testing: t.FailNow() in a deferred function masks test panic

[matthewmcnew]

When a test panics, any deferred functions will still process allowing the test to properly clean up. If a deferred function executes t.FailNow() the test output will not include any information on the panic or the panic's goroutine trace.

An example is shown here: https://play.golang.org/p/gWPDVluKaST
Despite that test clearly panicking with a t.FailNow() in the deferred function the test output only shows:

--- FAIL: TestPanic (0.00s)
FAIL

The desired behavior would be along the lines of this example: https://play.golang.org/p/qrLtmYnkqAh. (Where the deferred function calls runtime.Goexit() instead) This produces the following output:

--- FAIL: TestPanic (0.00s)
panic: Test Panic
	panic: test executed panic(nil) or runtime.Goexit

goroutine 5 [running]:
testing.tRunner.func1(0x4560b0, 0xf)
	/usr/local/go/src/testing/testing.go:792 +0x460
runtime.Goexit()
	/usr/local/go/src/runtime/panic.go:397 +0x140
main.TestPanic.func1()
	/tmp/sandbox284554843/main.go:18 +0x20
panic(0x121940, 0x165a58)
	/usr/local/go/src/runtime/panic.go:513 +0x240
main.TestPanic(0x4560b0, 0xbab699fc)
	/tmp/sandbox284554843/main.go:20 +0x60
testing.tRunner(0x4560b0, 0x1558f8)
	/usr/local/go/src/testing/testing.go:827 +0x140
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:878 +0x3c0

I believe this is the result of this line:

go/src/testing/testing.go

Line 813 in bbae8d5

if !t.finished && err == nil {

!t.finished will return false because t.Now() marks the test finished and err will be nil because the runtime.Goexit() in t.FatalNow() is what is being recovered (not the original panic).

This prevents the expected panic on this line:

go/src/testing/testing.go

Line 827 in bbae8d5

panic(err)

[bcmills]

Probably FailNow should check for a panic, but in order to distinguish a panic from a runtime.GoExit we must attempt to recover it (#25448 (comment)), and that destroys the stack information associated with the original panic. (I think I discussed that caveat with @aclements at GopherCon...)

[bcmills]

CC @mpvl @josharian

[ianlancetaylor]

I don't see how testing.FailNow could test for a panic. Calling recover won't work, as recover only returns non-nil if called directly from a deferred function, which would not be the case if it is called by testing.FailNow.

What does this come up, though? Why defer a call to FailNow or Fatal?

[bcmills]

Hmm, good point. FailNow itself can't check for a panic, but the test runner can.

Unfortunately, it appears that runtime.Goexit currently cancels a panic (https://play.golang.org/p/xO5kMrMQkVS). I'm not sure whether that is a bug or the intended behavior of Goexit.

[nhooyr]

What does this come up, though? Why defer a call to FailNow or Fatal?

Test helpers that are deferred. E.g. to cleanup temp files.

[nhooyr]

I could change calls to t.Fatal in deferred functions to be t.Error and then return, but imo this should just work to make error handling code in the test itself and deferred functions to maintain the same structure.