fs: WriteStream should handle partial writes

Given a buffer of length l, fs.write() will not necessarily write the
entire buffer to the file. This can occur if, for example, there is
insufficient space on the underlying physical medium.

WriteStream did not handle this case, and when partial write occurs,
it will errorneously report that the write is successful.

This commit changes the _write() behavior to continue the write
operation, picking up from where the last operation left off.

More information about the write() system call is available at
http://man7.org/linux/man-pages/man2/write.2.html

lib/internal/fs/streams.js

@@ -308,16 +308,28 @@ WriteStream.prototype._write = function(data, encoding, cb) {
     });
   }
 
-  fs.write(this.fd, data, 0, data.length, this.pos, (er, bytes) => {
+  const len = data.length;
+  let pos = this.pos;
+  let offset = 0;
+  const callback = (er, bytes) => {
     if (er) {
       if (this.autoClose) {
         this.destroy();
       }
       return cb(er);
     }
     this.bytesWritten += bytes;
-    cb();
-  });
+    offset += bytes;
+    if (offset < len) {
+      if (pos !== undefined) {
+        pos += bytes;
+      }
+      fs.write(this.fd, data, offset, len - offset, pos, callback);
+    } else {
+      cb();
+    }
+  };
+  fs.write(this.fd, data, offset, len, pos, callback);
 
   if (this.pos !== undefined)
     this.pos += data.length;

test/parallel/test-fs-write-stream-partial-write.js

@@ -0,0 +1,42 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+
+const tmpdir = require('../common/tmpdir');
+const file = path.join(tmpdir.path, 'out.txt');
+
+tmpdir.refresh();
+
+{
+  const _fs_write = fs.write;
+  let i = 0;
+  fs.write = function(fd, data, offset, length, pos, cb) {
+    if (i === 0) {
+      const half = Math.floor(length / 2);
+      _fs_write(fd, data, offset, half, pos, cb);
+    } else {
+      assert(
+        offset > 0,
+        `second call to fs.write() must provide non-zero offset, got ${offset}`
+      );
+      _fs_write(fd, data, offset, length, pos, cb);
+    }
+    i++;
+  };
+
+  const stream = fs.createWriteStream(file);
+  const data = Buffer.from('Hello World, how are you?', 'ascii');
+  stream.write(data, 'ascii', common.mustCall(function(err) {
+    assert.ifError(err);
+  }));
+  stream.end();
+
+  process.on('exit', function() {
+    assert.strictEqual(
+      i, 2, 'expected fs.write() to be called twice but got ' + i);
+    const content = fs.readFileSync(file);
+    assert.strictEqual(content.toString(), 'Hello World, how are you?');
+  });
+}