Ensure state of PartGenerator accepts buffers

This commit makes sure that the current state of the PartGenerator
is able to handle an incoming buffer, possibly requested because of
a request coming from the subscription. All states accept new buffers
except the WritingFileState.

Closes gh-29227

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

@@ -229,6 +229,7 @@ final class PartGenerator extends BaseSubscriber<MultipartParser.Token> {
 		if (upstream() != null &&
 				!this.sink.isCancelled() &&
 				this.sink.requestedFromDownstream() > 0 &&
+				this.state.get().canRequest() &&
 				this.requestOutstanding.compareAndSet(false, true)) {
 			request(1);
 		}
@@ -291,6 +292,13 @@ final class PartGenerator extends BaseSubscriber<MultipartParser.Token> {
 		default void error(Throwable throwable) {
 		}
 
+		/**
+		 * Indicates whether the current state is ready to accept a new token.
+		 */
+		default boolean canRequest() {
+			return true;
+		}
+
 		/**
 		 * Cleans up any state.
 		 */
@@ -814,6 +822,11 @@ final class PartGenerator extends BaseSubscriber<MultipartParser.Token> {
 			}
 		}
 
+		@Override
+		public boolean canRequest() {
+			return false;
+		}
+
 		@Override
 		public void dispose() {
 			this.disposed = true;