Count in memory buffered data against limit in PartGeenrator

The new PartGenerator for parsing multipart requests supports buffering
the content in memory and switching to a file after a configured size.
More specifically, when a multipart part exceeds maxInMemorySize and
InMemoryState switches it over to FileState, the bytes that were
already buffered in memory are flushed to the temp file.

Prior to this commit, this was done via FileState.writeBuffer(), meaning
that the in memory buffered data would not be counted against the
configured limit for writing to a file.
This commit fixes this by writing buffered data with FileState.onBody().

Fixes gh-37238

spring-web/src/main/java/org/springframework/http/converter/multipart/PartGenerator.java

@@ -293,7 +293,7 @@ final class PartGenerator implements MultipartParser.PartListener {
 
 		private void switchToFile(DataBuffer current, boolean last) {
 			FileState fileState = new FileState(this.headers, PartGenerator.this.fileStorageDirectory);
-			this.content.forEach(fileState::writeBuffer);
+			this.content.forEach(buffer -> fileState.onBody(buffer, false));
 			fileState.onBody(current, last);
 			PartGenerator.this.state = fileState;
 		}

spring-web/src/test/java/org/springframework/http/converter/multipart/MultipartHttpMessageConverterTests.java

@@ -236,6 +236,17 @@ class MultipartHttpMessageConverterTests {
 					.isInstanceOf(HttpMessageConversionException.class).hasMessage("Part exceeded the disk usage limit of 35 bytes");
 		}
 
+		@Test // gh-37238
+		void readMultipartMaxDiskUsageExceededAcrossBufferedPrefix() throws Exception {
+			MockHttpInputMessage response = createMultipartResponse("large-file.multipart", "B");
+			converter.setMaxInMemorySize(1935);
+			// just over the max in memory, will only fail if buffered memory is counted for when writing to file
+			converter.setMaxDiskUsagePerPart(1945);
+			assertThatThrownBy(() -> converter.read(ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, Part.class), response, null))
+					.isInstanceOf(HttpMessageConversionException.class)
+					.hasMessage("Part exceeded the disk usage limit of 1945 bytes");
+		}
+
 		@Test
 		void readMultipartUnnamedPart() throws Exception {
 			MockHttpInputMessage response = createMultipartResponse("simple.multipart", "simple-boundary");

spring-web/src/test/resources/org/springframework/http/multipart/large-file.multipart

@@ -0,0 +1,6 @@
+--B
+Content-Disposition: form-data; name="file"; filename="test.txt"
+Content-Type: application/octet-stream
+
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+--B--