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wasm: add recover support

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Add unwinding and recover support for wasm using WebAssembly exception
handling. This still has a few gotchas:

  * Many WASI systems don't support exception handling yet.
    For example, see:
    https://github.com/bytecodealliance/wasmtime/issues/2049
  * Asyncify doesn't support wasm exception handling:
    https://github.com/WebAssembly/binaryen/issues/4470
    This means it's not possible to use goroutines together with
    panic/recover.
  * The current way that exceptions are implemented pretend to be C++
    exceptions, but work slightly differently. If C++ code is called
    (for example through CGo) that raises an exception, that exception
    will be eaten by TinyGo and not be propagated. This is fixable, it
    just hasn't been implemented (because we don't actually support C++
    right now).

I hope that these issues will be resolved over time. At least for now,
people who need `recover()` have a way to use it.
This commit is contained in:
Ayke van Laethem
2024-08-03 09:56:19 +02:00
parent 2f7952ff29
commit 89ffb47a0c
28 changed files with 885 additions and 397 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/runtime/panic.go
+98 -28
View File
@@ -15,11 +15,20 @@ func trap()
// purposes of TinyGo. It restores the stack pointer and jumps to the given pc.
//
//export tinygo_longjmp
func tinygo_longjmp(frame *deferFrame)
func tinygo_longjmp(frame *deferFrameInlineAsm)
// Compiler intrinsic.
// Returns whether recover is supported on the current architecture.
func supportsRecover() bool
func supportsRecover() recoverType
type recoverType uint8
const (
// Note: these contants match recoverSupport in compiler/defer.go
recoverNone recoverType = iota
recoverInlineAsm
recoverWasmEH
)
const (
panicStrategyPrint = 1
@@ -31,33 +40,55 @@ const (
// using the -panic= compiler flag.
func panicStrategy() uint8
// DeferFrame is a stack allocated object that stores information for the
// current "defer frame", which is used in functions that use the `defer`
// LLVM compiler intrinsic: this inserts a wasm 'throw' instruction.
//
//export llvm.wasm.throw
func wasm_throw(uint32, unsafe.Pointer)
// DeferFrameInlineAsm is a stack allocated object that stores information for
// the current "defer frame", which is used in functions that use the `defer`
// keyword.
// The compiler knows about the JumpPC struct offset, so it should not be moved
// without also updating compiler/defer.go.
type deferFrame struct {
type deferFrameInlineAsm struct {
JumpSP unsafe.Pointer // stack pointer to return to
JumpPC unsafe.Pointer // pc to return to
ExtraRegs [deferExtraRegs]unsafe.Pointer // extra registers (depending on the architecture)
Previous *deferFrame // previous recover buffer pointer
Previous *deferFrameInlineAsm // previous recover buffer pointer
Panicking bool // true iff this defer frame is panicking
PanicValue interface{} // panic value, might be nil for panic(nil) for example
}
// DeferFrameWasmEH is like deferFrameInlineAsm but for WebAssembly exception
// handling.
type deferFrameWasmEH struct {
Previous *deferFrameWasmEH // previous recover buffer pointer
Panicking bool // true iff this defer frame is panicking
PanicValue interface{} // panic value, might be nil for panic(nil) for example
}
// Builtin function panic(msg), used as a compiler intrinsic.
func _panic(message interface{}) {
if panicStrategy() == panicStrategyTrap {
trap()
}
if supportsRecover() {
frame := (*deferFrame)(task.Current().DeferFrame)
switch supportsRecover() {
case recoverInlineAsm:
frame := (*deferFrameInlineAsm)(task.Current().DeferFrame)
if frame != nil {
frame.PanicValue = message
frame.Panicking = true
tinygo_longjmp(frame)
// unreachable
}
case recoverWasmEH:
frame := (*deferFrameWasmEH)(task.Current().DeferFrame)
