[PATCH] Fix faulty use of obstack_free() to *shrink* dont_print_statmem_obstack. Instead use obstack_blank_fast() with a "negative" size. A real stack data structured would be appropriate here. Added unit test gdb/testsuite/gdb.cp/printstaticrecursion.exp.

[Patrick Frants <osscontribute at gmail dot com>]

Recursion detection for static members was not working because when popping addresses of the stack, the stack was unintentionally cleared, leaving the stack empty. This was caused by a call to obstack_next_free(), which frees the current object.
The fix uses obstack_blank_fast with a negative size to shrink the current object with the desired amount.
A unit test (gdb.cp/printstaticrecursion.exp) was added. No new regression has been observed in testsuite/gdb.cp/*.exp.
---
 gdb/cp-valprint.c                             | 11 +++-----
 gdb/testsuite/gdb.cp/printstaticrecursion.cc  | 21 +++++++++++++++
 gdb/testsuite/gdb.cp/printstaticrecursion.exp | 39 +++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 8 deletions(-)
 create mode 100644 gdb/testsuite/gdb.cp/printstaticrecursion.cc
 create mode 100644 gdb/testsuite/gdb.cp/printstaticrecursion.exp

diff --git a/gdb/cp-valprint.c b/gdb/cp-valprint.c
index fb9bfd9..8f9658d 100644
--- a/gdb/cp-valprint.c
+++ b/gdb/cp-valprint.c
@@ -370,14 +370,9 @@ cp_print_value_fields (struct type *type, struct type *real_type,
 
 	  if (obstack_final_size > statmem_obstack_initial_size)
 	    {
-	      /* In effect, a pop of the printed-statics stack.  */
-
-	      void *free_to_ptr =
-		(char *) obstack_next_free (&dont_print_statmem_obstack) -
-		(obstack_final_size - statmem_obstack_initial_size);
-
-	      obstack_free (&dont_print_statmem_obstack,
-			    free_to_ptr);
+          /* In effect, a pop of the printed-statics stack.  */
+          size_t shrink_bytes = statmem_obstack_initial_size - obstack_final_size;
+          obstack_blank_fast(&dont_print_statmem_obstack, shrink_bytes);
 	    }
 
 	  if (last_set_recurse != recurse)
diff --git a/gdb/testsuite/gdb.cp/printstaticrecursion.cc b/gdb/testsuite/gdb.cp/printstaticrecursion.cc
new file mode 100644
index 0000000..616c00d
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/printstaticrecursion.cc
@@ -0,0 +1,21 @@
+#include <cstdio>
+
+struct Inner
+{
+    static Inner instance;
+};
+
+
+struct Outer
+{
+    Inner inner;
+    static Outer instance;
+};
+
+Inner Inner::instance;
+Outer Outer::instance;
+
+int main()
+{
+    return 0; /* break-here */
+}
\ No newline at end of file
diff --git a/gdb/testsuite/gdb.cp/printstaticrecursion.exp b/gdb/testsuite/gdb.cp/printstaticrecursion.exp
new file mode 100644
index 0000000..1407c12
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/printstaticrecursion.exp
@@ -0,0 +1,39 @@
+# Copyright 2008-2017 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/>.
+
+
+if { [skip_cplus_tests] } { continue }
+
+standard_testfile printstaticrecursion.cc
+
+# Create and source the file that provides information about the compiler
+# used to compile the test case.
+if [get_compiler_info "c++"] {
+    untested "couldn't find a valid c++ compiler"
+    return -1
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+    return -1
+}
+
+if ![runto_main] then {
+    perror "couldn't run to main"
+    continue
+} 
+
+gdb_test "print Outer::instance" "{inner = {static instance = {static instance = <same as static member of an already seen type>}}, static instance = {inner = {static instance = {static instance = <same as static member of an already seen type>}}, static instance = <same as static member of an already seen type>}}" "Print recursive static member"
+gdb_exit
+return 0
-- 
1.8.3.1