Simplified implementation of "requireBucketExists"

This implementation depends on multiple (reasonable) assumptions and insights gained from the source code of MinIO:
- "maxKeys" is implemented as page size: The iterator returned by "MinioClient.listObjects" automatically loads another batch of 0 <= count <= maxKeys results if the number of calls to the iterator exceeds the number of previously loaded results. The extension function "MinioClient.listObjectsLimit" limits the number of objects returned in total.
- The iterator returned by "MinioClient.listObjects" will only return up to one error result. If there is an error result, it will always be returned as the next and only remaining result.
- AWS as well as other S3 providers return the results of "listObjects" in a sorted fashion. This implementation breaks if a provider doesn't abide to this contract.

data/src/main/java/org/cryptomator/data/cloud/s3/S3Impl.kt

@@ -1,6 +1,5 @@
 package org.cryptomator.data.cloud.s3
 
-import io.minio.Result as ListObjectsResult
 import android.content.Context
 import org.cryptomator.data.cloud.s3.S3CloudApiExceptions.isAccessProblem
 import org.cryptomator.data.util.CopyStream
@@ -33,6 +32,7 @@ import io.minio.MinioClient
 import io.minio.PutObjectArgs
 import io.minio.RemoveObjectArgs
 import io.minio.RemoveObjectsArgs
+import io.minio.Result
 import io.minio.StatObjectArgs
 import io.minio.errors.ErrorResponseException
 import io.minio.messages.DeleteObject
@@ -346,33 +346,23 @@ internal class S3Impl(private val cloud: S3Cloud, private val client: MinioClien
 	@Throws(NoSuchBucketException::class, BackendException::class)
 	private fun requireBucketExists() {
 		try {
-			//Throw appropriate exception implicitly through "handleApiError"
-			val results: List<Result<Item?>> = ListObjectsArgs.builder() //
+			val returned: Sequence<Result<Item?>?> = ListObjectsArgs.builder() //
 				.bucket(cloud.s3Bucket()) //
-				.recursive(true) //
-				.maxKeys(1) //
+				.recursive(true) // //TODO
+				.maxKeys(1) // Batch size
 				.build() //
-				.let { client.listObjects(it) } //
-				.map { result -> result.runCatching(ListObjectsResult<Item?>::get) } //
-
-			if(results.isEmpty()) {
-				return
-			}
-
-			if (results.any { it.isSuccess }) {
-				results.asSequence().filter { it.isFailure }.forEach { Timber.d(it.exceptionOrNull(), "Non-critical exception while checking for bucket %s", cloud.s3Bucket()) }
-				return
-			}
-
-			val exceptions = results.map { it.exceptionOrNull()!! }
-			if(exceptions.any { it is ErrorResponseException && it.errorResponse().code() == S3CloudApiErrorCodes.NO_SUCH_BUCKET.value }) {
-				results.asSequence().filter { it.isFailure }.forEach { Timber.d(it.exceptionOrNull(), "Non-critical exception while checking for bucket %s", cloud.s3Bucket()) }
-				throw NoSuchBucketException(cloud.s3Bucket())
-			}
-			//No success and no NoSuchBucketException
-			val toThrow = exceptions.first()
-			results.asSequence().drop(1).filter { it.isFailure }.forEach { Timber.d(it.exceptionOrNull(), "Exception while checking for bucket %s", cloud.s3Bucket()) }
-			throw toThrow
+				.let { client.listObjectsLimit(it, 1) }
+			//returned
+			//	|-- <Empty>					No elements in bucket
+			//	|-- Result<Err>				Any error
+			//	|-- Result<Item>
+			//		|-- "name/.bzEmpty"		Web interface folder
+			//		|-- "name/"				0 Byte folder
+			//		|-- "name"				0 or X Byte file
+			//Note: This implementation depends on listObjects/listObjectsLimit returning elements in a sensible order
+
+			val result: Result<Item?> = returned.firstOrNull() ?: return //No item, but above all no exception, ergo: Bucket exists
+			result.get() //Throw appropriate exception (if any) implicitly through "handleApiError"
 		} catch (e: ErrorResponseException) {
 			throw handleApiError(e, cloud.s3Bucket())
 		}