make movePreviousSession idempotent wrt the previous session

Author: supervacuus

sentry/src/main/java/io/sentry/cache/EnvelopeCache.java

@@ -462,6 +462,10 @@ public void flushPreviousSession() {
   public void movePreviousSession(
       final @NotNull File currentSessionFile, final @NotNull File previousSessionFile) {
     try (final @NotNull ISentryLifecycleToken ignored = sessionLock.acquire()) {
+      if (!currentSessionFile.exists()) {
+        return;
+      }
+
       if (previousSessionFile.exists()) {
         options.getLogger().log(DEBUG, "Previous session file already exists, deleting it.");
         if (!previousSessionFile.delete()) {
@@ -471,19 +475,17 @@ public void movePreviousSession(
         }
       }
 
-      if (currentSessionFile.exists()) {
-        options.getLogger().log(INFO, "Moving current session to previous session.");
+      options.getLogger().log(INFO, "Moving current session to previous session.");
 
-        try {
-          final boolean renamed = currentSessionFile.renameTo(previousSessionFile);
-          if (!renamed) {
-            options.getLogger().log(WARNING, "Unable to move current session to previous session.");
-          }
-        } catch (Throwable e) {
-          options
-              .getLogger()
-              .log(SentryLevel.ERROR, "Error moving current session to previous session.", e);
+      try {
+        final boolean renamed = currentSessionFile.renameTo(previousSessionFile);
+        if (!renamed) {
+          options.getLogger().log(WARNING, "Unable to move current session to previous session.");
         }
+      } catch (Throwable e) {
+        options
+            .getLogger()
+            .log(SentryLevel.ERROR, "Error moving current session to previous session.", e);
       }
     }
   }

sentry/src/test/java/io/sentry/cache/EnvelopeCacheTest.kt

@@ -494,6 +494,37 @@ class EnvelopeCacheTest {
     assertFalse(previousSessionFile.exists())
   }
 
+  @Test
+  fun `movePreviousSession is idempotent wrt the previous session`() {
+    val cache = fixture.getSUT()
+
+    val currentSessionFile = EnvelopeCache.getCurrentSessionFile(fixture.options.cacheDirPath!!)
+    val previousSessionFile = EnvelopeCache.getPreviousSessionFile(fixture.options.cacheDirPath!!)
+
+    // Create a current session file
+    currentSessionFile.createNewFile()
+    currentSessionFile.writeText("session content from last run")
+
+    assertTrue(currentSessionFile.exists())
+    assertFalse(previousSessionFile.exists())
+
+    // First call: moves session.json -> previous_session.json
+    cache.movePreviousSession(currentSessionFile, previousSessionFile)
+
+    assertFalse(currentSessionFile.exists())
+    assertTrue(previousSessionFile.exists())
+    assertEquals("session content from last run", previousSessionFile.readText())
+
+    // Second call should be idempotent: previous_session.json should be preserved
+    // This simulates the race where both MovePreviousSession runnable and
+    // storeInternal (SessionStart) both call movePreviousSession
+    cache.movePreviousSession(currentSessionFile, previousSessionFile)
+
+    // previous_session.json should still exist with original content
+    assertTrue(previousSessionFile.exists())
+    assertEquals("session content from last run", previousSessionFile.readText())
+  }
+
   @Test
   fun `movePreviousSession deletes file and moves session when previous session file already exists`() {
     val cache = fixture.getSUT()