r18435: added a function talloc_move() which is like talloc_steal(), but is

meant for moving pointers between structures. The difference is that
talloc_move() will zero the source pointer, thus ensuring you don't
reference the pointer in the old context.

talloc_move() is appropriate in some, but not all cases where we use
talloc_steal() now. The interface came out of a discussion with
Jeremy.
(This used to be commit 200756017e1867faa207703eddc00a75ae4527df)

4 files changed, 61 insertions, 3 deletions

diff --git a/source4/lib/talloc/talloc.3.xml b/source4/lib/talloc/talloc.3.xml
index a448e8d978..76b54ddb80 100644
--- a/source4/lib/talloc/talloc.3.xml
+++ b/source4/lib/talloc/talloc.3.xml
@@ -369,7 +369,7 @@ talloc_realloc(ctx, ptr, type, 0)  ==> talloc_free(ptr);</programlisting>
 	  known so the type-safe talloc_realloc() cannot be used.
         </para>
     </refsect2>
-    <refsect2><title>void *talloc_steal(const void *<emphasis role="italic">new_ctx</emphasis>, const void *<emphasis role="italic">ptr</emphasis>);</title>
+    <refsect2><title>TYPE *talloc_steal(const void *<emphasis role="italic">new_ctx</emphasis>, const TYPE *<emphasis role="italic">ptr</emphasis>);</title>
         <para>
 	  The talloc_steal() function changes the parent context of a
 	  talloc pointer.  It is typically used when the context that the
@@ -387,6 +387,16 @@ talloc_realloc(ctx, ptr, type, 0)  ==> talloc_free(ptr);</programlisting>
 	  data if you do this.
         </para>
     </refsect2>
+    <refsect2><title>TYPE *talloc_move(const void *<emphasis role="italic">new_ctx</emphasis>, TYPE *<emphasis role="italic">ptr</emphasis>);</title>
+        <para>
+	  The talloc_move() function is a wrapper around
+	  talloc_steal() which zeros the source pointer after the
+	  move. This avoids a potential source of bugs where a
+	  programmer leaves a pointer in two structures, and uses the
+	  pointer from the old structure after it has been moved to a
+	  new one.
+        </para>
+    </refsect2>
     <refsect2><title>size_t talloc_total_size(const void *<emphasis role="italic">ptr</emphasis>);</title>
         <para>
 	  The talloc_total_size() function returns the total size in bytes
diff --git a/source4/lib/talloc/talloc.c b/source4/lib/talloc/talloc.c
index d2f7a5d828..2b80594e80 100644
--- a/source4/lib/talloc/talloc.c
+++ b/source4/lib/talloc/talloc.c
@@ -738,6 +738,17 @@ void *_talloc_steal(const void *new_ctx, const void *ptr)
 }
 
 /*
+  a wrapper around talloc_steal() for situations where you are moving a pointer
+  between two structures, and want the old pointer to be set to NULL
+*/
+void *_talloc_move(const void *new_ctx, const void **pptr)
+{
+	void *ret = _talloc_steal(new_ctx, *pptr);
+	(*pptr) = NULL;
+	return ret;
+}
+
+/*
   return the total size of a talloc pool (subtree)
 */
 size_t talloc_total_size(const void *ptr)
diff --git a/source4/lib/talloc/talloc.h b/source4/lib/talloc/talloc.h
index fdc0fd3494..b9ecd8f0b8 100644
--- a/source4/lib/talloc/talloc.h
+++ b/source4/lib/talloc/talloc.h
@@ -65,15 +65,16 @@ typedef void TALLOC_CTX;
 /* this extremely strange macro is to avoid some braindamaged warning
    stupidity in gcc 4.1.x */
 #define talloc_steal(ctx, ptr) ({ _TALLOC_TYPEOF(ptr) __talloc_steal_ret = (_TALLOC_TYPEOF(ptr))_talloc_steal((ctx),(ptr)); __talloc_steal_ret; })
-#define talloc_reference(ctx, ptr) (_TALLOC_TYPEOF(ptr))_talloc_reference((ctx),(ptr))
 #else
 #define talloc_set_destructor(ptr, function) \
 	_talloc_set_destructor((ptr), (int (*)(void *))(function))
 #define _TALLOC_TYPEOF(ptr) void *
 #define talloc_steal(ctx, ptr) (_TALLOC_TYPEOF(ptr))_talloc_steal((ctx),(ptr))
-#define talloc_reference(ctx, ptr) (_TALLOC_TYPEOF(ptr))_talloc_reference((ctx),(ptr))
 #endif
 
+#define talloc_reference(ctx, ptr) (_TALLOC_TYPEOF(ptr))_talloc_reference((ctx),(ptr))
+#define talloc_move(ctx, ptr) (_TALLOC_TYPEOF(ptr))_talloc_move((ctx),&(ptr))
+
 /* useful macros for creating type checked pointers */
 #define talloc(ctx, type) (type *)talloc_named_const(ctx, sizeof(type), #type)
 #define talloc_size(ctx, size) talloc_named_const(ctx, size, __location__)
@@ -127,6 +128,7 @@ int talloc_free(void *ptr);
 void talloc_free_children(void *ptr);
 void *_talloc_realloc(const void *context, void *ptr, size_t size, const char *name);
 void *_talloc_steal(const void *new_ctx, const void *ptr);
+void *_talloc_move(const void *new_ctx, const void **pptr);
 size_t talloc_total_size(const void *ptr);
 size_t talloc_total_blocks(const void *ptr);
 void talloc_report_depth_cb(const void *ptr, int depth, int max_depth,
diff --git a/source4/lib/talloc/testsuite.c b/source4/lib/talloc/testsuite.c
index 2faa6d64c4..38039c11bf 100644
--- a/source4/lib/talloc/testsuite.c
+++ b/source4/lib/talloc/testsuite.c
@@ -731,6 +731,40 @@ static bool test_steal(void)
 }
 
 /*
+  test move
+*/
+static bool test_move(void)
+{
+	void *root;
+	struct t_move {
+		char *p;
+		int *x;
+	} *t1, *t2;
+	printf("TESTING MOVE\n");
+
+	root = talloc_new(NULL);
+
+	t1 = talloc(root, struct t_move);
+	t2 = talloc(root, struct t_move);
+	t1->p = talloc_strdup(t1, "foo");
+	t1->x = talloc(t1, int);
+	*t1->x = 42;
+
+	t2->p = talloc_move(t2, t1->p);
+	t2->x = talloc_move(t2, t1->x);
+	if (t1->p != NULL || t1->x != NULL ||
+	    strcmp(t2->p, "foo") ||
+	    *t2->x != 42) {
+		printf("talloc move failed\n");
+		return false;
+	}
+
+	talloc_free(root);
+
+	return true;
+}
+
+/*
   test talloc_realloc_fn
 */
 static bool test_realloc_fn(void)
@@ -1022,6 +1056,7 @@ bool torture_local_talloc(struct torture_context *torture)
 	ret &= test_realloc();
 	ret &= test_realloc_child();
 	ret &= test_steal();
+	ret &= test_move();
 	ret &= test_unref_reparent();
 	ret &= test_realloc_fn();
 	ret &= test_type();