Fix overlay bug with multiple field patches

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

@@ -142,7 +142,16 @@ Map<DocumentKey, OverlayedDocument> getOverlayedDocuments(
     return computeViews(docs, overlays, new HashSet<>());
   }
 
-  /*Computes the local view for doc */
+  /**
+   * Computes the local view for the given documents.
+   *
+   * @param docs The documents to compute views for. It also has the base version of the documents.
+   * @param overlays The overlays that need to be applied to the given base version of the
+   *     documents.
+   * @param existenceStateChanged A set of documents that might there existence states changed. This
+   *     is used to determine if we need to re-calculate overlays from mutation queues.
+   * @return A map represents the local documents view.
+   */
   private Map<DocumentKey, OverlayedDocument> computeViews(
       Map<DocumentKey, MutableDocument> docs,
       Map<DocumentKey, Overlay> overlays,
@@ -161,7 +170,7 @@ private Map<DocumentKey, OverlayedDocument> computeViews(
           && (overlay == null || overlay.getMutation() instanceof PatchMutation)) {
         recalculateDocuments.put(doc.getKey(), doc);
       } else if (overlay != null) {
-        mutatedFields.put(doc.getKey(), overlay.getFieldMask());
+        mutatedFields.put(doc.getKey(), overlay.getMutation().getFieldMask());
         overlay.getMutation().applyToLocalView(doc, null, Timestamp.now());
       }
     }

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

@@ -20,16 +20,16 @@
 
 /** Represents a local view (overlay) of a document, and the fields that are locally mutated. */
 public class OverlayedDocument {
-  private Document overlay;
+  private Document overlayedDocument;
   private FieldMask mutatedFields;
 
-  OverlayedDocument(Document overlay, FieldMask mutatedFields) {
-    this.overlay = overlay;
+  OverlayedDocument(Document overlayedDocument, FieldMask mutatedFields) {
+    this.overlayedDocument = overlayedDocument;
     this.mutatedFields = mutatedFields;
   }
 
   public Document getDocument() {
-    return overlay;
+    return overlayedDocument;
   }
 
   /**

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/DeleteMutation.java

@@ -79,4 +79,9 @@ public void applyToRemoteDocument(MutableDocument document, MutationResult mutat
 
     return previousMask;
   }
+
+  @Override
+  public @Nullable FieldMask getFieldMask() {
+    return null;
+  }
 }

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/Mutation.java

@@ -164,6 +164,12 @@ public abstract void applyToRemoteDocument(
   public abstract @Nullable FieldMask applyToLocalView(
       MutableDocument document, @Nullable FieldMask previousMask, Timestamp localWriteTime);
 
+  /**
+   * Returns a {@code FieldMask} representing the fields that will be changed by applying this
+   * mutation. Returns {@code null} if the mutation will overwrite the entire document.
+   */
+  public abstract @Nullable FieldMask getFieldMask();
+
   /** Helper for derived classes to implement .equals(). */
   boolean hasSameKeyAndPrecondition(Mutation other) {
     return key.equals(other.key) && precondition.equals(other.precondition);

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/MutationBatch.java

@@ -100,14 +100,11 @@ public void applyToRemoteDocument(MutableDocument document, MutationBatchResult
   }
 
   /**
-   * Computes the local view of a document given all the mutations in this batch. Returns an {@link
-   * FieldMask} representing all the fields that are mutated.
+   * Computes the local view of a document given all the mutations in this batch.
+   *
+   * @param mutatedFields Fields that are already mutated before applying the current one.
+   * @return An {@link FieldMask} representing all the fields that are mutated.
    */
-  public FieldMask applyToLocalView(MutableDocument document) {
-    FieldMask mutatedFields = FieldMask.fromSet(new HashSet<>());
-    return applyToLocalView(document, mutatedFields);
-  }
-
   public FieldMask applyToLocalView(MutableDocument document, @Nullable FieldMask mutatedFields) {
     // First, apply the base state. This allows us to apply non-idempotent transform against a
     // consistent set of values.

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/Overlay.java

@@ -36,10 +36,4 @@ public static Overlay create(int largestBatchId, Mutation mutation) {
   public DocumentKey getKey() {
     return getMutation().getKey();
   }
-
-  public FieldMask getFieldMask() {
-    return getMutation() instanceof PatchMutation
-        ? ((PatchMutation) getMutation()).getMask()
-        : null;
-  }
 }

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/PatchMutation.java

@@ -101,7 +101,8 @@ public ObjectValue getValue() {
    * Returns the mask to apply to {@link #getValue}, where only fields that are in both the
    * fieldMask and the value will be updated.
    */
-  public FieldMask getMask() {
+  @Override
+  public FieldMask getFieldMask() {
     return mask;
   }
 

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/SetMutation.java

@@ -103,6 +103,11 @@ public FieldMask applyToLocalView(
     return null;
   }
 
+  @Override
+  public @Nullable FieldMask getFieldMask() {
+    return null;
+  }
+
   /** Returns the object value to use when setting the document. */
   public ObjectValue getValue() {
     return value;

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/VerifyMutation.java

@@ -66,4 +66,9 @@ public FieldMask applyToLocalView(
       MutableDocument document, @Nullable FieldMask previousMask, Timestamp localWriteTime) {
     throw Assert.fail("VerifyMutation should only be used in Transactions.");
   }
+
+  @Override
+  public @Nullable FieldMask getFieldMask() {
+    return null;
+  }
 }

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java

@@ -269,7 +269,7 @@ public com.google.firestore.v1.Write encodeMutation(Mutation mutation) {
       builder.setUpdate(encodeDocument(mutation.getKey(), ((SetMutation) mutation).getValue()));
     } else if (mutation instanceof PatchMutation) {
       builder.setUpdate(encodeDocument(mutation.getKey(), ((PatchMutation) mutation).getValue()));
-      builder.setUpdateMask(encodeDocumentMask(((PatchMutation) mutation).getMask()));
+      builder.setUpdateMask(encodeDocumentMask(((PatchMutation) mutation).getFieldMask()));
     } else if (mutation instanceof DeleteMutation) {
       builder.setDelete(encodeKey(mutation.getKey()));
     } else if (mutation instanceof VerifyMutation) {