[Spark] Add nextObserver and the PhaseLockingExecutionObserver for th…

…e Concurrency Testing Framework (delta-io#2932)

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#### Which Delta project/connector is this regarding?
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- [X] Spark
- [ ] Standalone
- [ ] Flink
- [ ] Kernel
- [ ] Other (fill in here)

## Description
Add the `nextObserver` and the `PhaseLockingExecutionObserver` for the
Concurrency Testing Framework, as we will need it to write Row Tracking
Backfill conflicts with other commands tests when we add Row Tracking
Backfill.
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- Describe what this PR changes.
- Describe why we need the change.
 
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## How was this patch tested?
Added UTs.
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## Does this PR introduce _any_ user-facing changes?
No.
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spark/src/main/scala/org/apache/spark/sql/delta/TransactionExecutionObserver.scala

@@ -16,14 +16,32 @@
 
 package org.apache.spark.sql.delta
 
+trait ChainableExecutionObserver[O] {
+  /**
+   * The next txn observer for this thread.
+   * The next observer is used to test threads that perform multiple transactions, i.e.
+   * commands that perform multiple commits.
+   */
+  @volatile protected var nextObserver: Option[O] = None
+
+  /** Set the next observer for this thread. */
+  def setNextObserver(nextTxnObserver: O): Unit = {
+    nextObserver = Some(nextTxnObserver)
+  }
+
+  /** Update the observer of this thread with the next observer. */
+  def advanceToNextThreadObserver(): Unit
+}
+
 /**
  * Track different stages of the execution of a transaction.
  *
  * This is mostly meant for test instrumentation.
  *
  * The default is a no-op implementation.
  */
-trait TransactionExecutionObserver {
+trait TransactionExecutionObserver
+  extends ChainableExecutionObserver[TransactionExecutionObserver] {
   /*
    * This is called outside the transaction object,
    * since it wraps its creation.
@@ -59,6 +77,11 @@ trait TransactionExecutionObserver {
    *         This occurs when there is an Exception thrown during the transaction's body.
    */
   def transactionAborted(): Unit
+
+  override def advanceToNextThreadObserver(): Unit = {
+    TransactionExecutionObserver.threadObserver.set(
+      nextObserver.getOrElse(NoOpTransactionExecutionObserver))
+  }
 }
 
 object TransactionExecutionObserver {

spark/src/main/scala/org/apache/spark/sql/delta/fuzzer/ExecutionPhaseLock.scala

@@ -65,4 +65,7 @@ case class ExecutionPhaseLock(
     waitToEnter()
     leave()
   }
+
+  /** Blocks at this point until the phase has been left. */
+  def waitToLeave(): Unit = exitBarrier.waitToPass()
 }

spark/src/main/scala/org/apache/spark/sql/delta/fuzzer/PhaseLockingExecutionObserver.scala

@@ -0,0 +1,29 @@
+/*
+ * Copyright (2021) The Delta Lake Project Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.delta.fuzzer
+
+/**
+ * Trait representing execution observers that rely on phases with entry and exit barriers to
+ * control the order of execution of the observed code paths. See [[ExecutionPhaseLock]].
+ */
+trait PhaseLockingExecutionObserver {
+
+  val phaseLocks: Seq[ExecutionPhaseLock]
+
+  /** Return `true` if we have left all phases, `false` otherwise. */
+  def allPhasesHavePassed: Boolean = phaseLocks.forall(_.hasLeft)
+}

spark/src/main/scala/org/apache/spark/sql/delta/fuzzer/PhaseLockingTransactionExecutionObserver.scala

@@ -20,7 +20,22 @@ import org.apache.spark.sql.delta.{OptimisticTransaction, TransactionExecutionOb
 
 private[delta] class PhaseLockingTransactionExecutionObserver(
     val phases: OptimisticTransactionPhases)
-  extends TransactionExecutionObserver {
+  extends TransactionExecutionObserver
+  with PhaseLockingExecutionObserver {
+
+  override val phaseLocks: Seq[ExecutionPhaseLock] = Seq(
+    phases.initialPhase,
+    phases.preparePhase,
+    phases.commitPhase)
+
+  /**
+   * When set to true this observer will automatically update the thread's current observer to
+   * the next one. Also, it will not unblock the exit barrier of the commit phase automatically.
+   * Instead, the caller will have to automatically unblock it. This allows writing tests that
+   * can capture errors caused by code written between the end of the last txn and the start of
+   * the next txn.
+   */
+  @volatile protected var autoAdvanceNextObserver: Boolean = false
 
   override def startingTransaction(f: => OptimisticTransaction): OptimisticTransaction =
     phases.initialPhase.execute(f)
@@ -29,14 +44,52 @@ private[delta] class PhaseLockingTransactionExecutionObserver(
 
   override def beginDoCommit(): Unit = phases.commitPhase.waitToEnter()
 
