Validate upload URL before initiating large file upload

There seems to be a situation where a large file upload can be marked as
non-local and attempt to POST file contents to an invalid URL (in this
case, the name of the local upload directory).

This obviously results in an error. This change ensures that the
designated upload URL is actually a valid URL, otherwise it forces the
local-only "dummy" mode to be enabled.

See #14

Author: benbusby

backend/server/transfer/upload.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"github.com/benbusby/b2"
 	"log"
+	"net/url"
 	db "yeetfile/backend/db"
 	"yeetfile/backend/service"
 	"yeetfile/backend/utils"
@@ -155,12 +156,22 @@ func InitLargeB2Upload(filename string, upload db.B2Upload) error {
 		return err
 	}
 
+	isDummy := info.Dummy
+	if !isDummy {
+		// Ensure that the dummy option is enabled if the request URI
+		// is not actually valid
+		_, err = url.ParseRequestURI(upload.UploadURL)
+		if err != nil {
+			isDummy = true
+		}
+	}
+
 	return db.UpdateUploadValues(
 		upload.MetadataID,
 		info.UploadURL,
 		info.AuthorizationToken,
 		info.FileID, // Multi-chunk files use the file ID for uploading
-		info.Dummy)
+		isDummy)
 }
 
 func ResetLargeUpload(b2FileID string, metadataID string) (b2.FilePartInfo, error) {