Fix ByteBufUtil#writeUtf8 subsequence split surrogate edge-case bug (#9437)

Motivation:

#9224 introduced overrides of ByteBufUtil#writeUtf8(...) and related
methods to operate on a sub-CharSequence directly to save having to
allocate substrings, but it missed an edge case where the subsequence
does not extend to the end of the CharSequence and the last char in the
sequence is a high surrogate.

Due to the catch-IndexOutOfBoundsException optimization that avoids an
additional bounds check, it would be possible to read past the specified
end char index and successfully decode a surrogate pair which would
otherwise result in a '?' byte being written.

Modifications:

- Check for end-of-subsequence before reading next char after a high
surrogate is encountered in the
writeUtf8(AbstractByteBuf,int,CharSequence,int,int) and
utf8BytesNonAscii methods
- Add unit test for this edge case

Result:

Bug is fixed.

This removes the bounds-check-avoidance optimization but it does not
appear to have a measurable impact on benchmark results, including when
the char sequence contains many surrogate pairs (which should be rare in
any case).

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -586,18 +586,13 @@ public final class ByteBufUtil {
                     buffer._setByte(writerIndex++, WRITE_UTF_UNKNOWN);
                     continue;
                 }
-                final char c2;
-                try {
-                    // Surrogate Pair consumes 2 characters. Optimistically try to get the next character to avoid
-                    // duplicate bounds checking with charAt. If an IndexOutOfBoundsException is thrown we will
-                    // re-throw a more informative exception describing the problem.
-                    c2 = seq.charAt(++i);
-                } catch (IndexOutOfBoundsException ignored) {
+                // Surrogate Pair consumes 2 characters.
+                if (++i == end) {
                     buffer._setByte(writerIndex++, WRITE_UTF_UNKNOWN);
                     break;
                 }
                 // Extra method to allow inlining the rest of writeUtf8 which is the most likely code path.
-                writerIndex = writeUtf8Surrogate(buffer, writerIndex, c, c2);
+                writerIndex = writeUtf8Surrogate(buffer, writerIndex, c, seq.charAt(i));
             } else {
                 buffer._setByte(writerIndex++, (byte) (0xe0 | (c >> 12)));
                 buffer._setByte(writerIndex++, (byte) (0x80 | ((c >> 6) & 0x3f)));
@@ -684,17 +679,13 @@ public final class ByteBufUtil {
                     // WRITE_UTF_UNKNOWN
                     continue;
                 }
-                final char c2;
-                try {
-                    // Surrogate Pair consumes 2 characters. Optimistically try to get the next character to avoid
-                    // duplicate bounds checking with charAt.
-                    c2 = seq.charAt(++i);
-                } catch (IndexOutOfBoundsException ignored) {
+                // Surrogate Pair consumes 2 characters.
+                if (++i == end) {
                     encodedLength++;
                     // WRITE_UTF_UNKNOWN
                     break;
                 }
-                if (!Character.isLowSurrogate(c2)) {
+                if (!Character.isLowSurrogate(seq.charAt(i))) {
                     // WRITE_UTF_UNKNOWN + (Character.isHighSurrogate(c2) ? WRITE_UTF_UNKNOWN : c2)
                     encodedLength += 2;
                     continue;

buffer/src/test/java/io/netty/buffer/ByteBufUtilTest.java

@@ -524,6 +524,20 @@ public class ByteBufUtilTest {
         buf2.release();
     }
 
+    @Test
+    public void testWriteUtf8SubsequenceSplitSurrogate() {
+        String usAscii = "\uD800\uDC00"; // surrogate pair: one code point, two chars
+        ByteBuf buf = Unpooled.buffer(16);
+        buf.writeBytes(usAscii.substring(0, 1).getBytes(CharsetUtil.UTF_8));
+        ByteBuf buf2 = Unpooled.buffer(16);
+        ByteBufUtil.writeUtf8(buf2, usAscii, 0, 1);
+
+        assertEquals(buf, buf2);
+
+        buf.release();
+        buf2.release();
+    }
+
     @Test
     public void testReserveAndWriteUtf8Subsequence() {
         String usAscii = "Some UTF-8 like äÄ∏ŒŒ";