fs: support pseudofiles in promises.readFile

PR-URL: #21497
Fixes: #21331
Refs: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
Refs: https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/fs/promises.js

@@ -125,6 +125,10 @@ async function writeFileHandle(filehandle, data, options) {
   } while (remaining > 0);
 }
 
+// Note: This is different from kReadFileBufferLength used for non-promisified
+// fs.readFile.
+const kReadFileMaxChunkSize = 16384;
+
 async function readFileHandle(filehandle, options) {
   const statFields = await binding.fstat(filehandle.fd, false, kUsePromises);
 
@@ -135,22 +139,19 @@ async function readFileHandle(filehandle, options) {
     size = 0;
   }
 
-  if (size === 0)
-    return options.encoding ? '' : Buffer.alloc(0);
-
   if (size > kMaxLength)
     throw new ERR_FS_FILE_TOO_LARGE(size);
 
   const chunks = [];
-  const chunkSize = Math.min(size, 16384);
-  let totalRead = 0;
+  const chunkSize = size === 0 ?
+    kReadFileMaxChunkSize :
+    Math.min(size, kReadFileMaxChunkSize);
   let endOfFile = false;
   do {
     const buf = Buffer.alloc(chunkSize);
     const { bytesRead, buffer } =
-      await read(filehandle, buf, 0, chunkSize, totalRead);
-    totalRead += bytesRead;
-    endOfFile = bytesRead !== chunkSize;
+      await read(filehandle, buf, 0, chunkSize, -1);
+    endOfFile = bytesRead === 0;
     if (bytesRead > 0)
       chunks.push(buffer.slice(0, bytesRead));
   } while (!endOfFile);

test/parallel/test-fs-promises-file-handle-readFile.js

@@ -28,5 +28,22 @@ async function validateReadFile() {
   assert.deepStrictEqual(buffer, readFileData);
 }
 
+async function validateReadFileProc() {
+  // Test to make sure reading a file under the /proc directory works. Adapted
+  // from test-fs-read-file-sync-hostname.js.
+  // Refs:
+  // - https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
+  // - https://github.com/nodejs/node/issues/21331
+
+  // Test is Linux-specific.
+  if (!common.isLinux)
+    return;
+
+  const fileHandle = await open('/proc/sys/kernel/hostname', 'r');
+  const hostname = await fileHandle.readFile();
+  assert.ok(hostname.length > 0);
+}
+
 validateReadFile()
+  .then(() => validateReadFileProc())
   .then(common.mustCall());

test/parallel/test-fs-promises-readfile.js

@@ -12,17 +12,35 @@ const fn = path.join(tmpdir.path, 'large-file');
 
 common.crashOnUnhandledRejection();
 
-// Creating large buffer with random content
-const buffer = Buffer.from(
-  Array.apply(null, { length: 16834 * 2 })
-    .map(Math.random)
-    .map((number) => (number * (1 << 8)))
-);
-
-// Writing buffer to a file then try to read it
-writeFile(fn, buffer)
-  .then(() => readFile(fn))
-  .then((readBuffer) => {
-    assert.strictEqual(readBuffer.equals(buffer), true);
-  })
+async function validateReadFile() {
+  // Creating large buffer with random content
+  const buffer = Buffer.from(
+    Array.apply(null, { length: 16834 * 2 })
+      .map(Math.random)
+      .map((number) => (number * (1 << 8)))
+  );
+
+  // Writing buffer to a file then try to read it
+  await writeFile(fn, buffer);
+  const readBuffer = await readFile(fn);
+  assert.strictEqual(readBuffer.equals(buffer), true);
+}
+
+async function validateReadFileProc() {
+  // Test to make sure reading a file under the /proc directory works. Adapted
+  // from test-fs-read-file-sync-hostname.js.
+  // Refs:
+  // - https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
+  // - https://github.com/nodejs/node/issues/21331
+
+  // Test is Linux-specific.
+  if (!common.isLinux)
+    return;
+
+  const hostname = await readFile('/proc/sys/kernel/hostname');
+  assert.ok(hostname.length > 0);
+}
+
+validateReadFile()
+  .then(() => validateReadFileProc())
   .then(common.mustCall());