Add direct Random parameter injection. Add AssertingRandom and synchronization-less Random factories.

Author: dweiss

etc/junit4-missing-features.txt

@@ -1,9 +1,5 @@
 [ai generated overview of junit4 features]
 
-*. injecting Random into test methods (based on the current context). Use non-synchronized
-Random implementation. Optionally (parameter?) enable verifying that this Random is not shared
-with other threads for reproducibility.
-
 4. Shuffled test execution order and seed annotations
    - @Seed on a class fixes the main seed, making execution fully deterministic
    - @Seeds / @Seed on a method pins a per-method seed for regression coverage

randomizedtesting-jupiter/src/main/java/com/carrotsearch/randomizedtesting/jupiter/AssertingRandom.java

@@ -0,0 +1,167 @@
+package com.carrotsearch.randomizedtesting.jupiter;
+
+import java.util.Locale;
+import java.util.Objects;
+import java.util.Random;
+
+/**
+ * A {@link Random} with a delegate, preventing {@link Random#setSeed(long)} and locked to only be
+ * usable by a single {@link Thread}.
+ */
+final class AssertingRandom extends Random {
+  private final Random delegate;
+  private final Thread ownerRef;
+  private final String ownerName;
+  private final StackTraceElement[] allocationStack;
+
+  /**
+   * Track out-of-context use of this {@link Random} instance. This introduces memory barriers and
+   * scheduling side effects but there's no other way to do it in any other way and sharing randoms
+   * across threads or test cases is very bad and worth tracking.
+   */
+  private volatile boolean valid = true;
+
+  /**
+   * Creates an instance to be used by <code>owner</code> thread and delegating to <code>delegate
+   * </code> until {@link #destroy()}ed.
+   */
+  public AssertingRandom(Thread owner, Random delegate) {
+    // Must be here, the only Random constructor. Has side effects on setSeed, see below.
+    super(0);
+
+    this.delegate = delegate;
+    this.ownerRef = Objects.requireNonNull(owner);
+    this.ownerName = owner.toString();
+    this.allocationStack = Thread.currentThread().getStackTrace();
+  }
+
+  @Override
+  protected int next(int bits) {
+    throw new RuntimeException("Shouldn't be reachable.");
+  }
+
+  @Override
+  public boolean nextBoolean() {
+    checkValid();
+    return delegate.nextBoolean();
+  }
+
+  @Override
+  public void nextBytes(byte[] bytes) {
+    checkValid();
+    delegate.nextBytes(bytes);
+  }
+
+  @Override
+  public double nextDouble() {
+    checkValid();
+    return delegate.nextDouble();
+  }
+
+  @Override
+  public float nextFloat() {
+    checkValid();
+    return delegate.nextFloat();
+  }
+
+  @Override
+  public double nextGaussian() {
+    checkValid();
+    return delegate.nextGaussian();
+  }
+
+  @Override
+  public int nextInt() {
+    checkValid();
+    return delegate.nextInt();
+  }
+
+  @Override
+  public int nextInt(int n) {
+    checkValid();
+    return delegate.nextInt(n);
+  }
+
+  @Override
+  public long nextLong() {
+    checkValid();
+    return delegate.nextLong();
+  }
+
+  @Override
+  public void setSeed(long seed) {
+    // This is an interesting case of observing uninitialized object from an instance method
+    // (this method is called from the superclass constructor).
+    if (seed == 0 && delegate == null) {
+      return;
+    }
+
+    throw noSetSeed();
+  }
+
+  @Override
+  public String toString() {
+    checkValid();
+    return delegate.toString();
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    checkValid();
+    return delegate.equals(obj);
+  }
+
+  @Override
+  public int hashCode() {
+    checkValid();
+    return delegate.hashCode();
+  }
+
+  /** This object will no longer be usable after this method is called. */
+  void destroy() {
+    this.valid = false;
+  }
+
+  private static final class StackTraceHolder extends Throwable {
+    public StackTraceHolder(String message) {
+      super(message);
+    }
+  }
+
+  /* */
+  private void checkValid() {
+    if (!valid) {
+      throw new RuntimeException(
+          "This Random instance has been invalidated and "
+              + "is probably used out of its allowed context (test or suite).");
+    }
+
+    if (Thread.currentThread() != ownerRef) {
+      Throwable allocationEx =
+          new StackTraceHolder(
+              "Original allocation stack for this Random (" + "allocated by " + ownerName + ")");
+      allocationEx.setStackTrace(allocationStack);
+      throw new RuntimeException(
+          String.format(
+              Locale.ROOT,
+              "This Random instance is tied to thread %s, can't access it from thread: %s "
+                  + "(Random instances must not be shared). Allocation stack is included as a nested exception.",
+              ownerName,
+              Thread.currentThread()),
+          allocationEx);
+    }
+  }
+
+  @Override
+  protected Object clone() throws CloneNotSupportedException {
+    checkValid();
+    throw new CloneNotSupportedException("Don't clone test Randoms.");
+  }
+
+  static RuntimeException noSetSeed() {
+    return new RuntimeException(
+        "Changing the seed of Random instances is forbidden, it breaks repeatability"
+            + " of tests. If you need a mutable instance of Random, create a new (local) instance,"
+            + " preferably with the initial seed acquired from this Random instance.");
+  }
+}

