Fix MI crash with Ada string access

I happened to notice that using -var-create at a certain spot in an
Ada program caused a crash.  This happens because
ada_get_decoded_value can return NULL -- in particular, deeper in the
code it can hit this return in ada_type_of_array:

      descriptor = desc_bounds (arr);
      if (value_as_long (descriptor) == 0)
        return NULL;

This patch avoids the crash by handling this NULL return.

gdb/ChangeLog
2020-08-17  Tom Tromey  <tromey@adacore.com>

	* ada-varobj.c (ada_varobj_decode_var): Handle case where
	ada_get_decoded_value returns NULL.

gdb/testsuite/ChangeLog
2020-08-17  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mi_var_access.exp: New file.
	* gdb.ada/mi_var_access/mi_access.adb: New file.
	* gdb.ada/mi_var_access/pck.adb: New file.
	* gdb.ada/mi_var_access/pck.ads: New file.

7 files changed, 154 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e15dd15..b097b32 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2020-08-17  Tom Tromey  <tromey@adacore.com>
 
+	* ada-varobj.c (ada_varobj_decode_var): Handle case where
+	ada_get_decoded_value returns NULL.
+
+2020-08-17  Tom Tromey  <tromey@adacore.com>
+
 	* python/py-inferior.c (infpy_search_memory): Use
 	gdb_py_object_from_ulongest.
 	* python/py-infevents.c (create_inferior_call_event_object)
diff --git a/gdb/ada-varobj.c b/gdb/ada-varobj.c
index f67fe50..a72653a 100644
--- a/gdb/ada-varobj.c
+++ b/gdb/ada-varobj.c
@@ -63,10 +63,10 @@ static void
 ada_varobj_decode_var (struct value **value_ptr, struct type **type_ptr)
 {
   if (*value_ptr)
-    {
-      *value_ptr = ada_get_decoded_value (*value_ptr);
-      *type_ptr = ada_check_typedef (value_type (*value_ptr));
-    }
+    *value_ptr = ada_get_decoded_value (*value_ptr);
+
+  if (*value_ptr != nullptr)
+    *type_ptr = ada_check_typedef (value_type (*value_ptr));
   else
     *type_ptr = ada_get_decoded_type (*type_ptr);
 }
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f0b7949..9fdbcb8 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2020-08-17  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.ada/mi_var_access.exp: New file.
+	* gdb.ada/mi_var_access/mi_access.adb: New file.
+	* gdb.ada/mi_var_access/pck.adb: New file.
+	* gdb.ada/mi_var_access/pck.ads: New file.
+
 2020-08-16  Tom de Vries  <tdevries@suse.de>
 
 	PR gdb/25350
diff --git a/gdb/testsuite/gdb.ada/mi_var_access.exp b/gdb/testsuite/gdb.ada/mi_var_access.exp
new file mode 100644
index 0000000..5ffc0ae
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/mi_var_access.exp
@@ -0,0 +1,52 @@
+# Copyright 2020 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+if {[skip_ada_tests]} {
+    return -1
+}
+
+standard_ada_testfile mi_access
+
+load_lib mi-support.exp
+set MIFLAGS "-i=mi"
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable debug] != "" } {
+    return -1
+}
+
+if {[mi_gdb_start]} {
+    continue
+}
+
+mi_delete_breakpoints
+mi_gdb_reinitialize_dir $srcdir/$subdir
+mi_gdb_load ${binfile}
+
+if ![mi_run_to_main] then {
+    fail "cannot run to main, testcase aborted"
+    return 0
+}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/mi_access.adb]
+mi_continue_to_line \
+    "mi_access.adb:$bp_location" \
+    "stop at start of mi_access"
+
+# The bug was that creating a varobj for A_String_Access would crash.
+mi_gdb_test "-var-create A_String_Access * A_String_Access" \
+    "\\^done,name=\"A_String_Access\",numchild=\"1\",.*" \
+    "Create varobj"
diff --git a/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb b/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb
new file mode 100644
index 0000000..d5f5f88
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb
@@ -0,0 +1,37 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with Pck; use Pck;
+
+procedure Mi_Access is
+   A_String : String (3 .. 5) := "345"; -- STOP
+   A_String_Access : String_Access;
+   A_Pointer : Pointer;
+begin
+   Do_Nothing (A_String'Address);
+   A_String (4) := '6';
+   A_String_Access := Copy (A_String);
+   A_Pointer.P := A_String_Access;
+   Do_Nothing (A_String_Access'Address);
+   A_String_Access (4) := 'a';
+   Do_Nothing (A_Pointer'Address);
+   A_String_Access := Copy("Hi");
+   A_Pointer.P := A_String_Access;
+   Do_Nothing (A_String_Access'Address);
+   A_String_Access := null;
+   A_Pointer.P := null;
+   Do_Nothing (A_Pointer'Address);
+   Do_Nothing (A_String'Address);
+end Mi_Access;
diff --git a/gdb/testsuite/gdb.ada/mi_var_access/pck.adb b/gdb/testsuite/gdb.ada/mi_var_access/pck.adb
new file mode 100644
index 0000000..9aff885
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/mi_var_access/pck.adb
@@ -0,0 +1,26 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package body Pck is
+   function Copy (S : String) return String_Access is
+   begin
+      return new String'(S);
+   end Copy;
+
+   procedure Do_Nothing (A : System.Address) is
+   begin
+      null;
+   end Do_Nothing;
+end Pck;
diff --git a/gdb/testsuite/gdb.ada/mi_var_access/pck.ads b/gdb/testsuite/gdb.ada/mi_var_access/pck.ads
new file mode 100644
index 0000000..307a0aa
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/mi_var_access/pck.ads
@@ -0,0 +1,23 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with System;
+package Pck is
+   type String_Access is access all String;
+   type Pointer is record P : String_Access; end record;
+
+   function Copy (S : String) return String_Access;
+   procedure Do_Nothing (A : System.Address);
+end Pck;