uplinkc: add download_range function (#3704)

lib/uplinkc/object.go

@@ -278,6 +278,31 @@ func download(bucketRef C.BucketRef, path *C.char, cErr **C.char) C.DownloaderRe
 	})}
 }
 
+//export download_range
+// download_range returns an Object's data from specified range
+func download_range(bucketRef C.BucketRef, path *C.char, start, limit int64, cErr **C.char) C.DownloaderRef {
+	bucket, ok := universe.Get(bucketRef._handle).(*Bucket)
+	if !ok {
+		*cErr = C.CString("invalid bucket")
+		return C.DownloaderRef{}
+	}
+
+	scope := bucket.scope.child()
+
+	rc, err := bucket.DownloadRange(scope.ctx, C.GoString(path), start, limit)
+	if err != nil {
+		if !errs2.IsCanceled(err) {
+			*cErr = C.CString(fmt.Sprintf("%+v", err))
+		}
+		return C.DownloaderRef{}
+	}
+
+	return C.DownloaderRef{universe.Add(&Download{
+		scope: scope,
+		rc:    rc,
+	})}
+}
+
 //export download_read
 // download_read reads data upto `length` bytes into `bytes` buffer and returns
 // the count of bytes read. The exact number of bytes returned depends on different

lib/uplinkc/testdata/object_test.c

@@ -187,6 +187,37 @@ void handle_project(ProjectRef project) {
             free_downloader(downloader);
         }
 
+        { // download range
+            long start = 100 + (i+1);
+            long limit = 1024 * (i+1);
+            DownloaderRef downloader = download_range(bucket, object_paths[i], start, limit, err);
+            require_noerror(*err);
+
+            uint8_t *downloaded_data = malloc(limit);
+            memset(downloaded_data, '\0', limit);
+            size_t downloaded_total = 0;
+
+            size_t size_to_read = 256 + i;
+            while (downloaded_total < limit) {
+                size_t read_size = download_read(downloader, &downloaded_data[downloaded_total], size_to_read, err);
+                require_noerror(*err);
+
+                if (read_size == 0) {
+                    break;
+                }
+
+                downloaded_total += read_size;
+            }
+
+            download_close(downloader, err);
+            require_noerror(*err);
+            require(limit == downloaded_total);
+            require(memcmp(&data[start], downloaded_data, limit) == 0);
+
+            free(downloaded_data);
+            free_downloader(downloader);
+        }
+
         if (data != NULL) {
             free(data);
         }