internal: use runtime.Pinner in PtrGuard

Since Go 1.21 there is a runtime.Pinner API that allows to safely
pass structures with embedded Go pointers to C code.  In earlier Go
version we know that the garbage collector is non-moving, so it is
safe to pass Go pointers to C as well. This change adds two
implementations of PtrGuard, one for pre 1.21 that is basically a
no-op, and one for 1.21+ that uses runtime.Pinner.

Signed-off-by: Sven Anderson <sven@redhat.com>

internal/cutil/ptrguard.go

@@ -1,90 +1,34 @@
+//go:build !go1.21
+// +build !go1.21
+
+// This code assumes a non-moving garbage collector, which is the case until at
+// least go 1.20
+
 package cutil
 
 import (
-	"sync"
 	"unsafe"
 )
 
 // PtrGuard respresents a guarded Go pointer (pointing to memory allocated by Go
 // runtime) stored in C memory (allocated by C)
 type PtrGuard struct {
-	// These mutexes will be used as binary semaphores for signalling events from
-	// one thread to another, which - in contrast to other languages like C++ - is
-	// possible in Go, that is a Mutex can be locked in one thread and unlocked in
-	// another.
-	stored, release sync.Mutex
-	released        bool
+	cPtr CPtr
 }
 
-// WARNING: using binary semaphores (mutexes) for signalling like this is quite
-// a delicate task in order to avoid deadlocks or panics. Whenever changing the
-// code logic, please review at least three times that there is no unexpected
-// state possible. Usually the natural choice would be to use channels instead,
-// but these can not easily passed to C code because of the pointer-to-pointer
-// cgo rule, and would require the use of a Go object registry.
-
 // NewPtrGuard writes the goPtr (pointing to Go memory) into C memory at the
 // position cPtr, and returns a PtrGuard object.
 func NewPtrGuard(cPtr CPtr, goPtr unsafe.Pointer) *PtrGuard {
 	var v PtrGuard
-	// Since the mutexes are used for signalling, they have to be initialized to
-	// locked state, so that following lock attempts will block.
-	v.release.Lock()
-	v.stored.Lock()
-	// Start a background go routine that lives until Release is called. This
-	// calls a special function that makes sure the garbage collector doesn't touch
-	// goPtr, stores it into C memory at position cPtr and then waits until it
-	// reveices the "release" signal, after which it nulls out the C memory at
-	// cPtr and then exits.
-	go func() {
-		storeUntilRelease(&v, (*CPtr)(cPtr), uintptr(goPtr))
-	}()
-	// Wait for the "stored" signal from the go routine when the Go pointer has
-	// been stored to the C memory. <--(1)
-	v.stored.Lock()
+	v.cPtr = cPtr
+	p := (*unsafe.Pointer)(unsafe.Pointer(cPtr))
+	*p = goPtr
 	return &v
 }
 
 // Release removes the guarded Go pointer from the C memory by overwriting it
 // with NULL.
 func (v *PtrGuard) Release() {
-	if !v.released {
-		v.released = true
-		v.release.Unlock() // Send the "release" signal to the go routine. -->(2)
-		v.stored.Lock()    // Wait for the second "stored" signal when the C memory
-		//                    has been nulled out. <--(3)
-
-	}
-}
-
-// The uintptrPtr() helper function below assumes that uintptr has the same size
-// as a pointer, although in theory it could be larger.  Therefore we use this
-// constant expression to assert size equality as a safeguard at compile time.
-// How it works: if sizes are different, either the inner or outer expression is
-// negative, which always fails with "constant ... overflows uintptr", because
-// unsafe.Sizeof() is a uintptr typed constant.
-const _ = -(unsafe.Sizeof(uintptr(0)) - PtrSize) // size assert
-func uintptrPtr(p *CPtr) *uintptr {
-	return (*uintptr)(unsafe.Pointer(p))
-}
-
-//go:uintptrescapes
-
-// From https://golang.org/src/cmd/compile/internal/gc/lex.go:
-// For the next function declared in the file any uintptr arguments may be
-// pointer values converted to uintptr. This directive ensures that the
-// referenced allocated object, if any, is retained and not moved until the call
-// completes, even though from the types alone it would appear that the object
-// is no longer needed during the call. The conversion to uintptr must appear in
-// the argument list.
-// Also see https://golang.org/cmd/compile/#hdr-Compiler_Directives
-
-func storeUntilRelease(v *PtrGuard, cPtr *CPtr, goPtr uintptr) {
-	uip := uintptrPtr(cPtr)
-	*uip = goPtr      // store Go pointer in C memory at c_ptr
-	v.stored.Unlock() // send "stored" signal to main thread -->(1)
-	v.release.Lock()  // wait for "release" signal from main thread when
-	//                   Release() has been called. <--(2)
-	*uip = 0          // reset C memory to NULL
-	v.stored.Unlock() // send second "stored" signal to main thread -->(3)
+	p := (*unsafe.Pointer)(unsafe.Pointer(v.cPtr))
+	*p = nil
 }

internal/cutil/ptrguard_pinner.go

@@ -0,0 +1,35 @@
+//go:build go1.21
+// +build go1.21
+
+package cutil
+
+import (
+	"runtime"
+	"unsafe"
+)
+
+// PtrGuard respresents a guarded Go pointer (pointing to memory allocated by Go
+// runtime) stored in C memory (allocated by C)
+type PtrGuard struct {
+	cPtr   CPtr
+	pinner runtime.Pinner
+}
+
+// NewPtrGuard writes the goPtr (pointing to Go memory) into C memory at the
+// position cPtr, and returns a PtrGuard object.
+func NewPtrGuard(cPtr CPtr, goPtr unsafe.Pointer) *PtrGuard {
+	var v PtrGuard
+	v.pinner.Pin(goPtr)
+	v.cPtr = cPtr
+	p := (*unsafe.Pointer)(unsafe.Pointer(cPtr))
+	*p = goPtr
+	return &v
+}
+
+// Release removes the guarded Go pointer from the C memory by overwriting it
+// with NULL.
+func (v *PtrGuard) Release() {
+	p := (*unsafe.Pointer)(unsafe.Pointer(v.cPtr))
+	*p = nil
+	v.pinner.Unpin()
+}