Fix corruption of supplementary characters in AppendableByteArray

A high surrogate left unconsumed by the encoder was dropped between
append calls, so surrogate pairs written a character at a time by
JsonValueWriter were replaced with `?`.

See gh-51464

Signed-off-by: JaeHyunAn <98042706+yyuneu@users.noreply.github.com>

core/spring-boot/src/main/java/org/springframework/boot/json/AppendableByteArray.java

@@ -56,6 +56,8 @@ class AppendableByteArray implements Appendable {
 
 	private ByteBuffer out;
 
+	private char highSurrogate;
+
 	AppendableByteArray(Charset charset) {
 		this(charset, DEFAULT_INITIAL_SIZE, DEFAULT_EXPANSION_SIZE);
 	}
@@ -89,8 +91,19 @@ class AppendableByteArray implements Appendable {
 	}
 
 	private AppendableByteArray append(CharBuffer in) throws IOException {
-		CoderResult result = this.encoder.encode(in, this.out, false);
+		if (this.highSurrogate != 0) {
+			CharBuffer pending = CharBuffer.allocate(in.remaining() + 1);
+			pending.put(this.highSurrogate).put(in).flip();
+			this.highSurrogate = 0;
+			in = pending;
+		}
+		return append(in, false);
+	}
+
+	private AppendableByteArray append(CharBuffer in, boolean endOfInput) throws IOException {
+		CoderResult result = this.encoder.encode(in, this.out, endOfInput);
 		if (result.isUnderflow()) {
+			this.highSurrogate = (in.hasRemaining()) ? in.get() : 0;
 			return this;
 		}
 		if (result.isOverflow()) {
@@ -98,13 +111,18 @@ class AppendableByteArray implements Appendable {
 			this.out = ByteBuffer.allocate(out.capacity() + this.expansionSize);
 			out.flip();
 			this.out.put(out);
-			return append(in);
+			return append(in, endOfInput);
 		}
 		result.throwException();
 		return this;
 	}
 
-	byte[] toByteArray() {
+	byte[] toByteArray() throws IOException {
+		if (this.highSurrogate != 0) {
+			CharBuffer in = CharBuffer.wrap(new char[] { this.highSurrogate });
+			this.highSurrogate = 0;
+			append(in, true);
+		}
 		this.out.flip();
 		int limit = this.out.limit();
 		int position = this.out.position();
@@ -120,6 +138,7 @@ class AppendableByteArray implements Appendable {
 	private void reset() {
 		this.out.clear();
 		this.encoder.reset();
+		this.highSurrogate = 0;
 	}
 
 	static byte[] toByteArray(Charset charset, ThrowingConsumer<Appendable> appendable) throws IOException {

core/spring-boot/src/test/java/org/springframework/boot/json/AppendableByteArrayTests.java

@@ -90,6 +90,22 @@ class AppendableByteArrayTests {
 		assertThat(reused).isEqualTo("clean".getBytes(StandardCharsets.UTF_8));
 	}
 
+	@Test
+	void writesSurrogatePairAppendedAsIndividualChars() throws Exception {
+		assertByteArray(StandardCharsets.UTF_8, (appendable) -> appendable.append('\uD83D').append('\uDE00'));
+		assertByteArray(StandardCharsets.UTF_16, (appendable) -> appendable.append('\uD83D').append('\uDE00'));
+	}
+
+	@Test
+	void writesSurrogatePairSplitAcrossAppendedStrings() throws Exception {
+		assertByteArray(StandardCharsets.UTF_8, (appendable) -> appendable.append("a\uD83D").append("\uDE00b"));
+	}
+
+	@Test
+	void writesUnpairedHighSurrogate() throws Exception {
+		assertByteArray(StandardCharsets.UTF_8, (appendable) -> appendable.append('\uD83D'));
+	}
+
 	private void assertByteArray(Charset charset, ThrowingConsumer<Appendable> action) throws Exception {
 		assertByteArray(4, 4, charset, action);
 	}

core/spring-boot/src/test/java/org/springframework/boot/json/WritableJsonTests.java

@@ -66,6 +66,17 @@ class WritableJsonTests {
 		assertThat(writable.toByteArray()).isEqualTo("{}".getBytes());
 	}
 
+	@Test
+	void toByteArrayWhenContentIsAppendedCharByCharWritesSupplementaryCharacters() {
+		String emoji = new String(Character.toChars(0x1F600));
+		WritableJson writable = (out) -> {
+			for (int i = 0; i < emoji.length(); i++) {
+				out.append(emoji.charAt(i));
+			}
+		};
+		assertThat(writable.toByteArray(StandardCharsets.UTF_8)).isEqualTo(emoji.getBytes(StandardCharsets.UTF_8));
+	}
+
 	@Test
 	void toResourceWritesJson() throws Exception {
 		File file = new File(this.temp, "out.json");