Fix: allow reading into a 0-length array (#3329)

cli/js/files.ts

@@ -50,6 +50,9 @@ export async function open(
  *
  */
 export function readSync(rid: number, p: Uint8Array): number | EOF {
+  if (p.length == 0) {
+    return 0;
+  }
   const nread = sendSyncMinimal(dispatch.OP_READ, rid, p);
   if (nread < 0) {
     throw new Error("read error");
@@ -70,6 +73,9 @@ export function readSync(rid: number, p: Uint8Array): number | EOF {
  *       const text = new TextDecoder().decode(buf);
  */
 export async function read(rid: number, p: Uint8Array): Promise<number | EOF> {
+  if (p.length == 0) {
+    return 0;
+  }
   const nread = await sendAsyncMinimal(dispatch.OP_READ, rid, p);
   if (nread < 0) {
     throw new Error("read error");

cli/js/files_test.ts

@@ -118,6 +118,10 @@ testPerm(
     const filename = tempDir + "hello.txt";
     const file = await Deno.open(filename, "w+");
 
+    // reading into an empty buffer should return 0 immediately
+    const bytesRead = await file.read(new Uint8Array(0));
+    assert(bytesRead === 0);
+
     // reading file into null buffer should throw an error
     let err;
     try {

cli/js/io.ts

@@ -18,12 +18,14 @@ export enum SeekMode {
 // https://golang.org/pkg/io/#Reader
 export interface Reader {
   /** Reads up to p.byteLength bytes into `p`. It resolves to the number
-   * of bytes read (`0` < `n` <= `p.byteLength`) and rejects if any error encountered.
+   * of bytes read (`0` <= `n` <= `p.byteLength`) and rejects if any error encountered.
    * Even if `read()` returns `n` < `p.byteLength`, it may use all of `p` as
    * scratch space during the call. If some data is available but not
    * `p.byteLength` bytes, `read()` conventionally returns what is available
    * instead of waiting for more.
    *
+   * When `p.byteLength` == `0`, `read()` returns `0` and has no other effects.
+   *
    * When `read()` encounters end-of-file condition, it returns EOF symbol.
    *
    * When `read()` encounters an error, it rejects with an error.