Copied getAndSub() over from Reactor

Operators.getAndSub was removed in Reactor 3.0.1, this commit copies the
implementation over to AbstractRequestBodyPublisher, which used it.

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractRequestBodyPublisher.java

@@ -106,7 +106,7 @@ abstract class AbstractRequestBodyPublisher implements Publisher<DataBuffer> {
 		while (hasDemand()) {
 			DataBuffer dataBuffer = read();
 			if (dataBuffer != null) {
-				Operators.getAndSub(this.demand, 1L);
+				getAndSub(this.demand, 1L);
 				this.subscriber.onNext(dataBuffer);
 			}
 			else {
@@ -116,6 +116,29 @@ abstract class AbstractRequestBodyPublisher implements Publisher<DataBuffer> {
 		return false;
 	}
 
+	/**
+	 * Concurrent substraction bound to 0 and Long.MAX_VALUE.
+	 * Any concurrent write will "happen" before this operation.
+	 *
+	 * @param sequence current atomic to update
+	 * @param toSub    delta to sub
+	 * @return value before subscription, 0 or Long.MAX_VALUE
+	 */
+	private static long getAndSub(AtomicLong sequence, long toSub) {
+		long r;
+		long u;
+		do {
+			r = sequence.get();
+			if (r == 0 || r == Long.MAX_VALUE) {
+				return r;
+			}
+			u = Operators.subOrZero(r, toSub);
+		} while (!sequence.compareAndSet(r, u));
+
+		return r;
+	}
+
+
 	protected abstract void checkOnDataAvailable();
 
 	/**