[Bug testsuite/27787] FAIL: gdb.python/flexible-array-member.exp: python print(zs['items'].type.range())

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27787

--- Comment #6 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Simon Marchi from comment #5)
> Ok, so we could simply fix the test to accept both values (see patch below).
> 

LGTM.  Please apply (given that our discussion may not end up somewhere
better).

> But do you think we should address the underlying issue and make GDB output
> something consistent in that case, whatever the compiler and the method used
> to specify the flexible array member?
> 

I think so, yes.

> - Choose either 0 or -1 to indicate "there's no high bound" and output that
> consistently.  It would be a user-visible behavior change for some cases but
> not others.
> - Emit "None" to indicate "there's no high bound".  This is probably a more
> Pythonic way of doing things, but the downside is that it's a user-visible
> change for all cases involved.
> 

I think the current way of representing things makes no sense given that the
same (0,0) is printed for wildly different things.

My guess would be that a reasonable api client checks type.Dynamic, and if true
only check the lower bound, and not the upper, so I'm not sure how serious
usage breakage would be.

Anyway, I'm in favor of maximum clarity, so I'd say: "None".

Having said that, I tried to implement it, like so:
...
diff --git a/gdb/python/py-type.c b/gdb/python/py-type.c
index 4f5f42529c2..0b9d972c42e 100644
--- a/gdb/python/py-type.c
+++ b/gdb/python/py-type.c
@@ -577,6 +577,7 @@ typy_range (PyObject *self, PyObject *args)
   struct type *type = ((type_object *) self)->type;
   /* Initialize these to appease GCC warnings.  */
   LONGEST low = 0, high = 0;
+  bool has_low = false, has_high = false;

   if (type->code () != TYPE_CODE_ARRAY
       && type->code () != TYPE_CODE_STRING
@@ -593,32 +594,48 @@ typy_range (PyObject *self, PyObject *args)
     case TYPE_CODE_STRING:
     case TYPE_CODE_RANGE:
       if (type->bounds ()->low.kind () == PROP_CONST)
-       low = type->bounds ()->low.const_val ();
-      else
-       low = 0;
+       {
+         low = type->bounds ()->low.const_val ();
+         has_low = true;
+       }

-      if (type->bounds ()->high.kind () == PROP_CONST)
-       high = type->bounds ()->high.const_val ();
-      else
-       high = 0;
+      if (type->bounds ()->high.kind () == PROP_CONST
+         && !is_dynamic_type (type))
+       {
+         high = type->bounds ()->high.const_val ();
+         has_high = true;
+       }
       break;
     }

-  gdbpy_ref<> low_bound = gdb_py_object_from_longest (low);
-  if (low_bound == NULL)
-    return NULL;
+  gdbpy_ref<> low_bound;
+  if (has_low)
+    {--
+      low_bound = gdb_py_object_from_longest (low);
+      if (low_bound == NULL)
+       return NULL;
+    }

-  gdbpy_ref<> high_bound = gdb_py_object_from_longest (high);
-  if (high_bound == NULL)
-    return NULL;
+  gdbpy_ref<> high_bound;
+  if (has_high)
+    {
+      high_bound = gdb_py_object_from_longest (high);
+      if (high_bound == NULL)
+       return NULL;
+    }

   gdbpy_ref<> result (PyTuple_New (2));
   if (result == NULL)
     return NULL;

-  if (PyTuple_SetItem (result.get (), 0, low_bound.release ()) != 0
-      || PyTuple_SetItem (result.get (), 1, high_bound.release ()) != 0)
+  PyObject *pyobject_low = has_low ? low_bound.release () : Py_None;
+  if (PyTuple_SetItem (result.get (), 0, pyobject_low) != 0)
+    return NULL;
+
+  PyObject *pyobject_high = has_high ? high_bound.release () : Py_None;
+  if (PyTuple_SetItem (result.get (), 1, pyobject_high) != 0)
     return NULL;
+
   return result.release ();
 }

diff --git a/gdb/testsuite/gdb.python/flexible-array-member.exp
b/gdb/testsuite/gdb.python/
flexible-array-member.exp
index 349670cb7e7..580626d64c7 100644
--- a/gdb/testsuite/gdb.python/flexible-array-member.exp
+++ b/gdb/testsuite/gdb.python/flexible-array-member.exp
@@ -75,10 +75,8 @@ gdb_test "python print(zs\['items'\]\[0\].address + 1 ==
zs\['items
'\]\[1\].addr
 gdb_test "python print(zso\['items'\] == zso\['items'\]\[0\].address)" "True"
 gdb_test "python print(zso\['items'\]\[0\].address + 1 ==
zso\['items'\]\[1\].address)" "True
"

-# Verify the range attribute.  It looks a bit inconsistent that the high bound
-# is sometimes 0, sometimes -1, but that's what GDB produces today, so that's
-# what we test.
+# Verify the range attribute.

-gdb_test "python print(ns\['items'\].type.range())" "\\(0, 0\\)"
-gdb_test "python print(zs\['items'\].type.range())" "\\(0, -1\\)"
-gdb_test "python print(zso\['items'\].type.range())" "\\(0, -1\\)"
+gdb_test "python print(ns\['items'\].type.range())" "\\(0, None\\)"
+gdb_test "python print(zs\['items'\].type.range())" "\\(0, None\\)"
+gdb_test "python print(zso\['items'\].type.range())" "\\(0, \None\\)"
e\\)"
...

And that works for gcc 7, but for say gcc 10, I get:
...
(gdb) python print(ns['items'].type.range())^M
(0, None)^M
(gdb) PASS: gdb.python/flexible-array-member.exp: python
print(ns['items'].type.range())
python print(zs['items'].type.range())^M
(0, -1)^M
(gdb) FAIL: gdb.python/flexible-array-member.exp: python
print(zs['items'].type.range())
python print(zso['items'].type.range())^M
(0, -1)^M
(gdb) FAIL: gdb.python/flexible-array-member.exp: python
print(zso['items'].type.range())
...

So, appearantly the underlying type for a zero-size array is represented in gdb
using a highbound with PROP_CONST, and it's not a dynamic type.  The latter
strikes me as wrong.  WDYT?

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