gdb/varobj: Reset varobj after relocations have been computed

[This patch is a followup to the discussion in
https://sourceware.org/pipermail/gdb-patches/2022-August/191188.html]

PR/29426 shows failures when running the gdb.mi/mi-var-invalidate-shlib
test when using a compiler which does not produce a PIE executable by
default.

In the testcase, a varobj is created to track a global variable, and
then the main binary is reloaded in GDB (using the file command).

During the load of the new binary, GDB tries to recreate the varobj to
track the global in the new binary (varobj_invalidate_iter).  At this
point, the old process is still in flight.  So when we try to access to
the value of the global, in a PIE executable we only have access to the
unrelocated address (the objfile's text_section_offset () is 0).  As a
consequence down the line read_value_memory fails to read the unrelated
address, so cannot evaluate the value of the global.  Note that the
expression used to access to the global’s value is valid, so the varobj
can be created.  When using a non PIE executable, the address of the
global GDB knows about at this point does not need relocation, so
read_value_memory can access the (old binary’s) value.

So at this point, in the case of a non-PIE executable the value field is
set, while it is cleared in the case of PIE executable.  Later when the
test issues a "-var-update global_var", the command sees no change in
the case of the non-PIE executable, while in the case of the PIE
executable install_new_value sees that value changes, leading to a
different output.

This patch makes sure that, as we do for breakpoints, we wait until
relocation has happened before we try to recreate varobjs.  This way we
have a consistent behavior between PIE and non-PIE binaries.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29426
Co-authored-by: Lancelot SIX <lancelot.six@amd.com>

gdb/symfile.c

@@ -1656,6 +1656,9 @@ symbol_file_command (const char *args, int from_tty)
 
       /* Now it's safe to re-add the breakpoints.  */
       breakpoint_re_set ();
+
+      /* Also, it's safe to re-add varobjs.  */
+      varobj_re_set ();
     }
 }
 
@@ -2870,9 +2873,6 @@ clear_symtab_users (symfile_add_flags add_flags)
   clear_pc_function_cache ();
   gdb::observers::new_objfile.notify (NULL);
 
-  /* Varobj may refer to old symbols, perform a cleanup.  */
-  varobj_invalidate ();
-
   /* Now that the various caches have been cleared, we can re_set
      our breakpoints without risking it using stale data.  */
   if ((add_flags & SYMFILE_DEFER_BP_RESET) == 0)

gdb/testsuite/gdb.mi/mi-var-invalidate-shlib.exp

@@ -99,7 +99,7 @@ proc do_test { separate_debuginfo } {
 	# When reloading the symbol file, only the var for the global in the main
 	# executable is re-created.
 	mi_gdb_test "-var-update global_var" \
-	    "\\^done,changelist=\\\[{name=\"global_var\",in_scope=\"true\",type_changed=\"false\",has_more=\"0\"}\\\]" \
+	    "\\^done,changelist=\\\[\\\]" \
 	    "global_var recreated"
 	mi_gdb_test "-var-update global_shlib_var" \
 	    "\\^done,changelist=\\\[{name=\"global_shlib_var\",in_scope=\"invalid\",has_more=\"0\"}\\\]" \

gdb/varobj.c

@@ -2353,18 +2353,14 @@ all_root_varobjs (gdb::function_view<void (struct varobj *var)> func)
     }
 }
 
-/* Invalidate varobj VAR if it is tied to locals and re-create it if it is
-   defined on globals.  It is a helper for varobj_invalidate.
-
-   This function is called after changing the symbol file, in this case the
-   pointers to "struct type" stored by the varobj are no longer valid.  All
-   varobj must be either re-evaluated, or marked as invalid here.  */
+/* Try to recreate the varobj VAR if it is a global or floating.  This is a
+   helper function for varobj_re_set.  */
 
 static void
-varobj_invalidate_iter (struct varobj *var)
+varobj_re_set_iter (struct varobj *var)
 {
-  /* global and floating var must be re-evaluated.  */
-  if (var->root->floating || var->root->global)
+  /* Invalidated globals and floating var must be re-evaluated.  */
+  if (var->root->global || var->root->floating)
     {
       struct varobj *tmp_var;
 
@@ -2389,14 +2385,12 @@ varobj_invalidate_iter (struct varobj *var)
     }
 }
 
-/* Invalidate the varobjs that are tied to locals and re-create the ones that
-   are defined on globals.
-   Invalidated varobjs will be always printed in_scope="invalid".  */
+/* See varobj.h.  */
 
 void 
-varobj_invalidate (void)
+varobj_re_set (void)
 {
-  all_root_varobjs (varobj_invalidate_iter);
+  all_root_varobjs (varobj_re_set_iter);
 }
 
 /* Ensure that no varobj keep references to OBJFILE.  */

gdb/varobj.h

@@ -314,7 +314,10 @@ extern void all_root_varobjs (gdb::function_view<void (struct varobj *var)>);
 extern std::vector<varobj_update_result>
   varobj_update (struct varobj **varp, bool is_explicit);
 
-extern void varobj_invalidate (void);
+/* Try to recreate any global or floating varobj.  This is called after
+   changing symbol files.  */
+
+extern void varobj_re_set (void);
 
 extern bool varobj_editable_p (const struct varobj *var);