don't allocate in cgo_context

cgo_context may be called from a signal handler. Allocating is
forbidden. Use thread local storage instead.

cgosymbolizer_darwin.c

@@ -6,10 +6,13 @@
 #include "cgosymbolizer_darwin.h"
 
 struct cgo_context {
+ bool used;
  unw_context_t unw_ctx;
  unw_cursor_t unw_cursor;
 };
 
+__thread struct cgo_context thread_ctx;
+
 void cgo_traceback(void* p) {
  struct {
  uintptr_t ctx;
@@ -46,21 +49,21 @@ void cgo_context(void* p) {
  }* arg = p;
 
  if (arg->ctx != 0) {
- free((struct cgo_context*) arg->ctx);
+ struct cgo_context* ctx = (struct cgo_context*) arg->ctx;
+ ctx->used = 0;
  return;
  }
 
- struct cgo_context* ctx = malloc(sizeof(cgo_context));
- if (ctx == NULL)
+ if (thread_ctx.used)
  return;
- if (unw_getcontext(&ctx->unw_ctx) != 0)
+ if (unw_getcontext(&thread_ctx.unw_ctx) != 0)
  return;
- if (unw_init_local(&ctx->unw_cursor, &ctx->unw_ctx) != 0)
+ if (unw_init_local(&thread_ctx.unw_cursor, &thread_ctx.unw_ctx) != 0)
  return;
  // Step the saved state past this frame. It will cease to exist momentarily.
- if (unw_step(&ctx->unw_cursor) <= 0)
+ if (unw_step(&thread_ctx.unw_cursor) <= 0)
  return;
- arg->ctx = (uintptr_t) ctx;
+ arg->ctx = (uintptr_t) &thread_ctx;
 }
 
 void cgo_symbolizer(void* p) {