feat(runtime): add truncate and truncateSync methods to Deno.File (#10130)

cli/dts/lib.deno.ns.d.ts

@@ -781,6 +781,8 @@ declare namespace Deno {
     constructor(rid: number);
     write(p: Uint8Array): Promise<number>;
     writeSync(p: Uint8Array): number;
+    truncate(len?: number): void;
+    truncateSync(len?: number): Promise<void>;
     read(p: Uint8Array): Promise<number | null>;
     readSync(p: Uint8Array): number | null;
     seek(offset: number, whence: SeekMode): Promise<number>;

cli/tests/unit/file_test.ts

@@ -104,6 +104,50 @@ unitTest(function fileUsingEmptyStringFileName(): void {
   testSecondArgument("", "");
 });
 
+unitTest(
+  { perms: { read: true, write: true } },
+  function fileTruncateSyncSuccess(): void {
+    const filename = Deno.makeTempDirSync() + "/test_fileTruncateSync.txt";
+    const file = Deno.openSync(filename, {
+      create: true,
+      read: true,
+      write: true,
+    });
+
+    file.truncateSync(20);
+    assertEquals(Deno.readFileSync(filename).byteLength, 20);
+    file.truncateSync(5);
+    assertEquals(Deno.readFileSync(filename).byteLength, 5);
+    file.truncateSync(-5);
+    assertEquals(Deno.readFileSync(filename).byteLength, 0);
+
+    file.close();
+    Deno.removeSync(filename);
+  },
+);
+
+unitTest(
+  { perms: { read: true, write: true } },
+  async function fileTruncateSuccess(): Promise<void> {
+    const filename = Deno.makeTempDirSync() + "/test_fileTruncate.txt";
+    const file = await Deno.open(filename, {
+      create: true,
+      read: true,
+      write: true,
+    });
+
+    await file.truncate(20);
+    assertEquals((await Deno.readFile(filename)).byteLength, 20);
+    await file.truncate(5);
+    assertEquals((await Deno.readFile(filename)).byteLength, 5);
+    await file.truncate(-5);
+    assertEquals((await Deno.readFile(filename)).byteLength, 0);
+
+    file.close();
+    await Deno.remove(filename);
+  },
+);
+
 unitTest({ perms: { read: true } }, function fileStatSyncSuccess(): void {
   const file = Deno.openSync("README.md");
   const fileInfo = file.statSync();

runtime/js/40_files.js

@@ -4,7 +4,7 @@
 ((window) => {
   const core = window.Deno.core;
   const { read, readSync, write, writeSync } = window.__bootstrap.io;
-  const { fstat, fstatSync } = window.__bootstrap.fs;
+  const { ftruncate, ftruncateSync, fstat, fstatSync } = window.__bootstrap.fs;
   const { pathFromURL } = window.__bootstrap.util;
 
   function seekSync(
@@ -88,6 +88,14 @@
       return writeSync(this.rid, p);
     }
 
+    truncate(len) {
+      return ftruncate(this.rid, len);
+    }
+
+    truncateSync(len) {
+      return ftruncateSync(this.rid, len);
+    }
+
     read(p) {
       return read(this.rid, p);
     }