Introduce an S3 object monitor.

service/src/main/java/org/whispersystems/textsecuregcm/util/S3ObjectMonitor.java

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+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import com.amazonaws.auth.InstanceProfileCredentialsProvider;
+import com.amazonaws.services.s3.AmazonS3;
+import com.amazonaws.services.s3.AmazonS3ClientBuilder;
+import com.amazonaws.services.s3.model.ObjectMetadata;
+import com.amazonaws.services.s3.model.S3Object;
+import com.google.common.annotations.VisibleForTesting;
+import io.dropwizard.lifecycle.Managed;
+import java.time.Duration;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Consumer;
+import org.apache.commons.lang3.StringUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * An S3 object monitor watches a specific object in an S3 bucket and notifies a listener if that object changes.
+ */
+public class S3ObjectMonitor implements Managed {
+
+  private final String s3Bucket;
+  private final String objectKey;
+
+  private final ScheduledExecutorService refreshExecutorService;
+  private final Duration refreshInterval;
+  private ScheduledFuture<?> refreshFuture;
+
+  private final Consumer<S3Object> changeListener;
+
+  private final AtomicReference<String> lastETag = new AtomicReference<>();
+
+  private final AmazonS3 s3Client;
+
+  private static final Logger log = LoggerFactory.getLogger(S3ObjectMonitor.class);
+
+  public S3ObjectMonitor(
+      final String s3Region,
+      final String s3Bucket,
+      final String objectKey,
+      final ScheduledExecutorService refreshExecutorService,
+      final Duration refreshInterval,
+      final Consumer<S3Object> changeListener) {
+
+    this(AmazonS3ClientBuilder.standard()
+            .withCredentials(InstanceProfileCredentialsProvider.getInstance())
+            .withRegion(s3Region)
+            .build(),
+        s3Bucket,
+        objectKey,
+        refreshExecutorService,
+        refreshInterval,
+        changeListener);
+  }
+
+  @VisibleForTesting
+  S3ObjectMonitor(
+      final AmazonS3 s3Client,
+      final String s3Bucket,
+      final String objectKey,
+      final ScheduledExecutorService refreshExecutorService,
+      final Duration refreshInterval,
+      final Consumer<S3Object> changeListener) {
+
+    this.s3Client = s3Client;
+    this.s3Bucket = s3Bucket;
+    this.objectKey = objectKey;
+
+    this.refreshExecutorService = refreshExecutorService;
+    this.refreshInterval = refreshInterval;
+
+    this.changeListener = changeListener;
+  }
+
+  @Override
+  public synchronized void start() {
+    if (refreshFuture != null) {
+      throw new RuntimeException("S3 object manager already started");
+    }
+
+    refreshFuture = refreshExecutorService
+        .scheduleAtFixedRate(this::refresh, refreshInterval.toMillis(), refreshInterval.toMillis(), TimeUnit.MILLISECONDS);
+  }
+
+  @Override
+  public synchronized void stop() {
+    if (refreshFuture != null) {
+      refreshFuture.cancel(true);
+    }
+  }
+
+  /**
+   * Polls S3 for object metadata and notifies the listener provided at construction time if and only if the object has
+   * changed. This method blocks until S3 object metadata has been retrieved and, if the object has changed, the
+   * listener returns. Callers may wish to call this method at initialization time if they need data from the monitored
+   * S3 object immediately and don't want to wait for the first scheduled update.
+   */
+  public void refresh() {
+    try {
+      final ObjectMetadata objectMetadata = s3Client.getObjectMetadata(s3Bucket, objectKey);
+
+      final String initialETag = lastETag.get();
+      final String refreshedETag = objectMetadata.getETag();
+
+      if (!StringUtils.equals(initialETag, refreshedETag) && lastETag.compareAndSet(initialETag, refreshedETag)) {
+        changeListener.accept(s3Client.getObject(s3Bucket, objectKey));
+      }
+    } catch (final Exception e) {
+      log.warn("Failed to refresh monitored object", e);
+    }
+  }
+}