Cleanup Concurrent RepositoryData Loading (#48329) (#48835)

The loading of `RepositoryData` is not an atomic operation.
It uses a list + get combination of calls.
This lead to accidentally returning an empty repository data
for generations >=0 which can never not exist unless the repository
is corrupted.
In the test #48122 (and other SLM tests) there was a low chance of
running into this concurrent modification scenario and the repository
actually moving two index generations between listing out the
index-N and loading the latest version of it. Since we only keep
two index-N around at a time this lead to unexpectedly absent
snapshots in status APIs.
Fixing the behavior to be more resilient is non-trivial but in the works.
For now I think we should simply throw in this scenario. This will also
help prevent corruption in the unlikely event but possible of running into this
issue in a snapshot create or delete operation on master failover on a
repository like S3 which doesn't have the "no overwrites" protection on
writing a new index-N.

Fixes #48122

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

@@ -849,9 +849,6 @@ public void endVerification(String seed) {
     public RepositoryData getRepositoryData() {
         try {
             return getRepositoryData(latestIndexBlobId());
-        } catch (NoSuchFileException ex) {
-            // repository doesn't have an index blob, its a new blank repo
-            return RepositoryData.EMPTY;
         } catch (IOException ioe) {
             throw new RepositoryException(metadata.name(), "Could not determine repository generation from root blobs", ioe);
         }
@@ -864,17 +861,12 @@ private RepositoryData getRepositoryData(long indexGen) {
         try {
             final String snapshotsIndexBlobName = INDEX_FILE_PREFIX + Long.toString(indexGen);
 
-            RepositoryData repositoryData;
             // EMPTY is safe here because RepositoryData#fromXContent calls namedObject
             try (InputStream blob = blobContainer().readBlob(snapshotsIndexBlobName);
                  XContentParser parser = XContentType.JSON.xContent().createParser(NamedXContentRegistry.EMPTY,
                      LoggingDeprecationHandler.INSTANCE, blob)) {
-                repositoryData = RepositoryData.snapshotsFromXContent(parser, indexGen);
+                return RepositoryData.snapshotsFromXContent(parser, indexGen);
             }
-            return repositoryData;
-        } catch (NoSuchFileException ex) {
-            // repository doesn't have an index blob, its a new blank repo
-            return RepositoryData.EMPTY;
         } catch (IOException ioe) {
             throw new RepositoryException(metadata.name(), "could not read repository data from index blob", ioe);
         }

x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/slm/SLMSnapshotBlockingIntegTests.java

@@ -410,18 +410,24 @@ private void testUnsuccessfulSnapshotRetention(boolean partialSuccess) throws Ex
             logger.info("--> waiting for {} snapshot [{}] to be deleted", expectedUnsuccessfulState, failedSnapshotName.get());
             assertBusy(() -> {
                 try {
+                    try {
+                        GetSnapshotsResponse snapshotsStatusResponse = client().admin().cluster()
+                            .prepareGetSnapshots(REPO).setSnapshots(failedSnapshotName.get()).get();
+                        assertThat(snapshotsStatusResponse.getSnapshots(), empty());
+                    } catch (SnapshotMissingException e) {
+                        // This is what we want to happen
+                    }
+                    logger.info("--> {} snapshot [{}] has been deleted, checking successful snapshot [{}] still exists",
+                        expectedUnsuccessfulState, failedSnapshotName.get(), successfulSnapshotName.get());
                     GetSnapshotsResponse snapshotsStatusResponse = client().admin().cluster()
-                        .prepareGetSnapshots(REPO).setSnapshots(failedSnapshotName.get()).get();
-                    assertThat(snapshotsStatusResponse.getSnapshots(), empty());
-                } catch (SnapshotMissingException e) {
-                    // This is what we want to happen
+                        .prepareGetSnapshots(REPO).setSnapshots(successfulSnapshotName.get()).get();
+                    SnapshotInfo snapshotInfo = snapshotsStatusResponse.getSnapshots().get(0);
+                    assertEquals(SnapshotState.SUCCESS, snapshotInfo.state());
+                } catch (RepositoryException re) {
+                    // Concurrent status calls and write operations may lead to failures in determining the current repository generation
+                    // TODO: Remove this hack once tracking the current repository generation has been made consistent
+                    throw new AssertionError(re);
                 }
-                logger.info("--> {} snapshot [{}] has been deleted, checking successful snapshot [{}] still exists",
-                    expectedUnsuccessfulState, failedSnapshotName.get(), successfulSnapshotName.get());
-                GetSnapshotsResponse snapshotsStatusResponse = client().admin().cluster()
-                    .prepareGetSnapshots(REPO).setSnapshots(successfulSnapshotName.get()).get();
-                SnapshotInfo snapshotInfo = snapshotsStatusResponse.getSnapshots().get(0);
-                assertEquals(SnapshotState.SUCCESS, snapshotInfo.state());
             });
         }
     }