Partially emulate arbitrary markers with GCS

S3 supports arbitrary keys for the marker while GCS only supports its
opaque marker.  Emulate the common case for GCS by mapping the last
key from a listing to the corresponding previously returned marker.

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -158,6 +158,11 @@ final class S3ProxyHandler extends AbstractHandler {
             "bucket-owner-full-control",
             "log-delivery-write"
     );
+    /** Blobstores with opaque markers. */
+    private static final Set<String> BLOBSTORE_OPAQUE_MARKERS = ImmutableSet.of(
+            "azureblob",
+            "google-cloud-storage"
+    );
 
     private final boolean anonymousIdentity;
     private final Optional<String> virtualHost;
@@ -169,9 +174,10 @@ final class S3ProxyHandler extends AbstractHandler {
     // TODO: hack to allow per-request anonymous access
     private final BlobStore defaultBlobStore;
     /**
-     * S3 supports arbitrary keys for the marker while Azure only supports its
-     * opaque marker.  Emulate the common case for Azure by mapping the last
-     * key from a listing to the corresponding previously returned marker.
+     * S3 supports arbitrary keys for the marker while some blobstores only
+     * support opaque markers.  Emulate the common case for these by mapping
+     * the last key from a listing to the corresponding previously returned
+     * marker.
      */
     private final Cache<Map.Entry<String, String>, String> lastKeyToMarker =
             CacheBuilder.newBuilder()
@@ -874,7 +880,7 @@ final class S3ProxyHandler extends AbstractHandler {
         }
         String marker = request.getParameter("marker");
         if (marker != null) {
-            if (blobStoreType.equals("azureblob")) {
+            if (BLOBSTORE_OPAQUE_MARKERS.contains(blobStoreType)) {
                 String realMarker = lastKeyToMarker.getIfPresent(
                         Maps.immutableEntry(containerName, marker));
                 if (realMarker != null) {
@@ -930,7 +936,7 @@ final class S3ProxyHandler extends AbstractHandler {
             if (nextMarker != null) {
                 writeSimpleElement(xml, "IsTruncated", "true");
                 writeSimpleElement(xml, "NextMarker", nextMarker);
-                if (blobStoreType.equals("azureblob")) {
+                if (BLOBSTORE_OPAQUE_MARKERS.contains(blobStoreType)) {
                     lastKeyToMarker.put(Maps.immutableEntry(containerName,
                             Iterables.getLast(set).getName()), nextMarker);
                 }