Fix StackOverflowError by deeply nested server-timestamps. (#4771)

* Fix StackOverflowError by deeply nested server-timestamps.

* Add comments about the Thread usage.

Author: ehsannas

firebase-firestore/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 * [feature] Add support for disjunctions in queries (`OR` queries).
+* [fixed] Fixed stack overflow caused by deeply nested server timestamps (#4702).
 
 # 24.4.4
 * [changed] Relaxed certain query validations performed by the SDK (#4231).

firebase-firestore/src/main/java/com/google/firebase/firestore/model/ServerTimestamps.java

@@ -62,6 +62,16 @@ public static Value valueOf(Timestamp localWriteTime, @Nullable Value previousVa
             .putFields(TYPE_KEY, encodedType)
             .putFields(LOCAL_WRITE_TIME_KEY, encodeWriteTime);
 
+    // We should avoid storing deeply nested server timestamp map values, because we never use the
+    // intermediate "previous values".
+    // For example:
+    // previous: 42L, add: t1, result: t1 -> 42L
+    // previous: t1,  add: t2, result: t2 -> 42L (NOT t2 -> t1 -> 42L)
+    // previous: t2,  add: t3, result: t3 -> 42L (NOT t3 -> t2 -> t1 -> 42L)
+    // previous: t3,  add: t4, result: t4 -> 42L (NOT t4 -> t3 -> t2 -> t1 -> 42L)
+    // `getPreviousValue` recursively traverses server timestamps to find the least recent Value.
+    previousValue =
+        isServerTimestamp(previousValue) ? getPreviousValue(previousValue) : previousValue;
     if (previousValue != null) {
       mapRepresentation.putFields(PREVIOUS_VALUE_KEY, previousValue);
     }

firebase-firestore/src/test/java/com/google/firebase/firestore/local/SQLiteLocalStoreTest.java

@@ -32,12 +32,29 @@
 import static java.util.Collections.emptyList;
 import static java.util.Collections.singletonList;
 
+import com.google.firebase.Timestamp;
 import com.google.firebase.firestore.FieldValue;
 import com.google.firebase.firestore.core.Query;
+import com.google.firebase.firestore.model.DocumentKey;
 import com.google.firebase.firestore.model.FieldIndex;
+import com.google.firebase.firestore.model.FieldPath;
+import com.google.firebase.firestore.model.ObjectValue;
+import com.google.firebase.firestore.model.ServerTimestamps;
+import com.google.firebase.firestore.model.mutation.FieldMask;
+import com.google.firebase.firestore.model.mutation.FieldTransform;
+import com.google.firebase.firestore.model.mutation.PatchMutation;
+import com.google.firebase.firestore.model.mutation.Precondition;
+import com.google.firebase.firestore.model.mutation.ServerTimestampOperation;
+import com.google.firestore.v1.Value;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
@@ -290,4 +307,63 @@ public void testIndexesServerTimestamps() {
     assertOverlayTypes(keyMap("coll/a", CountingQueryEngine.OverlayType.Set));
     assertQueryReturned("coll/a");
   }
+
+  @Test
+  public void testDeeplyNestedServerTimestamps() {
+    Timestamp timestamp = Timestamp.now();
+    Value initialServerTimestamp = ServerTimestamps.valueOf(timestamp, null);
+    Map<String, Value> fields =
+        new HashMap<String, Value>() {
+          {
+            put("timestamp", ServerTimestamps.valueOf(timestamp, initialServerTimestamp));
+          }
+        };
+    FieldPath path = FieldPath.fromSingleSegment("timestamp");
+    FieldMask mask =
+        FieldMask.fromSet(
+            new HashSet<FieldPath>() {
+              {
+                add(path);
+              }
+            });
+    FieldTransform fieldTransform =
+        new FieldTransform(path, ServerTimestampOperation.getInstance());
+    List<FieldTransform> fieldTransforms =
+        new ArrayList<FieldTransform>() {
+          {
+            add(fieldTransform);
+          }
+        };
+    // The purpose of this test is to ensure that deeply nested server timestamps do not result in
+    // a stack overflow error. Below we use a `Thread` object to create a large number of mutations
+    // because the `Thread` class allows us to specify the maximum stack size.
+    AtomicReference<Throwable> error = new AtomicReference<>();
+    Thread thread =
+        new Thread(
+            Thread.currentThread().getThreadGroup(),
+            () -> {
+              try {
+                for (int i = 0; i < 1000; ++i) {
+                  writeMutation(
+                      new PatchMutation(
+                          DocumentKey.fromPathString("some/object/for/test"),
+                          ObjectValue.fromMap(fields),
+                          mask,
+                          Precondition.NONE,
+                          fieldTransforms));
+                }
+              } catch (Throwable e) {
+                error.set(e);
+              }
+            },
+            /* name */ "test",
+            /* stackSize */ 1024 * 1024);
+    try {
+      thread.start();
+      thread.join();
+    } catch (InterruptedException e) {
+      throw new AssertionError(e);
+    }
+    assertThat(error.get()).isNull();
+  }
 }