fix: make ListMultipartUploads ordering deterministic and stabilize test timing

The `posix` and `azure` backends sorted `ListMultipartUploads` by `Key` and `Initiated` only, leaving uploads with identical values dependent on arbitrary directory/blob listing order. Add `UploadID` as a deterministic tertiary sort key in both backends.

Also apply the one-second delay in `ListMultipartUploads_keyMarker_not_from_list` to all backends so same-key uploads get distinct timestamps in CI, and fix an unrelated error message that referenced the wrong slice.

backend/azure/azure.go

@@ -1764,12 +1764,18 @@ func (az *Azure) ListMultipartUploads(ctx context.Context, input *s3.ListMultipa
 		}
 	}
 
-	// Sort once: Key asc, Initiated asc
-	sort.SliceStable(uploads, func(i, j int) bool {
+	// Sort once: Key asc, Initiated asc, UploadID asc
+	// UploadID is a tie-breaker for uploads that share both key and
+	// initiated time, since azure reports the same identical creation
+	// time for uploads created back to back.
+	sort.Slice(uploads, func(i, j int) bool {
 		if uploads[i].Key != uploads[j].Key {
 			return uploads[i].Key < uploads[j].Key
 		}
-		return uploads[i].Initiated.Before(uploads[j].Initiated)
+		if !uploads[i].Initiated.Equal(uploads[j].Initiated) {
+			return uploads[i].Initiated.Before(uploads[j].Initiated)
+		}
+		return uploads[i].UploadID < uploads[j].UploadID
 	})
 
 	result, err := backend.ListMultipartUploads(uploads, prefix, delimiter, keyMarker, uploadIDMarker, maxUploads)

backend/posix/posix.go

@@ -3147,12 +3147,18 @@ func (p *Posix) ListMultipartUploads(ctx context.Context, mpu *s3.ListMultipartU
 		}
 	}
 
-	// Sort once: Key asc, Initiated asc
-	sort.SliceStable(uploads, func(i, j int) bool {
+	// Sort once: Key asc, Initiated asc, UploadID asc
+	// UploadID is a tie-breaker for uploads that share both key and
+	// initiated time, since directory mtime resolution isn't always
+	// fine-grained enough to distinguish uploads created back to back.
+	sort.Slice(uploads, func(i, j int) bool {
 		if uploads[i].Key != uploads[j].Key {
 			return uploads[i].Key < uploads[j].Key
 		}
-		return uploads[i].Initiated.Before(uploads[j].Initiated)
+		if !uploads[i].Initiated.Equal(uploads[j].Initiated) {
+			return uploads[i].Initiated.Before(uploads[j].Initiated)
+		}
+		return uploads[i].UploadID < uploads[j].UploadID
 	})
 
 	result, err := backend.ListMultipartUploads(uploads, prefix, delimiter, keyMarker, uploadIDMarker, maxUploads)

tests/integration/ListMultipartUploads.go

@@ -302,12 +302,12 @@ func ListMultipartUploads_keyMarker_not_from_list(s *S3Conf) error {
 					UploadId:     out.UploadId,
 					StorageClass: types.StorageClassStandard,
 				})
-				if s.azureTests {
-					// add an artificial delay for azure tests
-					// as azure uploads all these mps with the same
-					// identical creation time
-					time.Sleep(time.Second)
-				}
+				// add an artificial delay, since some backends
+				// (e.g. azure, or posix on filesystems with coarse
+				// mtime resolution) can report the same identical
+				// creation/initiated time for uploads created back
+				// to back, making the resulting order non-deterministic
+				time.Sleep(time.Second)
 			}
 		}
 
@@ -323,7 +323,7 @@ func ListMultipartUploads_keyMarker_not_from_list(s *S3Conf) error {
 		}
 
 		if !compareMultipartUploads(uploads[3:], out.Uploads) {
-			return fmt.Errorf("expected the mp list to be %v, instead got %v", uploads[:3], out.Uploads)
+			return fmt.Errorf("expected the mp list to be %v, instead got %v", uploads[3:], out.Uploads)
 		}
 
 		// should start the listing after the specified uploadId marker