Check for Common FJP Having Zero Threads

src/main/java/oracle/r2dbc/OracleR2dbcOptions.java

@@ -56,9 +56,11 @@ private OracleR2dbcOptions() {}
    *   ...
    *   .build();
    * </pre>
-   * If this option is not configured, then Oracle R2DBC will use
-   * {@code ForkJoinPool}'s
-   * {@linkplain ForkJoinPool#commonPool() common pool} by default.
+   * If this option is not configured, then Oracle R2DBC will
+   * use the {@linkplain ForkJoinPool#commonPool() common ForkJoinPool} by
+   * default. However, if the common {@code ForkJoinPool} has a maximum pool
+   * size that is potentially zero, then a single-threaded {@code Executor} will
+   * be used by default.
    */
   public static final Option<Executor> EXECUTOR;
 

src/main/java/oracle/r2dbc/impl/OracleConnectionFactoryImpl.java

@@ -102,6 +102,20 @@
  */
 final class OracleConnectionFactoryImpl implements ConnectionFactory {
 
+  /**
+   * <p>
+   * The default executor when {@link OracleR2dbcOptions#EXECUTOR} is not
+   * configured. It will use the common {@code ForkJoinPool}, unless it has
+   * a maximum pool size of 0. See:
+   * https://github.com/oracle/oracle-r2dbc/issues/129
+   * </p>
+   */
+  private static final Executor DEFAULT_EXECUTOR =
+    "0".equals(System.getProperty(
+      "java.util.concurrent.ForkJoinPool.common.parallelism"))
+      ? new ForkJoinPool(1)
+      : ForkJoinPool.commonPool();
+
   /** JDBC data source that this factory uses to open connections */
   private final DataSource dataSource;
 
@@ -200,7 +214,7 @@ final class OracleConnectionFactoryImpl implements ConnectionFactory {
 
     Object executor = options.getValue(OracleR2dbcOptions.EXECUTOR);
     if (executor == null) {
-      this.executor = ForkJoinPool.commonPool();
+      this.executor = DEFAULT_EXECUTOR;
     }
     else if (executor instanceof Executor) {
       this.executor = (Executor) executor;
@@ -267,4 +281,5 @@ public Publisher<Connection> create() {
   public ConnectionFactoryMetadata getMetadata() {
     return () -> "Oracle Database";
   }
+
 }

src/test/java/oracle/r2dbc/impl/OracleStatementImplTest.java

@@ -3222,7 +3222,7 @@ private void verifyConcurrentFetch(Connection connection) {
       // Create many statements and execute them in parallel.
       @SuppressWarnings({"unchecked","rawtypes"})
       Publisher<Long>[] publishers =
-        new Publisher[Runtime.getRuntime().availableProcessors() * 4];
+        new Publisher[Runtime.getRuntime().availableProcessors() * 2];
 
       for (int i = 0; i < publishers.length; i++) {