module: warn on using unfinished circular dependency

Warn when a non-existent property of an unfinished module.exports
object is being accessed, as that very often indicates the presence
of a hard-to-detect and hard-to-debug problem.

This mechanism is only used if `module.exports` is still a
regular object at the point at which the second, circular `require()`
happens.

The downside is that, temporarily, `module.exports` will have a
prototype other than `Object.prototype`, and that there may
be valid uses of accessing non-existent properties of unfinished
`module.exports` objects.

Performance of circular require calls in general is not
noticeably impacted.

                                               confidence improvement accuracy (*)   (**)  (***)
     module/module-loader-circular.js n=10000                 3.96 %       ±5.12% ±6.82% ±8.89%

Example:

    $ cat a.js
    'use strict';
    const b = require('./b.js');

    exports.fn = () => {};
    $ cat b.js
    'use strict';
    const a = require('./a.js');

    a.fn();
    $ node a.js
    (node:1617) Warning: Accessing non-existent property 'fn' of module exports inside circular dependency
    /tmp/b.js:4
    a.fn();
      ^

    TypeError: a.fn is not a function
        at Object.<anonymous> (/tmp/b.js:4:3)
        [...]

PR-URL: https://github.com/nodejs/node/pull/29935
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/benchmark/module/module-loader-circular.js b/benchmark/module/module-loader-circular.js
new file mode 100644
index 0000000000..6d392e0e19
--- /dev/null
+++ b/benchmark/module/module-loader-circular.js
@@ -0,0 +1,34 @@
+'use strict';
+const fs = require('fs');
+const path = require('path');
+const common = require('../common.js');
+
+const tmpdir = require('../../test/common/tmpdir');
+const benchmarkDirectory =
+  path.resolve(tmpdir.path, 'benchmark-module-circular');
+
+const bench = common.createBenchmark(main, {
+  n: [1e4]
+});
+
+function main({ n }) {
+  tmpdir.refresh();
+
+  const aDotJS = path.join(benchmarkDirectory, 'a.js');
+  const bDotJS = path.join(benchmarkDirectory, 'b.js');
+
+  fs.mkdirSync(benchmarkDirectory);
+  fs.writeFileSync(aDotJS, 'require("./b.js");');
+  fs.writeFileSync(bDotJS, 'require("./a.js");');
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    require(aDotJS);
+    require(bDotJS);
+    delete require.cache[aDotJS];
+    delete require.cache[bDotJS];
+  }
+  bench.end(n);
+
+  tmpdir.refresh();
+}