[Spark] Rename lastCommitTimestamp to lastCommitFileModificationTimes…

…tamp (#2746)

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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

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- Describe what this PR changes.
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Renames LogSegment.lastCommitTimestamp to
lastCommitFileModificationTimestamp so as to distinguish this file
timestamp from in-commit-timestamp.

## How was this patch tested?

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Related to #2532 . 

Name-only change.

## Does this PR introduce _any_ user-facing changes?

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No

spark/src/main/scala/org/apache/spark/sql/delta/Snapshot.scala

@@ -105,7 +105,8 @@ class Snapshot(
    * is retrieved from the CommitInfo of the latest commit which
    * can result in an IO operation.
    */
-  def timestamp: Long = getInCommitTimestampOpt.getOrElse(logSegment.lastCommitTimestamp)
+  def timestamp: Long =
+    getInCommitTimestampOpt.getOrElse(logSegment.lastCommitFileModificationTimestamp)
 
   /**
    * Returns the inCommitTimestamp if ICT is enabled, otherwise returns None.

spark/src/main/scala/org/apache/spark/sql/delta/SnapshotManagement.scala

@@ -1136,7 +1136,7 @@ object SnapshotManagement {
     oldLogSegment.copy(
       version = committedVersion,
       deltas = oldLogSegment.deltas :+ commitFileStatus,
-      lastCommitTimestamp = commitFileStatus.getModificationTime)
+      lastCommitFileModificationTimestamp = commitFileStatus.getModificationTime)
   }
 }
 
@@ -1186,7 +1186,7 @@ object SerializableFileStatus {
  * @param version The Snapshot version to generate
  * @param deltas The delta commit files (.json) to read
  * @param checkpointProvider provider to give information about Checkpoint files.
- * @param lastCommitTimestamp The "unadjusted" file modification timestamp of the
+ * @param lastCommitFileModificationTimestamp The "unadjusted" file modification timestamp of the
  *          last commit within this segment. By unadjusted, we mean that the commit timestamps may
  *          not necessarily be monotonically increasing for the commits within this segment.
  */
@@ -1195,9 +1195,10 @@ case class LogSegment(
     version: Long,
     deltas: Seq[FileStatus],
     checkpointProvider: UninitializedCheckpointProvider,
-    lastCommitTimestamp: Long) {
+    lastCommitFileModificationTimestamp: Long) {
 
-  override def hashCode(): Int = logPath.hashCode() * 31 + (lastCommitTimestamp % 10000).toInt
+  override def hashCode(): Int =
+    logPath.hashCode() * 31 + (lastCommitFileModificationTimestamp % 10000).toInt
 
   /**
    * An efficient way to check if a cached Snapshot's contents actually correspond to a new
@@ -1206,7 +1207,8 @@ case class LogSegment(
   override def equals(obj: Any): Boolean = {
     obj match {
       case other: LogSegment =>
-        version == other.version && lastCommitTimestamp == other.lastCommitTimestamp &&
+        version == other.version &&
+          lastCommitFileModificationTimestamp == other.lastCommitFileModificationTimestamp &&
           logPath == other.logPath && checkpointProvider.version == other.checkpointProvider.version
       case _ => false
     }

spark/src/test/scala/org/apache/spark/sql/delta/DeltaLogSuite.scala

@@ -480,7 +480,7 @@ class DeltaLogSuite extends QueryTest
 
       // Store these for later usage
       val actions = deltaLog.snapshot.stateDS.collect()
-      val commitTimestamp = deltaLog.snapshot.logSegment.lastCommitTimestamp
+      val commitTimestamp = deltaLog.snapshot.logSegment.lastCommitFileModificationTimestamp
 
       checkAnswer(
         spark.read.format("delta").load(path),

spark/src/test/scala/org/apache/spark/sql/delta/InCommitTimestampSuite.scala

@@ -75,7 +75,7 @@ class InCommitTimestampSuite
         // File timestamp should be the same as snapshot.getTimestamp when inCommitTimestamp is not
         // enabled
         assert(
-          ver1Snapshot.logSegment.lastCommitTimestamp == ver1Snapshot.timestamp)
+          ver1Snapshot.logSegment.lastCommitFileModificationTimestamp == ver1Snapshot.timestamp)
 
         spark.sql(s"ALTER TABLE delta.`${tempDir.getAbsolutePath}`" +
           s"SET TBLPROPERTIES ('${DeltaConfigs.IN_COMMIT_TIMESTAMPS_ENABLED.key}' = 'true')")

spark/src/test/scala/org/apache/spark/sql/delta/OptimisticTransactionSuite.scala

@@ -432,15 +432,15 @@ class OptimisticTransactionSuite
 
       // preCommitLogSegment should not get updated until a commit is triggered
       assert(testTxn.preCommitLogSegment.version == 1)
-      assert(testTxn.preCommitLogSegment.lastCommitTimestamp < testTxnStartTs)
+      assert(testTxn.preCommitLogSegment.lastCommitFileModificationTimestamp < testTxnStartTs)
       assert(testTxn.preCommitLogSegment.deltas.size == 2)
       assert(testTxn.preCommitLogSegment.checkpointProvider.isEmpty)
 
       testTxn.commit(Seq.empty, ManualUpdate)
 
       // preCommitLogSegment should get updated to the version right before the txn commits
       assert(testTxn.preCommitLogSegment.version == 12)
-      assert(testTxn.preCommitLogSegment.lastCommitTimestamp < testTxnEndTs)
+      assert(testTxn.preCommitLogSegment.lastCommitFileModificationTimestamp < testTxnEndTs)
       assert(testTxn.preCommitLogSegment.deltas.size == 2)
       assert(testTxn.preCommitLogSegment.checkpointProvider.version == 10)
     }