On macOS, os.tmpdir() is not a real path

[novemberborn]

• v7.4.0:
• macOS Sierra 10.12.3:
• os:

os.tmpdir() returns a path like /var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T. Per

node/lib/os.js

Line 43 in 00c86cc

path = process.env.TMPDIR ||

that's the same path as the $TMPDIR environment variable. However on macOS this is a symlink. The real path is /private/var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T:

❯ node -pe 'os.tmpdir()'
/var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T
❯ node -pe 'fs.realpathSync(os.tmpdir())'
/private/var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T

On the face of it the current behavior seems correct, and it's just macOS that is annoying. On the other hand it makes for surprising failures when writing cross-platform code. See sindresorhus/temp-write#8 for example.

At the very least I think this pitfall should be documented (happy to open a PR), but that just means users need to always wrap os.tmpdir() with fs.realpathSync(). Should os.tmpdir() do this by itself? On macOS only, or all platforms?

(Note that this is different from #7545. When your shell is in a directory like /var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T then process.env.PWD is correctly set to that path.)

[sindresorhus]

I think os.tmpdir() should consistently return the resolved path on all platforms. You would never want the symlink path.

[sam-github]

Don't you think

proces.env.TEMP = '/some/path';
assert.equal(os.tmpdir(), process.env.TEMP)

should be an invariant? You would request that we don't return the string that the user/OS has set the temp dir string to, but instead return some other path we derive from it? What if TEMP points a symlink, and they change the symlink, won't an app that called os.tmpdir() once before the change be mysteriously stuck to the old path and people will report the opposite bug, that we didn't respect TEMP?

I think setting TEMP to a symlink may be surprising in itself, but node unilaterally deciding the user didin't really mean what they did might be even more surprising.

node/lib/os.js

Lines 34 to 52 in a196895

	exports.tmpdir = function() {
	var path;
	if (isWindows) {
	path = process.env.TEMP ||
	process.env.TMP ||
	(process.env.SystemRoot || process.env.windir) + '\\temp';
	if (path.length > 1 && path.endsWith('\\') && !path.endsWith(':\\'))
	path = path.slice(0, -1);
	} else {
	path = process.env.TMPDIR ||
	process.env.TMP ||
	process.env.TEMP ||
	'/tmp';
	if (path.length > 1 && path.endsWith('/'))
	path = path.slice(0, -1);
	}
	return path;
	};

is the code in question.

[sindresorhus]

Don't you think ... should be an invariant?

No, because that's an implementation detail, and it's already a non-invariant—Your example would fail if the user had TMPDIR set to a different directory on macOS, because it's checked before TEMP. If users want to get something that matches exactly what they set, they should get it from process.env directly.

[bnoordhuis]

One reason why I think this is unlikely to happen is that it would mean changing os.tmpdir() to take a callback (because silently sync operations in core? No.)

It's either that or resolve and cache at startup, but then changes to process.env.TMPDIR won't be visible. Neither option is very palatable.

[novemberborn]

One reason why I think this is unlikely to happen is that it would mean changing os.tmpdir() to take a callback

I don't think a os.tmpdirSync() is particular user friendly either.

It's either that or resolve and cache at startup, but then changes to process.env.TMPDIR won't be visible.

The implementation of os.tmpdir() (which environment variables it uses, and in what order) isn't documented though. It just says:

The os.tmpdir() method returns a string specifying the operating system's default directory for temporary files.

It should probably be treated as a breaking change, sure, but returning a real path may make the os.tmpdir() function align better with user expectations. (It would with mine, but that's just a sample of 1.)

If that's not a viable direction we should at least document that it may not return a real path, and that this is a known issue on macOS.

[bnoordhuis]

I did a quick survey and both python's tempfile.gettempdir() and ruby's Dir.tmpdir return the unresolved path, i.e., the symlink. Node isn't the outlier.

The implementation of os.tmpdir() (which environment variables it uses, and in what order) isn't documented though.

The environment variables and the order they're tried in is fairly standard for UNIX and Windows programs. I'd be okay with having that documented.

[Trott]

This issue has been inactive for sufficiently long that it seems like perhaps it should be closed. Feel free to re-open (or leave a comment requesting that it be re-opened) if you disagree. I'm just tidying up and not acting on a super-strong opinion or anything like that.

[novemberborn]

@Trott the documentation around this could be approved. Might be worth keeping the ticket around for that. I just haven't gotten around to it.