Do not check for S3 blob to exist before writing (#31128)

In #19749 an extra check was added before writing each blob to ensure that we would not be
overriding an existing blob. Due to S3's weak consistency model, this check was best effort. To
make matters worse, however, this resulted in a HEAD request to be done before every PUT, in
particular also when PUTTING a new object. The approach taken in #19749 worsened our
consistency guarantees for follow-up snapshot actions, as it made it less likely for new files that
had been written to be available for reads.

This commit therefore removes this extra check. Due to the weak consistency model, this check
was a best effort thing anyway, and there's currently no way to prevent accidental overrides on S3.

plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3BlobContainer.java

@@ -96,10 +96,6 @@ class S3BlobContainer extends AbstractBlobContainer {
 
     @Override
     public void writeBlob(String blobName, InputStream inputStream, long blobSize) throws IOException {
-        if (blobExists(blobName)) {
-            throw new FileAlreadyExistsException("Blob [" + blobName + "] already exists, cannot overwrite");
-        }
-
         SocketAccess.doPrivilegedIOException(() -> {
             if (blobSize <= blobStore.bufferSizeInBytes()) {
                 executeSingleUpload(blobStore, buildKey(blobName), inputStream, blobSize);

plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3BlobStoreContainerTests.java

@@ -64,6 +64,11 @@ public class S3BlobStoreContainerTests extends ESBlobStoreContainerTestCase {
         return randomMockS3BlobStore();
     }
 
+    @Override
+    public void testVerifyOverwriteFails() {
+        assumeFalse("not implemented because of S3's weak consistency model", true);
+    }
+
     public void testExecuteSingleUploadBlobSizeTooLarge() {
         final long blobSize = ByteSizeUnit.GB.toBytes(randomIntBetween(6, 10));
         final S3BlobStore blobStore = mock(S3BlobStore.class);