Fix functionality

The original implementation didn't work (at all really) on new files or
edits. Because `cat file` was running on the pre-saved file we were
always formatting the previous version. 🤦

This implementation uses the current contents of the document and writes
that to a temporary file to be formatted.

src/extension.ts

@@ -1,16 +1,31 @@
 import { languages, Range, TextDocument, TextEdit } from 'vscode';
-import * as cp from 'child_process';
-import * as path from 'path';
+import { spawnSync } from 'child_process';
+import { join, basename } from 'path';
+import { randomBytes } from 'crypto';
+import { tmpdir } from 'os';
+import { readFileSync, writeFileSync } from 'fs';
 
 function provideDocumentFormattingEdits(document: TextDocument): TextEdit[] {
-  let tempfile = cp.spawnSync("mktemp").stdout.toString().trim();
-  tempfile = `${tempfile}.cue`;
+  const dir = tmpdir();
+  const currentFileName = basename(document.fileName);
+  const random = randomBytes(16).toString('hex');
+  const tmpfileName = `vscode-cue-fmt-${random}-${currentFileName}`;
+  const tmpfile = join(dir, tmpfileName);
 
-  const copy = cp.execSync(`cat ${document.fileName} > ${tempfile}`)
-  const fmt = cp.spawnSync("cue", ["fmt", tempfile], {});
-  const formatted = cp.spawnSync("cat", [tempfile]).stdout.toString().trim();
+  // copy current contents to a tempfile
+  writeFileSync(tmpfile, document.getText());
+
+  // run `cue fmt` on temp file
+  // TODO: handle errror/stderr, report to user
+  const fmt = spawnSync("cue", ["fmt", tmpfile], {});
+
+  // read formatted file
+  const formatted = readFileSync(tmpfile).toString();
+
+  // create range representing the whole document to be replaced
   const range = document.validateRange(new Range(0, 0, Infinity, Infinity));
 
+  // write formatted CUE back to document
   return [new TextEdit(range, formatted)];
 }