Refine engine counter logic

Update counter logic to prevent negative values. Since the stop method
can now be called more than once, it was possible for the counter to
move into negative values.

See gh-8227

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainer.java

@@ -90,28 +90,34 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 		synchronized (this.monitor) {
 			try {
 				addInstanceIdToEngineName();
-
-				// Remove service connectors to that protocol binding doesn't happen yet
-				removeServiceConnectors();
-
-				// Start the server to trigger initialization listeners
-				this.tomcat.start();
-
-				// We can re-throw failure exception directly in the main thread
-				rethrowDeferredStartupExceptions();
-
-				Context context = findContext();
 				try {
-					ContextBindings.bindClassLoader(context, getNamingToken(context),
-							getClass().getClassLoader());
-				}
-				catch (NamingException ex) {
-					// Naming is not enabled. Continue
-				}
+					// Remove service connectors to that protocol binding doesn't happen
+					// yet
+					removeServiceConnectors();
 
-				// Unlike Jetty, all Tomcat threads are daemon threads. We create a
-				// blocking non-daemon to stop immediate shutdown
-				startDaemonAwaitThread();
+					// Start the server to trigger initialization listeners
+					this.tomcat.start();
+
+					// We can re-throw failure exception directly in the main thread
+					rethrowDeferredStartupExceptions();
+
+					Context context = findContext();
+					try {
+						ContextBindings.bindClassLoader(context, getNamingToken(context),
+								getClass().getClassLoader());
+					}
+					catch (NamingException ex) {
+						// Naming is not enabled. Continue
+					}
+
+					// Unlike Jetty, all Tomcat threads are daemon threads. We create a
+					// blocking non-daemon to stop immediate shutdown
+					startDaemonAwaitThread();
+				}
+				catch (Exception ex) {
+					containerCounter.decrementAndGet();
+					throw ex;
+				}
 			}
 			catch (Exception ex) {
 				throw new EmbeddedServletContainerException(
@@ -279,6 +285,7 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 	@Override
 	public void stop() throws EmbeddedServletContainerException {
 		synchronized (this.monitor) {
+			boolean wasStarted = this.started;
 			try {
 				this.started = false;
 				try {
@@ -294,7 +301,9 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 						"Unable to stop embedded Tomcat", ex);
 			}
 			finally {
-				containerCounter.decrementAndGet();
+				if (wasStarted) {
+					containerCounter.decrementAndGet();
+				}
 			}
 		}
 	}