Avoid face inheritance cycles

* src/xfaces.c (face_inheritance_cycle): New function.
(Finternal_set_lisp_face_attribute): Signal an error if the
':inherit' attribute of a face is modified in a way that will
cause it to inherit from itself.  (Bug#79672)

* test/src/xfaces-tests.el (xfaces-test-circular-inheritance): New
test.

* etc/NEWS: Announce the incompatible change.

1 files changed, 57 insertions, 1 deletions

diff --git a/src/xfaces.c b/src/xfaces.c
index 7626dfeb75c..ca1bee1d17a 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -2426,6 +2426,60 @@ face_inherited_attr (struct window *w, struct frame *f,
   return attr_val;
 }
 
+/* Chase the chain of inheritance for FACE on frame F, and return
+   non-zero if FACE inherits from its CHILD face, directly or
+   indirectly.  FACE is either a symbol or a list of face symbols, which
+   are two forms of values for the :inherit attribute of a face.  CHILD
+   must be a face symbol.  */
+static bool
+face_inheritance_cycle (struct frame *f, Lisp_Object face, Lisp_Object child)
+{
+  Lisp_Object face_attrs[LFACE_VECTOR_SIZE];
+  Lisp_Object parent_face;
+  bool ok, cycle_found = false;
+
+  eassert (SYMBOLP (child));
+  if (CONSP (face))
+    {
+      Lisp_Object tail;
+      for (tail = face; !NILP (tail); tail = XCDR (tail))
+        {
+          ok = get_lface_attributes (NULL, f, XCAR (tail), face_attrs,
+                                     false, NULL);
+          if (!ok)
+            break;
+          parent_face = face_attrs[LFACE_INHERIT_INDEX];
+          if (EQ (parent_face, child))
+            cycle_found = true;
+          else if (!NILP (parent_face)
+                   && !UNSPECIFIEDP (parent_face)
+                   && !IGNORE_DEFFACE_P (parent_face)
+                   && !RESET_P (parent_face))
+            cycle_found = face_inheritance_cycle (f, parent_face, child);
+          if (cycle_found)
+            break;
+        }
+    }
+  else
+    {
+      eassert (SYMBOLP (face));
+      ok = get_lface_attributes (NULL, f, face, face_attrs, false, NULL);
+      if (ok)
+        {
+          parent_face = face_attrs[LFACE_INHERIT_INDEX];
+          if (EQ (parent_face, child))
+            cycle_found = true;
+          else if (!NILP (parent_face)
+                   && !UNSPECIFIEDP (parent_face)
+                   && !IGNORE_DEFFACE_P (parent_face)
+                   && !RESET_P (parent_face))
+            cycle_found = face_inheritance_cycle (f, parent_face, child);
+        }
+    }
+
+  return cycle_found;
+}
+
 /* Merge the named face FACE_NAME on frame F, into the vector of face
    attributes TO.  Use NAMED_MERGE_POINTS to detect loops in face
    inheritance.  Return true if FACE_NAME is a valid face name and
@@ -3654,7 +3708,9 @@ FRAME 0 means change the face on all frames, and change the default
         for (tail = value; CONSP (tail); tail = XCDR (tail))
           if (!SYMBOLP (XCAR (tail)))
             break;
-      if (NILP (tail))
+      if (face_inheritance_cycle (f, value, face))
+        signal_error ("Face inheritance results in inheritance cycle", value);
+      else if (NILP (tail))
         ASET (lface, LFACE_INHERIT_INDEX, value);
       else
         signal_error ("Invalid face inheritance", value);