Calling function containing defer during an interrupt handler function causes a panic

[waj334]

var globalTestCrash testCrash

type testCrash struct {
}

func (t testCrash) test() {
	defer t.test2()
}

func (t testCrash) test2() {

}

func irqHandler(irq machine.Pin) {
	println("******************************************** ISR")

	globalTestCrash.test()
}

Above I have a basic interrupt handler set up that will panic once test() is called:

******************************************** ISR
panic: runtime error: nil pointer dereference

>tinygo version          
tinygo version 0.25.0 windows/amd64 (using go version go1.19 and LLVM version 14.0.0)

[dgryski]

Pretty sure this is because defer requires an allocation, which is currently unsafe during interrupt handlers.

[kenbell]

@dgryski I don't think it's an allocation issue... this test code shows no allocation, but does seem to 'not work'...

package main

import (
	"machine"
	"time"
)

func main() {
	time.Sleep(2 * time.Second)

	machine.LED.Configure(machine.PinConfig{Mode: machine.PinOutput})
	machine.GPIO12.Configure(machine.PinConfig{Mode: machine.PinInputPulldown})
	machine.GPIO12.SetInterrupt(machine.PinFalling, f)

	for {
		time.Sleep(1 * time.Second)
	}
}

func f(machine.Pin) {
	defer machine.LED.Set(!machine.LED.Get())
}

Output of tinygo flash -monitor -target feather-rp2040 -print-allocs .:

<snip>/tinygo/src/runtime/baremetal.go:42:14: object allocated on the heap: size is not constant
<snip>/tinygo/src/internal/task/task_stack.go:67:15: object allocated on the heap: size is not constant
<snip>/tinygo/src/internal/task/task_stack.go:102:12: object allocated on the heap: escapes at line 104
<snip>/tinygo/src/machine/machine_rp2040_clocks.go:87:15: object allocated on the heap: escapes at line 87

[xudongzheng]

I'm running into this as well and it seems like the panic happens even if the function never gets deferred.

package main

import (
	"machine"
	"runtime/interrupt"
)

var button = machine.D106

var i int

func test1() {
	i++
}

func test2() {
	i--
}

func main() {
	println("hello world")
	button.Configure(machine.PinConfig{Mode: machine.PinInputPullup})
	button.SetInterrupt(machine.PinToggle, func(p machine.Pin) {
		if !interrupt.In() {
			test1()
			defer test2()
		}
	})
}

[aykevl]

We don't support defer in interrupts. We could maybe add support for it, but right now it's not supported.

Here is a PR to improve the error message: #4522
With this PR, it will say something like:

panic: runtime error at 0x000005db: defer in interrupt

[aykevl]

I'm running into this as well and it seems like the panic happens even if the function never gets deferred.

Correct, any function with defer will set up a special hidden defer frame in the current goroutine. Because interrupts aren't running in any goroutine, this doesn't work.

The following would work though, because foo is never called inside an interrupt:

func foo() {
	test1()
	defer test2()
}

button.SetInterrupt(machine.PinToggle, func(p machine.Pin) {
	if !interrupt.In() {
		foo()
	}
})