mkdtemp doc issue?

[vtjnash]

During test-writing / manual fuzzing, I noticed that the doc for mkdtemp added in #6800 states that "Generates six random characters to be appended behind a required prefix to create a unique temporary directory". This is true on Linux and Windows, but misses an edge case on BSDs (including Apple). As seen below, it actually "replaces trailing X's with random characters and then appends six additional random characters to the prefix to create a unique directory name".

// on macOS
> fs.mkdtempSync(path.join(os.tmpdir(), 'foox')) // does as documented
'/tmp/fooxz5Ve7r'

> fs.mkdtempSync(path.join(os.tmpdir(), 'fooX')) // also replaces the X from the prefix
'/tmp/foo1PqEasA'
         ^-- not an 'X'

I think this is just a doc issue, but opening this as an issue to hear what others think.

[sam-github]

@nodejs/libuv Not sure if this is a libuv issue. Its just calling mkdtemp(), should it be shielding users from platform variations in that API?

If not, node docs could say "some systems may replace trailing 'X's. Do not rely on this for portability." or something of the like.

[vtjnash]

Thanks for confirming and clarifying the intended behavior!