randomizedtesting-jupiter/src/main/java/com/carrotsearch/randomizedtesting/jupiter/Hashing.java

@@ -1,6 +1,8 @@
 package com.carrotsearch.randomizedtesting.jupiter;
 
-public class Hashing {
+/** Static hashing utilities. */
+public final class Hashing {
+  /** Bit mixer for {@code long} values. */
   public static long mix64(long k) {
     k ^= k >>> 33;
     k *= 0xff51afd7ed558ccdL;
@@ -10,7 +12,7 @@ public static long mix64(long k) {
     return k;
   }
 
-  /** String hash function redistributing over a long range. */
+  /** String hash function redistributing over a {@code long}. */
   public static long longHash(String v) {
     long h = 0;
     int length = v.length();

randomizedtesting-jupiter/src/main/java/com/carrotsearch/randomizedtesting/jupiter/RandomFactory.java

@@ -0,0 +1,7 @@
+package com.carrotsearch.randomizedtesting.jupiter;
+
+import java.util.Random;
+import java.util.function.LongFunction;
+
+/** Supplier of {@link Random} instances, given the initial seed value. */
+public interface RandomFactory extends LongFunction<Random> {}

randomizedtesting-jupiter/src/main/java/com/carrotsearch/randomizedtesting/jupiter/RandomizedContext.java

@@ -2,47 +2,39 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.Locale;
 import java.util.Objects;
 import java.util.Random;
-import java.util.function.LongFunction;
 import org.junit.jupiter.api.extension.ExtensionContext;
 
 public final class RandomizedContext {
   private final RandomizedContext parent;
-  private final Thread owner;
   private final Seed seed;
   final String contextId;
 
   private final SeedChain remainingSeedChain;
 
   private final Random random;
-  private final LongFunction<Random> seedToRandomFn;
+  private final RandomFactory randomFactory;
 
   RandomizedContext(
       String contextId,
       RandomizedContext parent,
-      Thread owner,
-      LongFunction<Random> seedToRandomFn,
+      RandomFactory randomFactory,
       Seed seed,
       SeedChain remainingSeedChain) {
     this.contextId = contextId;
     this.parent = parent;
-    this.owner = owner;
     this.remainingSeedChain = remainingSeedChain;
-    this.seedToRandomFn = seedToRandomFn;
+    this.randomFactory = randomFactory;
 
     assert !seed.isUnspecified();
     this.seed = seed;
-    this.random = seedToRandomFn.apply(seed.value());
+    this.random = randomFactory.apply(seed.value());
   }
 
   @Override
   public String toString() {
-    return "Randomized context ["
-        + ("seedChain=" + getSeedChain() + ",")
-        + ("thread=" + Threads.threadName(owner))
-        + "]";
+    return "Randomized context [" + ("seedChain=" + getSeedChain() + ",") + "]";
   }
 
   SeedChain getSeedChain() {
@@ -67,20 +59,10 @@ private RandomizedContext getParent() {
   }
 
   public Random getRandom() {
-    if (Thread.currentThread() != owner) {
-      throw new RuntimeException(
-          String.format(
-              Locale.ROOT,
-              "This %s instance is bound to thread %s, can't access it from thread: %s",
-              RandomizedContext.class.getName(),
-              owner,
-              Thread.currentThread()));
-    }
-
     return random;
   }
 
-  RandomizedContext deriveNew(Thread thread, ExtensionContext extensionContext) {
+  RandomizedContext deriveNew(ExtensionContext extensionContext) {
     // sanity check.
     {
       var id = extensionContext.getUniqueId();
@@ -99,11 +81,6 @@ RandomizedContext deriveNew(Thread thread, ExtensionContext extensionContext) {
     }
 
     return new RandomizedContext(
-        extensionContext.getUniqueId(),
-        this,
-        thread,
-        seedToRandomFn,
-        nextSeed,
-        firstAndRest.rest());
+        extensionContext.getUniqueId(), this, randomFactory, nextSeed, firstAndRest.rest());
   }
 }