Today if we detect concurrent modification of a snapshot repository then we mark it with `CORRUPTED_REPO_GEN` which blocks further writes to try and prevent additional damage to its contents. You can remove the marker by calling the register-repository API again, but this API will discard no-op re-registrations without taking any action. You can work around this by unregistering the repository first (which may be impossible if it's still in use for searchable snapshots) or by re-registering it with some trivial setting change (which seems hacky).

We should not consider it a no-op re-registration if the repository is currently marked with `CORRUPTED_REPO_GEN`, because doing this re-registration will re-read the repo contents and clear the corruption marker and therefore is not a no-op.
server/src/internalClusterTest/java/org/elasticsearch/snapshots/CorruptedBlobStoreRepositoryIT.java
* Tests that a shard snapshot with a corrupted shard index file can still be used for restore and incremental snapshots.
     */
    public void testSnapshotWithCorruptedShardIndexFile() throws Exception {
        disableRepoConsistencyCheck("This test intentionally corrupts the repository contents");
        final Client client = client();
        final Path repo = randomRepoPath();
        final String indexName = "test-idx";
        final int nDocs = randomIntBetween(1, 10);
        logger.info("-->  creating index [{}] with [{}] documents in it", indexName, nDocs);
        assertAcked(prepareCreate(indexName).setSettings(indexSettingsNoReplicas(1)));
        final IndexRequestBuilder[] documents = new IndexRequestBuilder[nDocs];
        for (int j = 0; j < nDocs; j++) {
            documents[j] = client.prepareIndex(indexName).setSource("foo", "bar");
        }
        indexRandom(true, documents);
        flushAndRefresh();
        createRepository("test-repo", "fs", repo);
        final String snapshot1 = "test-snap-1";
        final SnapshotInfo snapshotInfo = createFullSnapshot("test-repo", snapshot1);
        assertThat(snapshotInfo.indices(), hasSize(1));
        final RepositoryData repositoryData = getRepositoryData("test-repo");
        final Map<String, IndexId> indexIds = repositoryData.getIndices();
        assertThat(indexIds.size(), equalTo(1));
        final IndexId corruptedIndex = indexIds.get(indexName);
        final Path shardIndexFile = repo.resolve("indices")
            .resolve(corruptedIndex.getId())
            .resolve("0")
            .resolve("index-" + repositoryData.shardGenerations().getShardGen(corruptedIndex, 0));
        logger.info("-->  truncating shard index file [{}]", shardIndexFile);
        try (SeekableByteChannel outChan = Files.newByteChannel(shardIndexFile, StandardOpenOption.WRITE)) {