fix(discovery): check container discovery domain socket readability b…

…efore start

src/main/java/io/cryostat/discovery/ContainerDiscovery.java

@@ -18,6 +18,7 @@
 import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URISyntaxException;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
@@ -36,6 +37,7 @@
 
 import io.cryostat.URIUtil;
 import io.cryostat.core.net.JFRConnectionToolkit;
+import io.cryostat.core.sys.FileSystem;
 import io.cryostat.targets.Target;
 import io.cryostat.targets.Target.Annotations;
 import io.cryostat.targets.Target.EventKind;
@@ -131,6 +133,7 @@ public abstract class ContainerDiscovery {
     public static final String JMX_PORT_LABEL = "io.cryostat.jmxPort";
 
     @Inject Logger logger;
+    @Inject FileSystem fs;
     @Inject Vertx vertx;
     @Inject WebClient webClient;
     @Inject JFRConnectionToolkit connectionToolkit;
@@ -146,6 +149,15 @@ void onStart(@Observes StartupEvent evt) {
             return;
         }
 
+        Path socketPath = Path.of(getSocket().path());
+        if (!(fs.isRegularFile(socketPath) && fs.isReadable(socketPath))) {
+            logger.errorv(
+                    "{0} enabled but socket {1} is not accessible!",
+                    getClass().getName(), socketPath);
+            return;
+        }
+        logger.infov("{0} started", getClass().getName());
+
         DiscoveryNode universe = DiscoveryNode.getUniverse();
         if (DiscoveryNode.getRealm(getRealm()).isEmpty()) {
             DiscoveryPlugin plugin = new DiscoveryPlugin();