Fix broken recursion detection when printing static members

Recursion detection for static members was broken.  The implementation
uses a growing (and shrinking) obstack object to simulate a stack of
addresses (CORE_ADDR).  Pushing addresses is implemented by calling
obstack_grow(), while popping is implemented by calling obstack_free().
The latter is problematic because obstack_free() expects a pointer to
the base of an object.  When popping elements of the stack however,
obstack_free() was called with the new top, which potentially is not the
same as the base of the stack.  This is unintended use and the effect is
that obstack->next_free and obstack->object_base members are assigned
the value of the new top, which equals an empty stack.  Summary: popping
elements would always result in an empty stack, which breaks the
recursion detection.

The fix shrinks the stack using obstack_blank_fast() with a negative
value as described at the bottom of this page:
https://gcc.gnu.org/onlinedocs/libiberty/Extra-Fast-Growing.html "You
can use obstack_blank_fast with a “negative” size argument to make the
current object smaller.  Just don’t try to shrink it beyond zero
length—there’s no telling what will happen if you do that. Earlier
versions of obstacks allowed you to use obstack_blank to shrink objects.
This will no longer work."

The reproducer is added to gdb.cp/classes.exp, which fails without this
patch.

gdb/ChangeLog:

	* cp-valprint.c (cp_print_value_fields): Use obstack_blank_fast
	to rewind obstack.

gdb/testsuite/ChangeLog:

	* gdb.cp/classes.exp (test_static_members): Test printing
	Outer::instance.
	* gdb.cp/classes.c (struct Inner, struct Outer): New.
	(Inner::instance, Outer::instance): New.

5 files changed, 33 insertions, 7 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index caa6484..1d18881 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2017-10-26  Patrick Frants  <osscontribute@gmail.com>
+
+	* cp-valprint.c (cp_print_value_fields): Use obstack_blank_fast
+	to rewind obstack.
+
 2017-10-26  Pedro Alves  <palves@redhat.com>
 
 	* remote.c (remote_async_terminal_ours_p): Delete.
diff --git a/gdb/cp-valprint.c b/gdb/cp-valprint.c
index fb9bfd9..609dabb 100644
--- a/gdb/cp-valprint.c
+++ b/gdb/cp-valprint.c
@@ -371,13 +371,9 @@ 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);
+	      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/ChangeLog b/gdb/testsuite/ChangeLog
index 0c6c93a..0e2bc23 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2017-10-26  Patrick Frants  <osscontribute@gmail.com>
+
+	* gdb.cp/classes.exp (test_static_members): Test printing
+	Outer::instance.
+	* gdb.cp/classes.c (struct Inner, struct Outer): New.
+	(Inner::instance, Outer::instance): New.
+
 2017-10-24  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/new-ui.exp (do_test): Split "delete all breakpoints on
diff --git a/gdb/testsuite/gdb.cp/classes.cc b/gdb/testsuite/gdb.cp/classes.cc
index 50f0740..a650846 100644
--- a/gdb/testsuite/gdb.cp/classes.cc
+++ b/gdb/testsuite/gdb.cp/classes.cc
@@ -665,6 +665,19 @@ void use_methods ()
   base1 b (3);
 }
 
+struct Inner
+{
+  static Inner instance;
+};
+
+struct Outer
+{
+  Inner inner;
+  static Outer instance;
+};
+
+Inner Inner::instance;
+Outer Outer::instance;
 
 int
 main()
diff --git a/gdb/testsuite/gdb.cp/classes.exp b/gdb/testsuite/gdb.cp/classes.exp
index 9e2630a..374b632 100644
--- a/gdb/testsuite/gdb.cp/classes.exp
+++ b/gdb/testsuite/gdb.cp/classes.exp
@@ -602,6 +602,11 @@ proc test_static_members {} {
     gdb_test "print cnsi" \
 	"{x = 30, y = 40, static null = {x = 0, y = 0, static null = <same as static member of an already seen type>, static yy = {z = 5, static xx = {x = 1, y = 2, static null = <same as static member of an already seen type>, static yy = <same as static member of an already seen type>}}}, static yy = <same as static member of an already seen type>}" \
 	"print cnsi with static members"
+
+    # Another case of infinite recursion.
+    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"
 }
 
 proc do_tests {} {