fix: enforce read-before-write in transaction execute() (#2358)

Ensures that Pipeline.execute() calls within a transaction follow the
requirement that all reads must occur before any writes. Previously,
this check was missing from the execute() methods in
ServerSideTransaction.

Author: wu-hui

google-cloud-firestore/src/main/java/com/google/cloud/firestore/ServerSideTransaction.java

@@ -271,13 +271,15 @@ public ApiFuture<AggregateQuerySnapshot> get(@Nonnull AggregateQuery query) {
   @Nonnull
   @Override
   public ApiFuture<Pipeline.Snapshot> execute(@Nonnull Pipeline pipeline) {
+    Preconditions.checkState(isEmpty(), READ_BEFORE_WRITE_ERROR_MSG);
     return execute(pipeline, new PipelineExecuteOptions());
   }
 
   @Nonnull
   @Override
   public ApiFuture<Pipeline.Snapshot> execute(
       @Nonnull Pipeline pipeline, @Nonnull PipelineExecuteOptions options) {
+    Preconditions.checkState(isEmpty(), READ_BEFORE_WRITE_ERROR_MSG);
     try (TraceUtil.Scope ignored = transactionTraceContext.makeCurrent()) {
       return pipeline.execute(new PipelineExecuteOptions(), transactionId, null);
     }

google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java

@@ -960,6 +960,27 @@ public void getShouldThrowWhenInvokedAfterAWriteWithAnAggregateQuery() throws Ex
     assertThat(executionException.getCause()).isInstanceOf(IllegalStateException.class);
   }
 
+  @Test
+  public void executeShouldThrowWhenInvokedAfterAWrite() throws Exception {
+    doReturn(beginResponse())
+        .when(firestoreMock)
+        .sendRequest(ArgumentMatchers.any(), ArgumentMatchers.any());
+
+    ApiFuture<Void> transaction =
+        firestoreMock.runTransaction(
+            t -> {
+              t.set(documentReference, LocalFirestoreHelper.SINGLE_FIELD_MAP);
+              t.execute(firestoreMock.pipeline().collection("test"));
+              return null;
+            });
+
+    ExecutionException executionException =
+        assertThrows(ExecutionException.class, transaction::get);
+    assertThat(executionException.getCause()).isInstanceOf(IllegalStateException.class);
+    assertThat(executionException.getCause().getMessage())
+        .contains("Firestore transactions require all reads to be executed before all writes");
+  }
+
   @Test
   public void givesProperErrorMessageForCommittedTransaction() throws Exception {
     doReturn(beginResponse())