Fix handling of bad utf16 passed out of JS

Summary:
JSC's utf16 -> utf8 conversion crashes on encountering bad utf16. Instead, use our own conversion (conveniently copied from fbjni).

Original fix thanks to rigdern (https://github.com/facebook/react-native/pull/9302)

Reviewed By: mhorowitz

Differential Revision: D3746947

fbshipit-source-id: 29887ca720f6a2b074f01f853bad28a083b273bc

ReactCommon/cxxreact/Unicode.cpp

@@ -0,0 +1,88 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+#include "Unicode.h"
+
+namespace facebook {
+namespace react {
+namespace unicode {
+namespace {
+
+// TODO(12827176): Don't duplicate this code here and fbjni.
+
+const uint16_t kUtf8OneByteBoundary       = 0x80;
+const uint16_t kUtf8TwoBytesBoundary      = 0x800;
+const uint16_t kUtf16HighSubLowBoundary   = 0xD800;
+const uint16_t kUtf16HighSubHighBoundary  = 0xDC00;
+const uint16_t kUtf16LowSubHighBoundary   = 0xE000;
+
+// Calculate how many bytes are needed to convert an UTF16 string into UTF8
+// UTF16 string
+size_t utf16toUTF8Length(const uint16_t* utf16String, size_t utf16StringLen) {
+  if (!utf16String || utf16StringLen == 0) {
+    return 0;
+  }
+
+  uint32_t utf8StringLen = 0;
+  auto utf16StringEnd = utf16String + utf16StringLen;
+  auto idx16 = utf16String;
+  while (idx16 < utf16StringEnd) {
+    auto ch = *idx16++;
+    if (ch < kUtf8OneByteBoundary) {
+      utf8StringLen++;
+    } else if (ch < kUtf8TwoBytesBoundary) {
+      utf8StringLen += 2;
+    } else if (
+        (ch >= kUtf16HighSubLowBoundary) && (ch < kUtf16HighSubHighBoundary) &&
+        (idx16 < utf16StringEnd) &&
+        (*idx16 >= kUtf16HighSubHighBoundary) && (*idx16 < kUtf16LowSubHighBoundary)) {
+      utf8StringLen += 4;
+      idx16++;
+    } else {
+      utf8StringLen += 3;
+    }
+  }
+
+  return utf8StringLen;
+}
+
+} // namespace
+
+std::string utf16toUTF8(const uint16_t* utf16String, size_t utf16StringLen) noexcept {
+  if (!utf16String || utf16StringLen <= 0) {
+    return "";
+  }
+
+  std::string utf8String(utf16toUTF8Length(utf16String, utf16StringLen), '\0');
+  auto idx8 = utf8String.begin();
+  auto idx16 = utf16String;
+  auto utf16StringEnd = utf16String + utf16StringLen;
+  while (idx16 < utf16StringEnd) {
+    auto ch = *idx16++;
+    if (ch < kUtf8OneByteBoundary) {
+      *idx8++ = (ch & 0x7F);
+    } else if (ch < kUtf8TwoBytesBoundary) {
+      *idx8++ = 0b11000000 | (ch >> 6);
+      *idx8++ = 0b10000000 | (ch & 0x3F);
+    } else if (
+        (ch >= kUtf16HighSubLowBoundary) && (ch < kUtf16HighSubHighBoundary) &&
+        (idx16 < utf16StringEnd) &&
+        (*idx16 >= kUtf16HighSubHighBoundary) && (*idx16 < kUtf16LowSubHighBoundary)) {
+      auto ch2 = *idx16++;
+      uint8_t trunc_byte = (((ch >> 6) & 0x0F) + 1);
+      *idx8++ = 0b11110000 | (trunc_byte >> 2);
+      *idx8++ = 0b10000000 | ((trunc_byte & 0x03) << 4) | ((ch >> 2) & 0x0F);
+      *idx8++ = 0b10000000 | ((ch & 0x03) << 4) | ((ch2 >> 6) & 0x0F);
+      *idx8++ = 0b10000000 | (ch2 & 0x3F);
+    } else {
+      *idx8++ = 0b11100000 | (ch >> 12);
+      *idx8++ = 0b10000000 | ((ch >> 6) & 0x3F);
+      *idx8++ = 0b10000000 | (ch & 0x3F);
+    }
+  }
+
+  return utf8String;
+}
+
+} // namespace unicode
+} // namespace react
+} // namespace facebook