fix(firestore): remote target ID mapping to avoid possible race condition on rapid subscribe and unsubscribe

firebase-firestore/src/main/java/com/google/firebase/firestore/core/SyncEngine.java

@@ -366,7 +366,7 @@ public Task<Map<String, Value>> runAggregateQuery(
 
   /** Called by FirestoreClient to notify us of a new remote event. */
   @Override
-  public void handleRemoteEvent(RemoteEvent event) {
+  public void handleRemoteEvent(RemoteEvent<Integer> event) {
     assertCallback("handleRemoteEvent");
 
     // Update `receivedDocument` as appropriate for any limbo targets.
@@ -465,8 +465,8 @@ public void handleRejectedListen(int targetId, Status error) {
       Map<DocumentKey, MutableDocument> documentUpdates =
           Collections.singletonMap(limboKey, result);
       Set<DocumentKey> limboDocuments = Collections.singleton(limboKey);
-      RemoteEvent event =
-          new RemoteEvent(
+      RemoteEvent<Integer> event =
+          new RemoteEvent<>(
               SnapshotVersion.NONE,
               /* targetChanges= */ Collections.emptyMap(),
               /* targetMismatches= */ Collections.emptyMap(),
@@ -673,7 +673,8 @@ private void removeLimboTarget(DocumentKey key) {
    * snapshot.
    */
   private void emitNewSnapsAndNotifyLocalStore(
-      ImmutableSortedMap<DocumentKey, Document> changes, @Nullable RemoteEvent remoteEvent) {
+      ImmutableSortedMap<DocumentKey, Document> changes,
+      @Nullable RemoteEvent<Integer> remoteEvent) {
     List<ViewSnapshot> newSnapshots = new ArrayList<>();
     List<LocalViewChanges> documentChangesInAllViews = new ArrayList<>();
 

firebase-firestore/src/main/java/com/google/firebase/firestore/core/TargetIdGenerator.java

@@ -53,6 +53,24 @@ public static TargetIdGenerator forSyncEngine() {
     return new TargetIdGenerator(SYNC_ENGINE_ID, 1);
   }
 
+  /**
+   * Creates and returns a new TargetIdGenerator for remote target cache watches.
+   *
+   * @return A new instance of TargetIdGenerator starting at 1000 (even).
+   */
+  public static TargetIdGenerator forRemoteTargetCache() {
+    return new TargetIdGenerator(QUERY_CACHE_ID, 1000);
+  }
+
+  /**
+   * Creates and returns a new TargetIdGenerator for remote sync engine watches.
+   *
+   * @return A new instance of TargetIdGenerator starting at 1001 (odd).
+   */
+  public static TargetIdGenerator forRemoteSyncEngine() {
+    return new TargetIdGenerator(SYNC_ENGINE_ID, 1001);
+  }
+
   private static final int QUERY_CACHE_ID = 0;
   private static final int SYNC_ENGINE_ID = 1;
 

firebase-firestore/src/main/java/com/google/firebase/firestore/local/BaseTargetData.java

