gdb: fix getting range of flexible array member in Python

As reported in bug 27757, we get an internal error when doing:

    $ cat test.c
    struct foo {
        int len;
        int items[];
    };

    struct foo *p;

    int main() {
        return 0;
    }
    $ gcc test.c -g -O0 -o test
    $ ./gdb -q -nx --data-directory=data-directory ./test -ex 'python gdb.parse_and_eval("p").type.target()["items"].type.range()'
    Reading symbols from ./test...
    /home/simark/src/binutils-gdb/gdb/gdbtypes.h:435: internal-error: LONGEST dynamic_prop::const_val() const: Assertion `m_kind == PROP_CONST' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n)

This is because the Python code (typy_range) blindly reads the high
bound of the type of `items` as a constant value.  Since it is a
flexible array member, it has no high bound, the property is undefined.
Since commit 8c2e4e0689 ("gdb: add accessors to struct dynamic_prop"),
the getters check that you are not getting a property value of the wrong
kind, so this causes a failed assertion.

Fix it by checking if the property is indeed a constant value before
accessing it as such.  Otherwise, use 0.  This restores the previous GDB
behavior: because the structure was zero-initialized, this is what was
returned before.  But now this behavior is explicit and not accidental.

Add a test, gdb.python/flexible-array-member.exp, that is derived from
gdb.base/flexible-array-member.exp.  It tests the same things, but
through the Python API.  It also specifically tests getting the range
from the various kinds of flexible array member types (AFAIK it wasn't
possible to do the equivalent through the CLI).

gdb/ChangeLog:

	PR gdb/27757
	* python/py-type.c (typy_range): Check that bounds are constant
	before accessing them as such.
	* guile/scm-type.c (gdbscm_type_range): Likewise.

gdb/testsuite/ChangeLog:

	PR gdb/27757
	* gdb.python/flexible-array-member.c: New test.
	* gdb.python/flexible-array-member.exp: New test.
	* gdb.guile/scm-type.exp (test_range): Add test for flexible
	array member.
	* gdb.guile/scm-type.c (struct flex_member): New.
	(main): Use it.

Change-Id: Ibef92ee5fd871ecb7c791db2a788f203dff2b841

2 files changed, 24 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.guile/scm-type.c b/gdb/testsuite/gdb.guile/scm-type.c
index 782b8fe..64f5f02 100644
--- a/gdb/testsuite/gdb.guile/scm-type.c
+++ b/gdb/testsuite/gdb.guile/scm-type.c
@@ -15,6 +15,8 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include <stdlib.h>
+
 struct s
 {
   int a;
@@ -53,6 +55,12 @@ enum E
 struct s vec_data_1 = {1, 1};
 struct s vec_data_2 = {1, 2};
 
+struct flex_member
+{
+  int n;
+  int items[];
+};
+
 int
 main ()
 {
@@ -72,6 +80,10 @@ main ()
   st.b = 5;
 
   e = v2;
+
+  struct flex_member *f = (struct flex_member *) malloc (100);
+  f->items[0] = 111;
+  f->items[1] = 222;
   
   return 0;      /* break to inspect struct and array.  */
 }
diff --git a/gdb/testsuite/gdb.guile/scm-type.exp b/gdb/testsuite/gdb.guile/scm-type.exp
index 8778cdb..517c99f 100644
--- a/gdb/testsuite/gdb.guile/scm-type.exp
+++ b/gdb/testsuite/gdb.guile/scm-type.exp
@@ -261,6 +261,18 @@ proc test_range {} {
 		"ERROR: .*: Wrong type argument in position 1 \\(expecting ranged type\\): .*" \
 		"check range for non ranged type"
 	}
+
+	with_test_prefix "on flexible array member" {
+	    gdb_scm_test_silent_cmd "print f" "print value (f)"
+	    gdb_scm_test_silent_cmd "guile (define f (history-ref 0))" \
+		"get value (f) from history"
+	    gdb_test "guile (print (type-range (field-type (type-field (value-type (value-dereference f)) \"items\"))))" \
+		"= \\(0 0\\)"
+	    gdb_test "guile (print (value-subscript (value-field (value-dereference f) \"items\") 0))" \
+		"= 111"
+	    gdb_test "guile (print (value-subscript (value-field (value-dereference f) \"items\") 1))" \
+		"= 222"
+	}
     }
 }