Merge branch '4.1.x'

Closes gh-51521

loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/nio/file/NestedByteChannel.java

@@ -85,7 +85,7 @@ class NestedByteChannel implements SeekableByteChannel {
 		while (dst.remaining() > 0) {
 			int count = this.resources.getData().read(dst, this.position);
 			if (count <= 0) {
-				return (total != 0) ? 0 : count;
+				return (total != 0) ? total : count;
 			}
 			total += count;
 			this.position += count;

loader/spring-boot-loader/src/test/java/org/springframework/boot/loader/nio/file/NestedByteChannelTests.java

@@ -118,11 +118,19 @@ class NestedByteChannelTests {
 	void readReadsBytesAndIncrementsPosition() throws IOException {
 		ByteBuffer dst = ByteBuffer.allocate(10);
 		assertThat(this.channel.position()).isZero();
-		this.channel.read(dst);
+		assertThat(this.channel.read(dst)).isEqualTo(10);
 		assertThat(this.channel.position()).isEqualTo(10L);
 		assertThat(dst.array()).isNotEqualTo(ByteBuffer.allocate(10).array());
 	}
 
+	@Test
+	void whenDataCannotFillBufferReadReturnsBytesRead() throws IOException {
+		ByteBuffer dst = ByteBuffer.allocate(8192);
+		assertThat(this.channel.position()).isZero();
+		assertThat(this.channel.read(dst)).isEqualTo(638);
+		assertThat(this.channel.position()).isEqualTo(638L);
+	}
+
 	@Test // gh-38592
 	void readReadsAsManyBytesAsPossible() throws Exception {
 		// ZipFileSystem checks that the number of bytes read matches an expected value