defer GC work while in cabi_realloc

wit/runtime/runtime.go

@@ -109,6 +109,19 @@ func Unpin() {
 //go:wasmimport wasi_snapshot_preview1 adapter_monotonic_clock_set_paused
 func adapterMonotonicClockSetPaused(paused bool)
 
+// `procPin` increments the current M's lock count, which prevents the Go
+// runtime from starting a GC cycle or performing GC assist work until
+// `procUnpin` is called. The runtime explicitly supports linking to these;
+// see https://go.dev/issue/67401.
+
+//nolint:unused
+//go:linkname procPin runtime.procPin
+func procPin() int
+
+//nolint:unused
+//go:linkname procUnpin runtime.procUnpin
+func procUnpin()
+
 //nolint:unused
 //go:wasmexport cabi_realloc
 func cabiRealloc(oldPointer unsafe.Pointer, oldSize, align, newSize uintptr) unsafe.Pointer {
@@ -117,19 +130,25 @@ func cabiRealloc(oldPointer unsafe.Pointer, oldSize, align, newSize uintptr) uns
 	}
 
 	if useGCAllocations {
-		// Here we call `adapter_monotonic_clock_set_paused` before and
-		// after allocating since the Go garbage collector calls
-		// `clock_time_get` to measure time spent in various stages of
-		// GC, but calls to imports from `cabi_realloc` are forbidden by
-		// the component model, so we must tell the
-		// `wasi_snapshot_preview1` adapter to use a cached value
-		// instead of calling `monotonic_clock::now`.
+		// Calls to imports from `cabi_realloc` are forbidden by the
+		// component model, so we must not let the Go garbage collector
+		// run here. A GC cycle may call imports in several ways: it
+		// reads the monotonic and realtime clocks, polls for I/O when
+		// restarting the world, and may switch to other goroutines
+		// (e.g. to park an assist), any of which may then call imports.
+		// Pinning the M defers any GC work until the next allocation
+		// made outside of `cabi_realloc`.
 		//
-		// See https://github.com/bytecodealliance/wasmtime/pull/13563
-		// for more details.
+		// We additionally call `adapter_monotonic_clock_set_paused`
+		// before and after allocating in case anything else reads the
+		// monotonic clock while allocating.  See
+		// https://github.com/bytecodealliance/wasmtime/pull/13563 for
+		// details.
+		procPin()
 		adapterMonotonicClockSetPaused(true)
 		pointer := Allocate(&pinner, newSize, align)
 		adapterMonotonicClockSetPaused(false)
+		procUnpin()
 		return pointer
 	} else {
 		alignedSize := newSize + offset(newSize, align)