Document and enforce some properties for strings created by modules.

When creating multibyte or unibyte strings, we should guarantee the
following invariants:

- When creating empty strings, a NULL data pointer should be allowed.
  This often arises in practice if the string length isn't known in
  advance, and we don't want to unnecessarily trigger undefined
  behavior.  Since functions like memcpy might not accept NULL
  pointers, use the canonical empty string objects in this case.

- Nonzero strings should be guaranteed to be unique and mutable.
  These are the same guarantees expected from Lisp functions such as
  'make-string' or 'unibyte-string'.  On the other hand, empty strings
  might not be unique.

* src/emacs-module.c (module_make_string)
(module_make_unibyte_string): Correctly handle empty strings.

* test/src/emacs-module-resources/mod-test.c (Fmod_test_make_string):
New test function.
(emacs_module_init): Expose it.

* test/src/emacs-module-tests.el (mod-test-make-string/empty)
(mod-test-make-string/nonempty): New unit tests.

* doc/lispref/internals.texi (Module Values): Document properties and
corner cases for strings.

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index fb24544c91757a2f99d9c231a010e96e54682974..28a5fdb3492e9a79082da1e146ab507bbe0a314f 100644 (file)
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -1864,7 +1864,10 @@ byte, is @var{len}.  The original string in @var{str} can be either an
 it can include embedded null bytes, and doesn't have to end in a
 terminating null byte at @code{@var{str}[@var{len}]}.  The function
 raises the @code{overflow-error} error condition if @var{len} is
-negative or exceeds the maximum length of an Emacs string.
+negative or exceeds the maximum length of an Emacs string.  If
+@var{len} is zero, then @var{str} can be @code{NULL}, otherwise it
+must point to valid memory.  For nonzero @var{len}, @code{make_string}
+returns unique mutable string objects.
 @end deftypefn
 
 @deftypefn Function emacs_value make_unibyte_string (emacs_env *@var{env}, const char *@var{str}, ptrdiff_t @var{len})

diff --git a/src/emacs-module.c b/src/emacs-module.c
index 0f3ef59fd8c39471d1fc34e6a0e5d5047dda8b7f..b7cd835c83ce737fcce89734d8a96f7a796c5212 100644 (file)
--- a/src/emacs-module.c
+++ b/src/emacs-module.c
@@ -784,7 +784,8 @@ module_make_string (emacs_env *env, const char *str, ptrdiff_t len)
   MODULE_FUNCTION_BEGIN (NULL);
   if (! (0 <= len && len <= STRING_BYTES_BOUND))
     overflow_error ();
-  Lisp_Object lstr = module_decode_utf_8 (str, len);
+  Lisp_Object lstr
+    = len == 0 ? empty_multibyte_string : module_decode_utf_8 (str, len);
   return lisp_to_value (env, lstr);
 }
 
@@ -794,9 +795,8 @@ module_make_unibyte_string (emacs_env *env, const char *str, ptrdiff_t length)
   MODULE_FUNCTION_BEGIN (NULL);
   if (! (0 <= length && length <= STRING_BYTES_BOUND))
     overflow_error ();
-  Lisp_Object lstr = make_uninit_string (length);
-  memcpy (SDATA (lstr), str, length);
-  SDATA (lstr)[length] = 0;
+  Lisp_Object lstr
+    = length == 0 ? empty_unibyte_string : make_unibyte_string (str, length);
   return lisp_to_value (env, lstr);
 }
 

diff --git a/test/src/emacs-module-resources/mod-test.c b/test/src/emacs-module-resources/mod-test.c
index f855e9b6da08f9b256eeb06d097cce802200f572..30ad352cf8b98d56e065ef72a2fac77c9f251015 100644 (file)
--- a/test/src/emacs-module-resources/mod-test.c
+++ b/test/src/emacs-module-resources/mod-test.c
@@ -24,6 +24,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
 #include <limits.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -699,6 +700,34 @@ Fmod_test_funcall (emacs_env *env, ptrdiff_t nargs, emacs_value *args,
   return env->funcall (env, args[0], nargs - 1, args + 1);
 }
 
+static emacs_value
+Fmod_test_make_string (emacs_env *env, ptrdiff_t nargs,
+                       emacs_value *args, void *data)
+{
+  assert (nargs == 2);
+  intmax_t length_arg = env->extract_integer (env, args[0]);
+  if (env->non_local_exit_check (env) != emacs_funcall_exit_return)
+    return args[0];
+  if (length_arg < 0 || SIZE_MAX < length_arg)
+    {
+      signal_error (env, "Invalid string length");
+      return args[0];
+    }
+  size_t length = (size_t) length_arg;
+  bool multibyte = env->is_not_nil (env, args[1]);
+  char *buffer = length == 0 ? NULL : malloc (length);
+  if (buffer == NULL && length != 0)
+    {
+      memory_full (env);
+      return args[0];
+    }
+  memset (buffer, 'a', length);
+  emacs_value ret = multibyte ? env->make_string (env, buffer, length)
+                              : env->make_unibyte_string (env, buffer, length);
+  free (buffer);
+  return ret;
+}
+
 /* Lisp utilities for easier readability (simple wrappers).  */
 
 /* Provide FEATURE to Emacs.  */
@@ -790,6 +819,7 @@ emacs_module_init (struct emacs_runtime *ert)
   DEFUN ("mod-test-async-pipe", Fmod_test_async_pipe, 1, 1, NULL, NULL);
   DEFUN ("mod-test-funcall", Fmod_test_funcall, 1, emacs_variadic_function,
          NULL, NULL);
+  DEFUN ("mod-test-make-string", Fmod_test_make_string, 2, 2, NULL, NULL);
 
 #undef DEFUN
 

diff --git a/test/src/emacs-module-tests.el b/test/src/emacs-module-tests.el
index 99d4cafb4af68239ec7e3604ad02fb512c40f092..bf26ffb935c60901389f375c4b772cbccba83147 100644 (file)
--- a/test/src/emacs-module-tests.el
+++ b/test/src/emacs-module-tests.el
@@ -30,6 +30,7 @@
 (require 'ert)
 (require 'ert-x)
 (require 'help-fns)
+(require 'subr-x)
 
 (defconst mod-test-emacs
   (expand-file-name invocation-name invocation-directory)
@@ -556,4 +557,23 @@ See Bug#36226."
     (thread-join thread-1)
     (thread-join thread-2)))
 
+(ert-deftest mod-test-make-string/empty ()
+  (dolist (multibyte '(nil t))
+    (ert-info ((format "Multibyte: %s" multibyte))
+      (let ((got (mod-test-make-string 0 multibyte)))
+        (should (stringp got))
+        (should (string-empty-p got))
+        (should (eq (multibyte-string-p got) multibyte))))))
+
+(ert-deftest mod-test-make-string/nonempty ()
+  (dolist (multibyte '(nil t))
+    (ert-info ((format "Multibyte: %s" multibyte))
+      (let ((first (mod-test-make-string 1 multibyte))
+            (second (mod-test-make-string 1 multibyte)))
+        (should (stringp first))
+        (should (eql (length first) 1))
+        (should (eq (multibyte-string-p first) multibyte))
+        (should (string-equal first second))
+        (should-not (eq first second))))))
+
 ;;; emacs-module-tests.el ends here