all: ensure that tests do not write to the current directory

[bcmills]

In #27957, @hyangah noticed that the tests for compress/bzip2 fail when GOROOT is not writable, and those tests are run whenever we run go test all in module mode (which is intended to be a useful default).

As noted in #28386, tests should not assume that they can write to the current directory. We should ensure that none of the tests in the standard library make that mistake.

[tkivisik]

Does that mean the following pattern would be fine (create and use a temporary folder)
e.g.

wd, _ := os.Getwd()
dir, err := ioutil.TempDir("", "testing")
if err != nil {
	log.Fatal(err)
}
defer os.RemoveAll(dir) // clean up
// change and restore finally
if err := os.Chdir(dir); err != nil {
	return
}
defer os.Chdir(wd)
// Create necessary files for a test
// Run tests

[dpinela]

@bcmills It isn't the tests for compress/bzip2 that fail in #27957, it's the ones for compress/flate, and in that particular case it only happens when they would fail anyway - they write the actual output to testdata/*.got when actual != expected. Considering these files need to outlive the test, I'm not sure where else we should put them. On the other hand, this case might be fine - if your GOROOT is read-only, then you're probably using a stable release of Go, which presumably means the tests are passing; if they're not, you're using a dev tree, which is almost certainly writable.

[gopherbot]

Change https://golang.org/cl/145283 mentions this issue: os: ensure tests pass even if GOROOT is read-only

[bcmills]

@tkivisik, yes, but in most cases you shouldn't even need to bother with Getwd and Chdir.

I've found this pattern pretty helpful:

	dir, err := ioutil.TempDir("", path.Base(t.Name()))
	if err != nil {
		t.Fatal(err)
	}
	defer os.RemoveAll(dir)

I wonder whether we should just make that a method on *testing.T: the os.RemoveAll doesn't strictly need to be deferred right there at the call site (we can run it at any point after the test function returns), and it might be useful to have a standard flag (along the lines of -work) to leave the temporary directory in place.

[bcmills]

@dpinela

Considering these files need to outlive the test, I'm not sure where else we should put them.

Do they belong in side-files at all, given that the -update flag is not set? It seems like most regressions will be fairly small diffs, in which case we could log the first few lines of the diff — and that would also make the failure logs more useful if the reader doesn't have filesystem access (such as in the case of a TryBot failure).

If someone wants to dig in and fix a massive regression, they can always use -update and look at the diff using git diff instead of a separate side file.

[bcmills]

if your GOROOT is read-only, then you're probably using a stable release of Go, which presumably means the tests are passing

That's not a valid assumption in general. Platform- or architecture-dependent bugs and non-hermetic tests can all lead to failures even with a stable release.

[gopherbot]

Change https://golang.org/cl/146119 mentions this issue: go/internal/gcimporter: ensure tests pass even if GOROOT is read-only