Add PhasedObservation

Observation itself does not protect against start and stop being called
multiple times. This commit aligns all observation instances to instead
use an implementation that does have these guards in place.

Closes gh-14082

Author: jzheaux

web/src/main/java/org/springframework/security/web/server/ObservationWebFilterChainDecorator.java

@@ -258,21 +258,21 @@ default Observation after() {
 
 		class SimpleAroundWebFilterObservation implements AroundWebFilterObservation {
 
-			private final ObservationReference before;
+			private final PhasedObservation before;
 
-			private final ObservationReference after;
+			private final PhasedObservation after;
 
-			private final AtomicReference<ObservationReference> currentObservation = new AtomicReference<>(
-					ObservationReference.NOOP);
+			private final AtomicReference<PhasedObservation> currentObservation = new AtomicReference<>(
+					PhasedObservation.NOOP);
 
 			SimpleAroundWebFilterObservation(Observation before, Observation after) {
-				this.before = new ObservationReference(before);
-				this.after = new ObservationReference(after);
+				this.before = new PhasedObservation(before);
+				this.after = new PhasedObservation(after);
 			}
 
 			@Override
 			public Observation start() {
-				if (this.currentObservation.compareAndSet(ObservationReference.NOOP, this.before)) {
+				if (this.currentObservation.compareAndSet(PhasedObservation.NOOP, this.before)) {
 					this.before.start();
 					return this.before.observation;
 				}
@@ -389,73 +389,6 @@ public String toString() {
 				return this.currentObservation.get().observation.toString();
 			}
 
-			private static final class ObservationReference {
-
-				private static final ObservationReference NOOP = new ObservationReference(Observation.NOOP);
-
-				private final Lock lock = new ReentrantLock();
-
-				private final AtomicInteger state = new AtomicInteger(0);
-
-				private final Observation observation;
-
-				private ObservationReference(Observation observation) {
-					this.observation = observation;
-				}
-
-				private void start() {
-					try {
-						this.lock.lock();
-						if (this.state.compareAndSet(0, 1)) {
-							this.observation.start();
-						}
-					}
-					finally {
-						this.lock.unlock();
-					}
-				}
-
-				private void error(Throwable ex) {
-					try {
-						this.lock.lock();
-						if (this.state.get() == 1) {
-							this.observation.error(ex);
-						}
-					}
-					finally {
-						this.lock.unlock();
-					}
-				}
-
-				private void stop() {
-					try {
-						this.lock.lock();
-						if (this.state.compareAndSet(1, 2)) {
-							this.observation.stop();
-						}
-					}
-					finally {
-						this.lock.unlock();
-					}
-				}
-
-				private void close() {
-					try {
-						this.lock.lock();
-						if (this.state.compareAndSet(1, 3)) {
-							this.observation.stop();
-						}
-						else {
-							this.state.set(3);
-						}
-					}
-					finally {
-						this.lock.unlock();
-					}
-				}
-
-			}
-
 		}
 
 	}
@@ -537,10 +470,10 @@ default WebFilterChain wrap(WebFilterChain chain) {
 
 		class SimpleWebFilterObservation implements WebFilterObservation {
 
-			private final Observation observation;
+			private final PhasedObservation observation;
 
 			SimpleWebFilterObservation(Observation observation) {
-				this.observation = observation;
+				this.observation = new PhasedObservation(observation);
 			}
 
 			@Override
@@ -728,4 +661,116 @@ public boolean supportsContext(Observation.Context context) {
 
 	}
 
+	private static final class PhasedObservation implements Observation {
+
+		private static final PhasedObservation NOOP = new PhasedObservation(Observation.NOOP);
+
+		private final Lock lock = new ReentrantLock();
+
+		private final AtomicInteger phase = new AtomicInteger(0);
+
+		private final Observation observation;
+
+		private PhasedObservation(Observation observation) {
+			this.observation = observation;
+		}
+
+		@Override
+		public Observation contextualName(String contextualName) {
+			return this.observation.contextualName(contextualName);
+		}
+
+		@Override
+		public Observation parentObservation(Observation parentObservation) {
+			return this.observation.parentObservation(parentObservation);
+		}
+
+		@Override
+		public Observation lowCardinalityKeyValue(KeyValue keyValue) {
+			return this.observation.lowCardinalityKeyValue(keyValue);
+		}
+
+		@Override
+		public Observation highCardinalityKeyValue(KeyValue keyValue) {
+			return this.observation.highCardinalityKeyValue(keyValue);
+		}
+
+		@Override
+		public Observation observationConvention(ObservationConvention<?> observationConvention) {
+			return this.observation.observationConvention(observationConvention);
+		}
+
+		@Override
+		public Observation event(Event event) {
+			return this.observation.event(event);
+		}
+
+		@Override
+		public Context getContext() {
+			return this.observation.getContext();
+		}
+
+		@Override
+		public Scope openScope() {
+			return this.observation.openScope();
+		}
+
+		@Override
+		public PhasedObservation start() {
+			try {
+				this.lock.lock();
+				if (this.phase.compareAndSet(0, 1)) {
+					this.observation.start();
+				}
+			}
+			finally {
+				this.lock.unlock();
+			}
+			return this;
+		}
+
+		@Override
+		public PhasedObservation error(Throwable ex) {
+			try {
+				this.lock.lock();
+				if (this.phase.get() == 1) {
+					this.observation.error(ex);
+				}
+			}
+			finally {
+				this.lock.unlock();
+			}
+			return this;
+		}
+
+		@Override
+		public void stop() {
+			try {
+				this.lock.lock();
+				if (this.phase.compareAndSet(1, 2)) {
+					this.observation.stop();
+				}
+			}
+			finally {
+				this.lock.unlock();
+			}
+		}
+
+		void close() {
+			try {
+				this.lock.lock();
+				if (this.phase.compareAndSet(1, 3)) {
+					this.observation.stop();
+				}
+				else {
+					this.phase.set(3);
+				}
+			}
+			finally {
+				this.lock.unlock();
+			}
+		}
+
+	}
+
 }