ffi/unsafe: fix make-sized-byte-string on a #f argument

In particular, a #f argument can make sense if the length is 0.
Technically, a byte string's byte array is supposed to be nul-terminated,
but many uses of byte strings get away without that terminator. I've
adjust the documentation to note that `bytes-copy` will work with a
non-terminated byte string.

Merge to v6.1.1

pkgs/racket-pkgs/racket-doc/scribblings/foreign/pointers.scrbl

@@ -389,6 +389,12 @@ Returns a byte string made of the given pointer and the given length.
 No copying is done.  This can be used as an alternative to make
 pointer values accessible in Racket when the size is known.
 
+Beware that the representation of a Racket byte string normally
+requires a nul terminator at the end of the byte string (after
+@racket[length] bytes), but some function work with a byte-string
+representation that has no such terminator---notably
+@racket[bytes-copy].
+
 If @racket[cptr] is an offset pointer created by @racket[ptr-add], the
 offset is immediately added to the pointer. Thus, this function cannot
 be used with @racket[ptr-add] to create a substring of a Racket byte

pkgs/racket-pkgs/racket-test/tests/racket/foreign-test.rktl

@@ -15,6 +15,8 @@
 (test #f malloc 0 _int)
 (test #f malloc _int 0)
 
+(test 0 bytes-length (make-sized-byte-string #f 0))
+
 ;; Check integer-range checking:
 (let ()
   (define (try-int-boundary N _int _uint)

racket/src/foreign/foreign.c

@@ -3178,9 +3178,7 @@ static Scheme_Object *foreign_make_sized_byte_string(int argc, Scheme_Object *ar
     scheme_wrong_contract(MYNAME, "cpointer?", 0, argc, argv);
   if (!scheme_get_int_val(argv[1],&len))
     wrong_intptr(MYNAME, 1, argc, argv);
-  if (SCHEME_FALSEP(cp)) return scheme_false;
+  return scheme_make_sized_byte_string(SCHEME_FFIANYPTR_OFFSETVAL(cp),
-  else return
-         scheme_make_sized_byte_string(SCHEME_FFIANYPTR_OFFSETVAL(cp),
                                        len, 0);
 }
 #undef MYNAME

racket/src/foreign/foreign.rktc

@@ -2346,9 +2346,7 @@ static Scheme_Object *do_memop(const char *who, int mode,
     scheme_wrong_contract(MYNAME, "cpointer?", 0, argc, argv);
   if (!scheme_get_int_val(argv[1],&len))
     wrong_intptr(MYNAME, 1, argc, argv);
-  if (SCHEME_FALSEP(cp)) return scheme_false;
+  return scheme_make_sized_byte_string(SCHEME_FFIANYPTR_OFFSETVAL(cp),
-  else return
-         scheme_make_sized_byte_string(SCHEME_FFIANYPTR_OFFSETVAL(cp),
                                        len, 0);
 }
 
@@ -2415,6 +2413,10 @@ static void ffi_call_in_orig_place(ffi_cif *cif, void *c_func, intptr_t cfoff,
   void *sh;
   int ready;
 
+  if (cached_orig_place_todo) {
+    todo = cached_orig_place_todo;
+    cached_orig_place_todo = NULL;
+  } else
     todo = (FFI_Orig_Place_Call *)malloc(sizeof(FFI_Orig_Place_Call));
   sh = scheme_get_signal_handle();
   todo->signal_handle = sh;
@@ -2466,6 +2468,9 @@ static void ffi_call_in_orig_place(ffi_cif *cif, void *c_func, intptr_t cfoff,
     if (!todo->signal_handle) {
       /* Done */
       mzrt_mutex_unlock(orig_place_mutex);
+      if (!cached_orig_place_todo)
+        cached_orig_place_todo = todo;
+      else
         free(todo);
       break;
     } else {