SOLR-14969: Prevent creating multiple cores with the same name which leads to instabilities (race condition)

solr/CHANGES.txt

@@ -167,6 +167,9 @@ Bug Fixes
 
 * SOLR-14940: ReplicationHandler memory leak through SolrCore.closeHooks with unstable ZK connection. (Anver Sotnikov, Mike Drob)
 
+* SOLR-14969: Prevent creating multiple cores with the same name which leads to instabilities (race condition)
+  (Erick Erickson, Andreas Hubold)
+
 Other Changes
 ---------------------
 

solr/core/src/java/org/apache/solr/core/CoreContainer.java

@@ -30,6 +30,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Date;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
@@ -1244,6 +1245,7 @@ public class CoreContainer {
     return create(coreName, cfg.getCoreRootDirectory().resolve(coreName), parameters, false);
   }
 
+  Set<String> inFlightCreations = new HashSet<>(); // See SOLR-14969
   /**
    * Creates a new core in a specified instance directory, publishing the core state to the cluster
    *
@@ -1253,13 +1255,23 @@ public class CoreContainer {
    * @return the newly created core
    */
   public SolrCore create(String coreName, Path instancePath, Map<String, String> parameters, boolean newCollection) {
-
+    SolrCore core = null;
+    boolean iAdded = false;
+    try {
+      synchronized (inFlightCreations) {
+        if (inFlightCreations.add(coreName)) {
+          iAdded = true;
+        } else {
+          String msg = "Already creating a core with name '" + coreName + "', call aborted '";
+          log.warn(msg);
+          throw new SolrException(ErrorCode.SERVER_ERROR, msg);
+        }
+      }
       CoreDescriptor cd = new CoreDescriptor(coreName, instancePath, parameters, getContainerProperties(), getZkController());
 
-    // TODO: There's a race here, isn't there?
       // Since the core descriptor is removed when a core is unloaded, it should never be anywhere when a core is created.
       if (getAllCoreNames().contains(coreName)) {
-      log.warn("Creating a core with existing name is not allowed");
+        log.warn("Creating a core with existing name is not allowed: '{}'", coreName);
         // TODO: Shouldn't this be a BAD_REQUEST?
         throw new SolrException(ErrorCode.SERVER_ERROR, "Core with name '" + coreName + "' already exists.");
       }
@@ -1281,7 +1293,6 @@ public class CoreContainer {
         // first and clean it up if there's an error.
         coresLocator.create(this, cd);
 
-      SolrCore core = null;
         try {
           solrCores.waitAddPendingCoreOps(cd.getName());
           core = createFromDescriptor(cd, true, newCollection);
@@ -1325,6 +1336,13 @@ public class CoreContainer {
         throw new SolrException(SolrException.ErrorCode.BAD_REQUEST,
             "Error CREATEing SolrCore '" + coreName + "': " + ex.getMessage() + rootMsg, ex);
       }
+    } finally {
+      synchronized (inFlightCreations) {
+        if (iAdded) {
+          inFlightCreations.remove(coreName);
+        }
+      }
+    }
   }
 
   /**
@@ -1777,6 +1795,7 @@ public class CoreContainer {
     }
 
     if (cd == null) {
+      log.warn("Cannot unload non-existent core '{}'", name);
       throw new SolrException(ErrorCode.BAD_REQUEST, "Cannot unload non-existent core [" + name + "]");
     }
 

solr/core/src/test/org/apache/solr/core/TestCoreContainer.java

@@ -20,6 +20,7 @@ import java.io.File;
 import java.io.FileOutputStream;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
@@ -117,22 +118,27 @@ public class TestCoreContainer extends SolrTestCaseJ4 {
     }
   }
 
-  @Test
-  public void testReloadThreaded() throws Exception {
-    final CoreContainer cc = init(CONFIGSETS_SOLR_XML);
-    cc.create("core1", ImmutableMap.of("configSet", "minimal"));
-
-    class TestThread extends Thread {
+  private static class TestReloadThread extends Thread {
+    private final CoreContainer cc;
+    TestReloadThread(CoreContainer cc) {
+      this.cc = cc;
+    }
     @Override
     public void run() {
       cc.reload("core1");
     }
   }
 
