Name Snapshot Data Blobs by UUID (elastic#40652)

* Name Snapshot Data Blobs by UUID

* There is no functional reason why we need incremental naming for these files but
  * As explained in elastic#38941 it is a possible source of corrupting the repository
  * It wastes API calls for the list operation
  * Is just needless complication
* Since we store the exact names of the data blobs in all the metadata anyway, we can make this change without any BwC considerations
  * Even on the worst case scenario of a downgrade the functionality would continue working since the incremental names wouldn't conflict with the uuids and the number parsing for finding the next incremental name suppresses the exception when encountring a non-numeric value after the double underscore prefix

server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java

@@ -135,11 +135,11 @@
  *      |- Ac1342-B_x/ - data for index "foo" which was assigned the unique id of Ac1342-B_x in the repository
  *      |  |- meta-20131010.dat - JSON Serialized IndexMetaData for index "foo"
  *      |  |- 0/ - data for shard "0" of index "foo"
- *      |  |  |- __1 \
- *      |  |  |- __2 |
- *      |  |  |- __3 |- files from different segments see snapshot-* for their mappings to real segment files
- *      |  |  |- __4 |
- *      |  |  |- __5 /
+ *      |  |  |- __1                      \  (files with numeric names were created by older ES versions)
+ *      |  |  |- __2                      |
+ *      |  |  |- __VPO5oDMVT5y4Akv8T_AO_A |- files from different segments see snap-* for their mappings to real segment files
+ *      |  |  |- __1gbJy18wS_2kv1qI7FgKuQ |
+ *      |  |  |- __R8JvZAHlSMyMXyZc2SS8Zg /
  *      |  |  .....
  *      |  |  |- snap-20131010.dat - JSON serialized BlobStoreIndexShardSnapshot for snapshot "20131010"
  *      |  |  |- snap-20131011.dat - JSON serialized BlobStoreIndexShardSnapshot for snapshot "20131011"
@@ -1065,41 +1065,6 @@ protected void finalize(final List<SnapshotFiles> snapshots,
             }
         }
 
-        /**
-         * Generates blob name
-         *
-         * @param generation the blob number
-         * @return the blob name
-         */
-        protected String fileNameFromGeneration(long generation) {
-            return DATA_BLOB_PREFIX + Long.toString(generation, Character.MAX_RADIX);
-        }
-
-        /**
-         * Finds the next available blob number
-         *
-         * @param blobs list of blobs in the repository
-         * @return next available blob number
-         */
-        protected long findLatestFileNameGeneration(Map<String, BlobMetaData> blobs) {
-            long generation = -1;
-            for (String name : blobs.keySet()) {
-                if (!name.startsWith(DATA_BLOB_PREFIX)) {
-                    continue;
-                }
-                name = canonicalName(name);
-                try {
-                    long currentGen = Long.parseLong(name.substring(DATA_BLOB_PREFIX.length()), Character.MAX_RADIX);
-                    if (currentGen > generation) {
-                        generation = currentGen;
-                    }
-                } catch (NumberFormatException e) {
-                    logger.warn("file [{}] does not conform to the '{}' schema", name, DATA_BLOB_PREFIX);
-                }
-            }
-            return generation;
-        }
-
         /**
          * Loads all available snapshots in the repository
          *
@@ -1192,7 +1157,6 @@ public void snapshot(final IndexCommit snapshotIndexCommit) {
                 throw new IndexShardSnapshotFailedException(shardId, "failed to list blobs", e);
             }
 
-            long generation = findLatestFileNameGeneration(blobs);
             Tuple<BlobStoreIndexShardSnapshots, Integer> tuple = buildBlobStoreIndexShardSnapshots(blobs);
             BlobStoreIndexShardSnapshots snapshots = tuple.v1();
             int fileListGeneration = tuple.v2();
@@ -1260,7 +1224,7 @@ public void snapshot(final IndexCommit snapshotIndexCommit) {
                         indexIncrementalSize += md.length();
                         // create a new FileInfo
                         BlobStoreIndexShardSnapshot.FileInfo snapshotFileInfo =
-                            new BlobStoreIndexShardSnapshot.FileInfo(fileNameFromGeneration(++generation), md, chunkSize());
+                            new BlobStoreIndexShardSnapshot.FileInfo(DATA_BLOB_PREFIX + UUIDs.randomBase64UUID(), md, chunkSize());
                         indexCommitPointFiles.add(snapshotFileInfo);
                         filesToSnapshot.add(snapshotFileInfo);
                     } else {