Fix InputStream violation in DataBufferInputStream

This commit fixes an issue in DataBufferInputStream::mark, which before
did not follow the contract defined by InputStream.

Closes gh-29642

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferInputStream.java

@@ -77,8 +77,9 @@ final class DataBufferInputStream extends InputStream {
 	}
 
 	@Override
-	public void mark(int mark) {
-		this.mark = mark;
+	public void mark(int readLimit) {
+		Assert.isTrue(readLimit > 0, "readLimit must be greater than 0");
+		this.mark = this.dataBuffer.readPosition();
 	}
 
 	@Override

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -318,6 +318,9 @@ class DataBufferTests extends AbstractDataBufferAllocatingTests {
 		assertThat(result).isEqualTo((byte) 'b');
 		assertThat(inputStream.available()).isEqualTo(3);
 
+		assertThat(inputStream.markSupported()).isTrue();
+		inputStream.mark(2);
+
 		byte[] bytes = new byte[2];
 		int len = inputStream.read(bytes);
 		assertThat(len).isEqualTo(2);
@@ -333,6 +336,12 @@ class DataBufferTests extends AbstractDataBufferAllocatingTests {
 		assertThat(inputStream.read()).isEqualTo(-1);
 		assertThat(inputStream.read(bytes)).isEqualTo(-1);
 
+		inputStream.reset();
+		bytes = new byte[3];
+		len = inputStream.read(bytes);
+		assertThat(len).isEqualTo(3);
+		assertThat(bytes).containsExactly('c', 'd', 'e');
+
 		release(buffer);
 	}