ctxlock: use safe implementation of keyify

The benchmarks for the unsafe implementation of keyify don't show a
significant improvement over the safe implementation. These changes
also fix a bug where keyify panics when an empty string input is
received.

Signed-off-by: Brad Lugo <[email protected]>

pkg/ctxlock/keyify.go

@@ -1,22 +1,13 @@
-//go:build !safe
-
 package ctxlock
 
 import (
 	"hash/fnv"
-	"reflect"
-	"unsafe"
 )
 
 // Keyify returns an int64 serialized into a []byte.
 func keyify(key string) []byte {
-	const maxsize = 0x7fff0000
-	l := len(key)
 	h := fnv.New64a()
-	// This is (obviously) unsafe -- it provides mutable access to "key".
-	// However, it doesn't outlive this Write call, and the implementation
-	// can be read to ensure it doesn't modify it.
-	h.Write((*[maxsize]byte)(unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(&key)).Data))[:l:l])
+	h.Write([]byte(key))
 	b := make([]byte, 0, 8)
 	return h.Sum(b)
 }