-  override def transactionCommitted(): Unit = phases.commitPhase.leave()
+  override def transactionCommitted(): Unit = {
+    if (nextObserver.nonEmpty && autoAdvanceNextObserver) {
+      waitForCommitPhaseAndAdvanceToNextObserver()
+    } else {
+      phases.commitPhase.leave()
+    }
+  }
 
   override def transactionAborted(): Unit = {
     if (!phases.commitPhase.hasEntered) {
-      // If an Exception was thrown earlier we may not have called `beginDoCommit`, yet.
-      phases.commitPhase.passThrough()
+      phases.commitPhase.waitToEnter()
+    }
+    if (nextObserver.nonEmpty && autoAdvanceNextObserver) {
+      waitForCommitPhaseAndAdvanceToNextObserver()
     } else {
       phases.commitPhase.leave()
     }
   }
+
+  /*
+   * Wait for the commit phase to pass but do not unblock it so that callers can write tests
+   * that capture errors caused by code between the end of the last txn and the start of the
+   * new txn. After the commit phase is passed, update the thread observer of the thread to
+   * the next observer.
+   */
+  def waitForCommitPhaseAndAdvanceToNextObserver(): Unit = {
+    require(nextObserver.nonEmpty)
+    phases.commitPhase.waitToLeave()
+    advanceToNextThreadObserver()
+  }
+
+  /**
+   * Set the next observer, which will replace the txn observer on the thread after a successful
+   * commit. This method only works as expected if we haven't entered the commit phase yet.
+   *
+   * Note that when a next observer is set, the caller needs to manually unblock the exit barrier
+   * of the commit phase.
+   *
+   * For example, see [[waitForCommitPhaseAndAdvanceToNextObserver]].
+   */
+  def setNextObserver(
+      nextTxnObserver: TransactionExecutionObserver,
+      autoAdvance: Boolean): Unit = {
+    setNextObserver(nextTxnObserver)
+    autoAdvanceNextObserver = autoAdvance
+  }
+
+  override def advanceToNextThreadObserver(): Unit = super.advanceToNextThreadObserver()
 }

spark/src/test/scala/org/apache/spark/sql/delta/concurrency/TransactionExecutionObserverSuite.scala

@@ -34,9 +34,10 @@ import org.apache.spark.sql.test.SharedSparkSession
  */
 class TransactionExecutionObserverSuite extends QueryTest with SharedSparkSession
   with DeltaSQLCommandTest
-  with PhaseLockingTestMixin {
+  with PhaseLockingTestMixin
+  with TransactionExecutionTestMixin {
 
-  val timeout: FiniteDuration = 10000.millis
+  override val timeout: FiniteDuration = 10000.millis
 
   test("Phase Locking - sequential") {
     withTempDir { tempFile =>
@@ -221,4 +222,51 @@ class TransactionExecutionObserverSuite extends QueryTest with SharedSparkSessio
       assert(res.isEmpty)
     }
   }
+
+  test("Phase Locking - set next observer after commit") {
+    withTempDir { tempFile =>
+      val tempPath = tempFile.toString
+
+      spark.range(end = 1).write.format("delta").save(tempPath)
+
+      val observer = new PhaseLockingTransactionExecutionObserver(
+        OptimisticTransactionPhases.forName("test-txn"))
+      val deltaLog = DeltaLog.forTable(spark, tempPath)
+      val initialTableVersion = deltaLog.update().version
+
+      // get things started
+      val replacementObserver = new PhaseLockingTransactionExecutionObserver(
+        OptimisticTransactionPhases.forName("test-replacement-txn"))
+
+      observer.setNextObserver(replacementObserver, autoAdvance = true)
+      unblockAllPhases(observer)
+
+      TransactionExecutionObserver.withObserver(observer) {
+        deltaLog.withNewTransaction { txn =>
+          observer.phases.commitPhase.exitBarrier.unblock()
+          val removedFiles = txn.snapshot.allFiles.collect().map(_.remove).toSeq
+          txn.commit(removedFiles, DeltaOperations.ManualUpdate)
+        }
+        val tableVersionAfterFirstTxn = deltaLog.update().version
+        assert(tableVersionAfterFirstTxn === initialTableVersion + 1,
+          "expected a successful commit")
+        // Check that we cannot re-use the old observer, with unblocks.
+        assertThrows[IllegalStateTransitionException] {
+          observer.phases.preparePhase.entryBarrier.unblock()
+        }
+
+        // Check that we can use the replaced observer to control a subsequent commit on the same
+        // thread.
+        val oldMetadata = deltaLog.update().metadata
+        val newMetadata = oldMetadata.copy(configuration = Map("foo" -> "bar"))
+        unblockAllPhases(replacementObserver)
+        deltaLog.withNewTransaction { txn =>
+          txn.commit(Seq(newMetadata), DeltaOperations.ManualUpdate)
+        }
+        assert(deltaLog.update().version === tableVersionAfterFirstTxn + 1,
+          "expected a successful commit")
+        assert(replacementObserver.allPhasesHavePassed)
+      }
+    }
+  }
 }