return;
     }
 

src/node_internals.h

@@ -158,7 +158,11 @@ v8::MaybeLocal<v8::Object> New(Environment* env,
                                char* data,
                                size_t length,
                                bool uses_malloc);
-
+// Creates a Buffer instance over an existing Uint8Array.
+v8::MaybeLocal<v8::Uint8Array> New(Environment* env,
+                                   v8::Local<v8::ArrayBuffer> ab,
+                                   size_t byte_offset,
+                                   size_t length);
 // Construct a Buffer from a MaybeStackBuffer (and also its subclasses like
 // Utf8Value and TwoByteValue).
 // If |buf| is invalidated, an empty MaybeLocal is returned, and nothing is

test/wpt/status/encoding.json

@@ -22,10 +22,7 @@
     "fail": "iso-2022-jp decoder state handling bug: https://encoding.spec.whatwg.org/#iso-2022-jp-decoder"
   },
   "textdecoder-byte-order-marks.any.js": {
-    "fail": "Mismatching BOM should not be ignored"
-  },
-  "textdecoder-copy.any.js": {
-    "fail": "Should not have output BOM: https://encoding.spec.whatwg.org/#concept-td-serialize"
+    "requires": ["small-icu"]
   },
   "textdecoder-fatal-single-byte.any.js": {
     "requires": ["full-icu"],