@@ -0,0 +1,147 @@
+// Copyright 2026 Google LLC
+//
+// 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 com.google.firebase.firestore.local;
+
+import static com.google.firebase.firestore.util.Preconditions.checkNotNull;
+
+import androidx.annotation.Nullable;
+import com.google.firebase.firestore.core.TargetOrPipeline;
+import com.google.firebase.firestore.model.SnapshotVersion;
+import com.google.protobuf.ByteString;
+import java.util.Objects;
+
+/** An abstract set of metadata that the store will need to keep track of for each target. */
+public abstract class BaseTargetData {
+  private final TargetOrPipeline target;
+  private final long sequenceNumber;
+  private final QueryPurpose purpose;
+  private final SnapshotVersion snapshotVersion;
+  private final SnapshotVersion lastLimboFreeSnapshotVersion;
+  private final ByteString resumeToken;
+  private final @Nullable Integer expectedCount;
+
+  /** Creates a new BaseTargetData with the given values. */
+  protected BaseTargetData(
+      TargetOrPipeline target,
+      long sequenceNumber,
+      QueryPurpose purpose,
+      SnapshotVersion snapshotVersion,
+      SnapshotVersion lastLimboFreeSnapshotVersion,
+      ByteString resumeToken,
+      @Nullable Integer expectedCount) {
+    this.target = checkNotNull(target);
+    this.sequenceNumber = sequenceNumber;
+    this.lastLimboFreeSnapshotVersion = lastLimboFreeSnapshotVersion;
+    this.purpose = purpose;
+    this.snapshotVersion = checkNotNull(snapshotVersion);
+    this.resumeToken = checkNotNull(resumeToken);
+    this.expectedCount = expectedCount;
+  }
+
+  /** Creates a new target data instance with an updated sequence number. */
+  public abstract BaseTargetData withSequenceNumber(long sequenceNumber);
+
+  /** Creates a new target data instance with an updated resume token and snapshot version. */
+  public abstract BaseTargetData withResumeToken(
+      ByteString resumeToken, SnapshotVersion snapshotVersion);
+
+  /** Creates a new target data instance with an updated expected count. */
+  public abstract BaseTargetData withExpectedCount(@Nullable Integer expectedCount);
+
+  /** Creates a new target data instance with an updated last limbo free snapshot version number. */
+  public abstract BaseTargetData withLastLimboFreeSnapshotVersion(
+      SnapshotVersion lastLimboFreeSnapshotVersion);
+
+  public TargetOrPipeline getTarget() {
+    return target;
+  }
+
+  public long getSequenceNumber() {
+    return sequenceNumber;
+  }
+
+  public QueryPurpose getPurpose() {
+    return purpose;
+  }
+
+  public SnapshotVersion getSnapshotVersion() {
+    return snapshotVersion;
+  }
+
+  public ByteString getResumeToken() {
+    return resumeToken;
+  }
+
+  @Nullable
+  public Integer getExpectedCount() {
+    return expectedCount;
+  }
+
+  public SnapshotVersion getLastLimboFreeSnapshotVersion() {
+    return lastLimboFreeSnapshotVersion;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+
+    BaseTargetData that = (BaseTargetData) o;
+    return target.equals(that.target)
+        && sequenceNumber == that.sequenceNumber
+        && purpose.equals(that.purpose)
+        && snapshotVersion.equals(that.snapshotVersion)
+        && lastLimboFreeSnapshotVersion.equals(that.lastLimboFreeSnapshotVersion)
+        && resumeToken.equals(that.resumeToken)
+        && Objects.equals(expectedCount, that.expectedCount);
+  }
+
+  @Override
+  public int hashCode() {
+    int result = target.hashCode();
+    result = 31 * result + (int) sequenceNumber;
+    result = 31 * result + purpose.hashCode();
+    result = 31 * result + snapshotVersion.hashCode();
+    result = 31 * result + lastLimboFreeSnapshotVersion.hashCode();
+    result = 31 * result + resumeToken.hashCode();
+    result = 31 * result + Objects.hashCode(expectedCount);
+    return result;
+  }
+
+  @Override
+  public String toString() {
+    return getClass().getSimpleName()
+        + "{"
+        + "target="
+        + target
+        + ", sequenceNumber="
+        + sequenceNumber
+        + ", purpose="
+        + purpose
+        + ", snapshotVersion="
+        + snapshotVersion
+        + ", lastLimboFreeSnapshotVersion="
+        + lastLimboFreeSnapshotVersion
+        + ", resumeToken="
+        + resumeToken
+        + ", expectedCount="
+        + expectedCount
+        + '}';
+  }
+}

firebase-firestore/src/main/java/com/google/firebase/firestore/local/LocalStore.java

@@ -413,7 +413,8 @@ public void setSessionsToken(ByteString sessionToken) {
    *
    * <p>LocalDocuments are re-calculated if there are remaining mutations in the queue.
    */
-  public ImmutableSortedMap<DocumentKey, Document> applyRemoteEvent(RemoteEvent remoteEvent) {
+  public ImmutableSortedMap<DocumentKey, Document> applyRemoteEvent(
+      RemoteEvent<Integer> remoteEvent) {
     SnapshotVersion remoteVersion = remoteEvent.getSnapshotVersion();
 
     // TODO: Call queryEngine.handleDocumentChange() appropriately.