Fix FileNotFoundEx crash when Fold happens on untitled: scheme doc (#839)

* Fix FileNotFoundEx crash when Fold happens on untitled: scheme doc

* Address PR feedback, use TryGetFile() instead of modifying GetFile

Author: rkeithhill

src/PowerShellEditorServices.Protocol/Server/LanguageServer.cs

@@ -1376,15 +1376,24 @@ protected Task HandleEvaluateRequest(
 
         #region Event Handlers
 
-        private FoldingRange[] Fold(
-            string documentUri)
+        private FoldingRange[] Fold(string documentUri)
         {
             // TODO Should be using dynamic registrations
             if (!this.currentSettings.CodeFolding.Enable) { return null; }
+
+            // Avoid crash when using untitled: scheme or any other scheme where the document doesn't
+            // have a backing file.  https://github.com/PowerShell/vscode-powershell/issues/1676
+            // Perhaps a better option would be to parse the contents of the document as a string
+            // as opposed to reading a file but the senario of "no backing file" probably doesn't
+            // warrant the extra effort.
+            ScriptFile scriptFile;
+            if (!editorSession.Workspace.TryGetFile(documentUri, out scriptFile)) { return null; }
+
             var result = new List<FoldingRange>();
-            foreach (FoldingReference fold in TokenOperations.FoldableRegions(
-                editorSession.Workspace.GetFile(documentUri).ScriptTokens,
-                this.currentSettings.CodeFolding.ShowLastLine))
+            FoldingReference[] foldableRegions = 
+                TokenOperations.FoldableRegions(scriptFile.ScriptTokens, this.currentSettings.CodeFolding.ShowLastLine);
+
+            foreach (FoldingReference fold in foldableRegions)
             {
                 result.Add(new FoldingRange {
                     EndCharacter   = fold.EndCharacter,
@@ -1394,6 +1403,7 @@ private FoldingRange[] Fold(
                     StartLine      = fold.StartLine
                 });
             }
+
             return result.ToArray();
         }
 

src/PowerShellEditorServices/Workspace/Workspace.cs

@@ -91,8 +91,9 @@ public ScriptFile CreateScriptFileFromFileBuffer(string filePath, string initial
         }
 
         /// <summary>
-        /// Gets an open file in the workspace.  If the file isn't open but
-        /// exists on the filesystem, load and return it.
+        /// Gets an open file in the workspace. If the file isn't open but exists on the filesystem, load and return it.
+        /// <para>IMPORTANT: Not all documents have a backing file e.g. untitled: scheme documents.  Consider using
+        /// <see cref="Workspace.TryGetFile(string, out ScriptFile)"/> instead.</para>
         /// </summary>
         /// <param name="filePath">The file path at which the script resides.</param>
         /// <exception cref="FileNotFoundException">
@@ -154,9 +155,7 @@ e is DirectoryNotFoundException ||
                 e is PathTooLongException ||
                 e is UnauthorizedAccessException)
             {
-                this.logger.WriteException(
-                    $"Failed to set breakpoint on file: {filePath}",
-                    e);
+                this.logger.WriteHandledException($"Failed to get file for {nameof(filePath)}: '{filePath}'", e);
                 scriptFile = null;
                 return false;
             }