+  @Test
+  public void testReloadThreaded() throws Exception {
+    final CoreContainer cc = init(CONFIGSETS_SOLR_XML);
+    cc.create("core1", ImmutableMap.of("configSet", "minimal"));
+
+
     List<Thread> threads = new ArrayList<>();
     int numThreads = 4;
     for (int i = 0; i < numThreads; i++) {
-      threads.add(new TestThread());
+      threads.add(new TestReloadThread(cc));
     }
 
     for (Thread thread : threads) {
@@ -147,6 +153,89 @@ public class TestCoreContainer extends SolrTestCaseJ4 {
 
   }
 
+  private static class TestCreateThread extends Thread {
+    final CoreContainer cc;
+    final String coreName;
+    boolean foundExpectedError = false;
+    SolrCore core = null;
+
+    TestCreateThread(CoreContainer cc, String coreName) {
+      this.cc = cc;
+      this.coreName = coreName;
+    }
+    @Override
+    public void run() {
+      try {
+        core = cc.create(coreName, ImmutableMap.of("configSet", "minimal"));
+      } catch (SolrException e) {
+        String msg = e.getMessage();
+        foundExpectedError = msg.contains("Already creating a core with name") || msg.contains("already exists");
+      }
+    }
+
+    int verifyMe() {
+      if (foundExpectedError) {
+        assertNull("failed create should have returned null for core", core);
+        return 0;
+      } else {
+        assertNotNull("Core should not be null if there was no error", core);
+        return 1;
+      }
+    }
+  }
+
+  @Test
+  public void testCreateThreaded() throws Exception {
+    final CoreContainer cc = init(CONFIGSETS_SOLR_XML);
+    final int NUM_THREADS = 3;
+
+
+    // Try this a few times to increase the chances of failure.
+    for (int idx = 0; idx < 3; ++idx) {
+      TestCreateThread[] threads = new TestCreateThread[NUM_THREADS];
+      // Let's add a little stress in here by using the same core name each
+      // time around. The final unload in the loop should delete the core and
+      // allow the next time around to succeed.
+      // This also checks the bookkeeping in CoreContainer.create
+      // that prevents muliple simulatneous creations,
+      // currently "inFlightCreations"
+      String testName = "coreToTest";
+      for (int thread = 0; thread < NUM_THREADS; ++thread) {
+        threads[thread] = new TestCreateThread(cc, testName);
+      }
+      // only one of these should succeed.
+      for (int thread = 0; thread < NUM_THREADS; ++thread) {
+        threads[thread].start();
+      }
+
+      for (int thread = 0; thread < NUM_THREADS; ++thread) {
+        threads[thread].join();
+      }
+
+      // We can't guarantee that which thread failed, so go find it
+      int goodCount = 0;
+      for (int thread = 0; thread < NUM_THREADS; ++thread) {
+        goodCount += threads[thread].verifyMe();
+      }
+      assertEquals("Only one create should have succeeded", 1, goodCount);
+
+
+      // Check bookkeeping by removing and creating the core again, making sure
+      // we didn't leave the record of trying to create this core around.
+      // NOTE: unloading the core closes it too.
+      cc.unload(testName, true, true, true);
+      cc.create(testName, ImmutableMap.of("configSet", "minimal"));
+      // This call should fail with a different error because the core was
+      // created successfully.
+      SolrException thrown = expectThrows(SolrException.class, () ->
+          cc.create(testName, ImmutableMap.of("configSet", "minimal")));
+      assertTrue("Should have 'already exists' error", thrown.getMessage().contains("already exists"));
+
+      cc.unload(testName, true, true, true);
+    }
+    cc.shutdown();
+  }
+
   @Test
   public void testNoCores() throws Exception {