Fix up IcebergIO Write path

 - remove Read path (will propose separately)
 - re-enable checking, fix type errors
 - some style adjustments

buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy

@@ -1151,7 +1151,7 @@ class BeamModulePlugin implements Plugin<Project> {
         options.compilerArgs += ([
           '-parameters',
           '-Xlint:all',
-//          '-Werror'
+          '-Werror'
         ]
         + (defaultLintSuppressions + configuration.disableLintWarnings).collect { "-Xlint:-${it}" })
       }

sdks/java/io/iceberg/src/main/java/org/apache/beam/io/iceberg/AppendFilesToTables.java

@@ -0,0 +1,102 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.beam.io.iceberg;
+
+import org.apache.beam.sdk.coders.KvCoder;
+import org.apache.beam.sdk.coders.SerializableCoder;
+import org.apache.beam.sdk.coders.StringUtf8Coder;
+import org.apache.beam.sdk.transforms.DoFn;
+import org.apache.beam.sdk.transforms.GroupByKey;
+import org.apache.beam.sdk.transforms.PTransform;
+import org.apache.beam.sdk.transforms.ParDo;
+import org.apache.beam.sdk.transforms.SerializableFunction;
+import org.apache.beam.sdk.transforms.WithKeys;
+import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
+import org.apache.beam.sdk.values.KV;
+import org.apache.beam.sdk.values.PCollection;
+import org.apache.iceberg.AppendFiles;
+import org.apache.iceberg.Snapshot;
+import org.apache.iceberg.Table;
+import org.apache.iceberg.catalog.Catalog;
+import org.apache.iceberg.catalog.TableIdentifier;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
+
+class AppendFilesToTables
+    extends PTransform<PCollection<FileWriteResult>, PCollection<KV<String, Snapshot>>> {
+
+  private final IcebergCatalogConfig catalogConfig;
+
+  AppendFilesToTables(IcebergCatalogConfig catalogConfig) {
+    this.catalogConfig = catalogConfig;
+  }
+
+  @Override
+  public PCollection<KV<String, Snapshot>> expand(PCollection<FileWriteResult> writtenFiles) {
+
+    // Apply any sharded writes and flatten everything for catalog updates
+    return writtenFiles
+        .apply(
+            "Key metadata updates by table",
+            WithKeys.of(
+                new SerializableFunction<FileWriteResult, String>() {
+                  @Override
+                  public String apply(FileWriteResult input) {
+                    return input.getTableIdentifier().toString();
+                  }
+                }))
+        .apply("Group metadata updates by table", GroupByKey.create())
+        .apply(
+            "Append metadata updates to tables",
+            ParDo.of(new AppendFilesToTablesDoFn(catalogConfig)))
+        .setCoder(KvCoder.of(StringUtf8Coder.of(), SerializableCoder.of(Snapshot.class)));
+  }
+
+  private static class AppendFilesToTablesDoFn
+      extends DoFn<KV<String, Iterable<FileWriteResult>>, KV<String, Snapshot>> {
+
+    private final IcebergCatalogConfig catalogConfig;
+
+    private transient @MonotonicNonNull Catalog catalog;
+
+    private AppendFilesToTablesDoFn(IcebergCatalogConfig catalogConfig) {
+      this.catalogConfig = catalogConfig;
+    }
+
+    private Catalog getCatalog() {
+      if (catalog == null) {
+        catalog = catalogConfig.catalog();
+      }
+      return catalog;
+    }
+
+    @ProcessElement
+    public void processElement(
+        @Element KV<String, Iterable<FileWriteResult>> element,
+        OutputReceiver<KV<String, Snapshot>> out,
+        BoundedWindow window) {
+      Table table = getCatalog().loadTable(TableIdentifier.parse(element.getKey()));
+      AppendFiles update = table.newAppend();
+      for (FileWriteResult writtenFile : element.getValue()) {
+        update.appendFile(writtenFile.getDataFile());
+      }
+      update.commit();
+      out.outputWithTimestamp(
+          KV.of(element.getKey(), table.currentSnapshot()), window.maxTimestamp());
+    }
+  }
+}