readfilecontext: added test for reading file with unknown length

lib/readfilecontext.js

@@ -62,7 +62,7 @@ exports.patchReadFileContext = function(prototype) {
           bytesRead === kReadFileUnknownBufferLength
             ? context.buffer
             : context.buffer.slice(0, bytesRead);
-        Array.prototype.push.apply(context.buffers, buffer);
+        context.buffers.push(buffer);
       }
       context.read();
     }

test/lib/readfilecontext.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const constants = require('constants');
 const helper = require('../helper');
 const fs = require('fs');
 const mock = require('../../lib/index');
@@ -145,4 +146,30 @@ describe('fs.readFile() with ReadFileContext', function() {
       done();
     });
   });
+
+  it('allows file reads with unknown size', function(done) {
+    mock({
+      'unknown-size.txt': function() {
+        const file = mock.file({
+          content: Buffer.from('unknown size')
+        })();
+
+        // Override getStats to drop the S_IFREG flag
+        const origGetStats = file.getStats;
+        file.getStats = function() {
+          const stats = origGetStats.apply(this, arguments);
+          stats[1] ^= constants.S_IFREG;
+          return stats;
+        };
+        return file;
+      }
+    });
+    // This isn't actually supported by mock-fs, but let's make sure the call goes through
+    // It also covers the case of reading an empty file and reading with encoding
+    fs.readFile('unknown-size.txt', 'utf-8', function(err, data) {
+      assert.isNull(err);
+      assert.equal(data, 'unknown size');
+      done();
+    });
+  });
 });