Follow-up to 4e2e9be6a - Move HeapSnapshot DevTools-only Modules back to DevTools

I am so done with this.

Resolves #316

devtools/shared/heapsnapshot/HeapSnapshotFileUtils.js

@@ -0,0 +1,95 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Heap snapshots are always saved in the temp directory, and have a regular
+// naming convention. This module provides helpers for working with heap
+// snapshot files in a safe manner. Because we attempt to avoid unnecessary
+// copies of the heap snapshot files by checking the local filesystem for a heap
+// snapshot file with the given snapshot id, we want to ensure that we are only
+// attempting to open heap snapshot files and not `~/.ssh/id_rsa`, for
+// example. Therefore, the RDP only talks about snapshot ids, or transfering the
+// bulk file data. A file path can be recovered from a snapshot id, which allows
+// one to check for the presence of the heap snapshot file on the local file
+// system, but we don't have to worry about opening arbitrary files.
+//
+// The heap snapshot file path conventions permits the following forms:
+//
+//     $TEMP_DIRECTORY/XXXXXXXXXX.fxsnapshot
+//     $TEMP_DIRECTORY/XXXXXXXXXX-XXXXX.fxsnapshot
+//
+// Where the strings of "X" are zero or more digits.
+
+"use strict";
+
+const { Ci } = require("chrome");
+loader.lazyRequireGetter(this, "FileUtils",
+                         "resource://gre/modules/FileUtils.jsm", true);
+loader.lazyRequireGetter(this, "OS", "resource://gre/modules/osfile.jsm", true);
+
+function getHeapSnapshotFileTemplate() {
+  return OS.Path.join(OS.Constants.Path.tmpDir, `${Date.now()}.fxsnapshot`);
+}
+
+/**
+ * Get a unique temp file path for a new heap snapshot. The file is guaranteed
+ * not to exist before this call.
+ *
+ * @returns String
+ */
+exports.getNewUniqueHeapSnapshotTempFilePath = function () {
+  let file = new FileUtils.File(getHeapSnapshotFileTemplate());
+  // The call to createUnique will append "-N" after the leaf name (but before
+  // the extension) until a new file is found and create it. This guarantees we
+  // won't accidentally choose the same file twice.
+  file.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o666);
+  return file.path;
+};
+
+function isValidSnapshotFileId(snapshotId) {
+  return /^\d+(\-\d+)?$/.test(snapshotId);
+}
+
+/**
+ * Get the file path for the given snapshot id.
+ *
+ * @param {String} snapshotId
+ *
+ * @returns String | null
+ */
+exports.getHeapSnapshotTempFilePath = function (snapshotId) {
+  // Don't want anyone sneaking "../../../.." strings into the snapshot id and
+  // trying to make us open arbitrary files.
+  if (!isValidSnapshotFileId(snapshotId)) {
+    return null;
+  }
+  return OS.Path.join(OS.Constants.Path.tmpDir, snapshotId + ".fxsnapshot");
+};
+
+/**
+ * Return true if we have the heap snapshot file for the given snapshot id on
+ * the local file system. False is returned otherwise.
+ *
+ * @returns Promise<Boolean>
+ */
+exports.haveHeapSnapshotTempFile = function (snapshotId) {
+  const path = exports.getHeapSnapshotTempFilePath(snapshotId);
+  if (!path) {
+    return Promise.resolve(false);
+  }
+
+  return OS.File.stat(path).then(() => true,
+                                 () => false);
+};
+
+/**
+ * Given a heap snapshot's file path, extricate the snapshot id.
+ *
+ * @param {String} path
+ *
+ * @returns String
+ */
+exports.getSnapshotIdFromPath = function (path) {
+  return path.slice(OS.Constants.Path.tmpDir.length + 1,
+                    path.length - ".fxsnapshot".length);
+};