fs: refactor fs module

PR-URL: #20764
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

Backport-PR-URL: #21172

lib/internal/fs/read_file_context.js

@@ -0,0 +1,108 @@
+'use strict';
+
+const { Buffer } = require('buffer');
+const { FSReqWrap, close, read } = process.binding('fs');
+
+const kReadFileBufferLength = 8 * 1024;
+
+function readFileAfterRead(err, bytesRead) {
+  const context = this.context;
+
+  if (err)
+    return context.close(err);
+
+  if (bytesRead === 0)
+    return context.close();
+
+  context.pos += bytesRead;
+
+  if (context.size !== 0) {
+    if (context.pos === context.size)
+      context.close();
+    else
+      context.read();
+  } else {
+    // unknown size, just read until we don't get bytes.
+    context.buffers.push(context.buffer.slice(0, bytesRead));
+    context.read();
+  }
+}
+
+function readFileAfterClose(err) {
+  const context = this.context;
+  const callback = context.callback;
+  let buffer = null;
+
+  if (context.err || err)
+    return callback(context.err || err);
+
+  try {
+    if (context.size === 0)
+      buffer = Buffer.concat(context.buffers, context.pos);
+    else if (context.pos < context.size)
+      buffer = context.buffer.slice(0, context.pos);
+    else
+      buffer = context.buffer;
+
+    if (context.encoding)
+      buffer = buffer.toString(context.encoding);
+  } catch (err) {
+    return callback(err);
+  }
+
+  callback(null, buffer);
+}
+
+class ReadFileContext {
+  constructor(callback, encoding) {
+    this.fd = undefined;
+    this.isUserFd = undefined;
+    this.size = undefined;
+    this.callback = callback;
+    this.buffers = null;
+    this.buffer = null;
+    this.pos = 0;
+    this.encoding = encoding;
+    this.err = null;
+  }
+
+  read() {
+    let buffer;
+    let offset;
+    let length;
+
+    if (this.size === 0) {
+      buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
+      offset = 0;
+      length = kReadFileBufferLength;
+    } else {
+      buffer = this.buffer;
+      offset = this.pos;
+      length = Math.min(kReadFileBufferLength, this.size - this.pos);
+    }
+
+    const req = new FSReqWrap();
+    req.oncomplete = readFileAfterRead;
+    req.context = this;
+
+    read(this.fd, buffer, offset, length, -1, req);
+  }
+
+  close(err) {
+    const req = new FSReqWrap();
+    req.oncomplete = readFileAfterClose;
+    req.context = this;
+    this.err = err;
+
+    if (this.isUserFd) {
+      process.nextTick(function tick() {
+        req.oncomplete(null);
+      });
+      return;
+    }
+
+    close(this.fd, req);
+  }
+}
+
+module.exports = ReadFileContext;