Avoid multiple 'getcontext failed' warnings if getcontext is broken

Issue #133 (bdwgc).

* mach_dep.c [!HAVE_PUSH_REGS && UNIX_LIKE && !NO_GETCONTEXT]
(GC_with_callee_saves_pushed): Define getcontext_works static variable;
do not call getcontext() if getcontext_works < 0; add comment; set
getcontext_works to 1 or -1 (depending whether getcontext is working)
if getcontext_works is zero.
* misc.c [THREADS && UNIX_LIKE && !NO_GETCONTEXT]
(callee_saves_pushed_dummy_fn): New static function (empty).
* misc.c [THREADS && UNIX_LIKE && !NO_GETCONTEXT] (GC_init): Call
GC_with_callee_saves_pushed(callee_saves_pushed_dummy_fn) if GC_dont_gc
or GC_dont_precollect (otherwise it is called by GC_gcollect_inner);
add comment.

1 files changed, 12 insertions, 6 deletions

diff --git a/mach_dep.c b/mach_dep.c
index b65e6dbe..95848971 100644
--- a/mach_dep.c
+++ b/mach_dep.c
@@ -237,6 +237,7 @@ GC_INNER void GC_with_callee_saves_pushed(void (*fn)(ptr_t, void *),
       /* Older versions of Darwin seem to lack getcontext().    */
       /* ARM and MIPS Linux often doesn't support a real        */
       /* getcontext().                                          */
+      static signed char getcontext_works = 0; /* (-1) - broken, 1 - works */
       ucontext_t ctxt;
 #     ifdef GETCONTEXT_FPU_EXCMASK_BUG
         /* Workaround a bug (clearing the FPU exception mask) in        */
@@ -255,12 +256,17 @@ GC_INNER void GC_with_callee_saves_pushed(void (*fn)(ptr_t, void *),
 #       endif
 #     endif
 
-      if (getcontext(&ctxt) < 0) {
-        WARN("getcontext failed:"
-             " using another register retrieval method...\n", 0);
-        /* E.g., to workaround a bug in Docker ubuntu_32bit.    */
-      } else {
-        context = &ctxt;
+      if (getcontext_works >= 0) {
+        if (getcontext(&ctxt) < 0) {
+          WARN("getcontext failed:"
+               " using another register retrieval method...\n", 0);
+          /* getcontext() is broken, do not try again.          */
+          /* E.g., to workaround a bug in Docker ubuntu_32bit.  */
+        } else {
+          context = &ctxt;
+        }
+        if (EXPECT(0 == getcontext_works, FALSE))
+          getcontext_works = context != NULL 1 : -1;
       }
 #     ifdef GETCONTEXT_FPU_EXCMASK_BUG
 #       ifdef X86_64