if frame != nil {
frame.PanicValue = message
frame.Panicking = true
wasm_throw(0, nil)
// unreachable
}
}
printstring("panic: ")
printitf(message)
@@ -94,21 +125,43 @@ func runtimePanicAt(addr unsafe.Pointer, msg string) {
//
//go:inline
//go:nobounds
func setupDeferFrame(frame *deferFrame, jumpSP unsafe.Pointer) {
func setupDeferFrameInlineAsm(frame *deferFrameInlineAsm, jumpSP unsafe.Pointer) {
currentTask := task.Current()
frame.Previous = (*deferFrame)(currentTask.DeferFrame)
frame.Previous = (*deferFrameInlineAsm)(currentTask.DeferFrame)
frame.JumpSP = jumpSP
frame.Panicking = false
currentTask.DeferFrame = unsafe.Pointer(frame)
}
//go:inline
//go:nobounds
func setupDeferFrameWasmEH(frame *deferFrameWasmEH) {
currentTask := task.Current()
frame.Previous = (*deferFrameWasmEH)(currentTask.DeferFrame)
frame.Panicking = false
currentTask.DeferFrame = unsafe.Pointer(frame)
}
// Called right before the return instruction. It pops the defer frame from the
// linked list of defer frames. It also re-raises a panic if the goroutine is
// still panicking.
//
//go:inline
//go:nobounds
func destroyDeferFrame(frame *deferFrame) {
func destroyDeferFrameInlineAsm(frame *deferFrameInlineAsm) {
task.Current().DeferFrame = unsafe.Pointer(frame.Previous)
if frame.Panicking {
// We're still panicking!
// Re-raise the panic now.
_panic(frame.PanicValue)
}
}
// Like destroyDeferFrameInlineAsm but for wasm.
//
//go:inline
//go:nobounds
func destroyDeferFrameWasmEH(frame *deferFrameWasmEH) {
task.Current().DeferFrame = unsafe.Pointer(frame.Previous)
if frame.Panicking {
// We're still panicking!
@@ -122,27 +175,44 @@ func destroyDeferFrame(frame *deferFrame) {
// useParentFrame is set when the caller of runtime._recover has a defer frame
// itself. In that case, recover() shouldn't check that frame but one frame up.
func _recover(useParentFrame bool) interface{} {
if !supportsRecover() {
// Compiling without stack unwinding support, so make this a no-op.
return nil
}
// TODO: somehow check that recover() is called directly by a deferred
// function in a panicking goroutine. Maybe this can be done by comparing
// the frame pointer?
frame := (*deferFrame)(task.Current().DeferFrame)
if useParentFrame {
// Don't recover panic from the current frame (which can't be panicking
// already), but instead from the previous frame.
frame = frame.Previous
switch supportsRecover() {
case recoverInlineAsm:
frame := (*deferFrameInlineAsm)(task.Current().DeferFrame)
if useParentFrame {
// Don't recover panic from the current frame (which can't be
// panicking already), but instead from the previous frame.
frame = frame.Previous
}
if frame != nil && frame.Panicking {
// Only the first call to recover returns the panic value. It also
// stops the panicking sequence, hence setting panicking to false.
frame.Panicking = false
return frame.PanicValue
}
// Not panicking, so return a nil interface.
return nil
case recoverWasmEH:
frame := (*deferFrameWasmEH)(task.Current().DeferFrame)
if useParentFrame {
// Don't recover panic from the current frame (which can't be
// panicking already), but instead from the previous frame.
frame = frame.Previous
}
if frame != nil && frame.Panicking {
// Only the first call to recover returns the panic value. It also
// stops the panicking sequence, hence setting panicking to false.
frame.Panicking = false
return frame.PanicValue
}
// Not panicking, so return a nil interface.
return nil
default:
// Compiling without stack unwinding support, so make this a no-op.
return nil
}
if frame != nil && frame.Panicking {
// Only the first call to recover returns the panic value. It also stops
// the panicking sequence, hence setting panicking to false.
frame.Panicking = false
return frame.PanicValue
}
// Not panicking, so return a nil interface.
return nil
}
// Panic when trying to dereference a nil pointer.