[mypp/examples] Repro GC crash bug

This is issue #1986.

Our rooting logic for context managers is simply wrong (and has always
been wrong).  The bug is apparently triggered by:

- ctx_Foo() that has member variable that's a GC object
- Invoking GC with mylib.MaybeCollect() while the context manager is
  "live" (in scope)

In Oils, it is exposed by our Str => replace() implementation, which
calls ctx_Shvar().

mycpp/examples/scoped_resource.py

@@ -7,6 +7,7 @@
 import os
 import sys
 
+from mycpp import mylib
 from mycpp.mylib import log
 from typing import List, Optional, Any
 
@@ -48,13 +49,18 @@ def __init__(self, state, entry):
         self.state = state
         state.Push(entry)
 
+        # Bug #1986: add heap-allocated member of context manager
+        #self.restored = []  # type: List[str]
+        #self.restored.append('foo')
+
     def __enter__(self):
         # type: () -> None
         """no-op, but it has to exist to be used as context manager."""
         pass
 
     def __exit__(self, type, value, traceback):
         # type: (Any, Any, Any) -> None
+        #self.restored.pop()
         self.state.Pop()
 
 
@@ -182,8 +188,12 @@ def run_benchmarks():
     # type: () -> None
     d = DirStack()
     for i in xrange(1000000):
+        # Does NOT trigger the bug!
+        #mylib.MaybeCollect()
         try:
             with ctx_DirStack(d, 'foo') as _:
+                # Bug #1986: add collection in this loop
+                mylib.MaybeCollect()
                 if i % 10000 == 0:
                     raise MyError()
         except MyError:

test/bug-1986.sh

@@ -0,0 +1,73 @@
+#!/usr/bin/env bash
+#
+# Potentially Long-running stress tests
+#
+# Usage:
+#   test/stress.sh <function name>
+
+# GC bug?
+
+replace-ysh() {
+  #local ysh=_bin/cxx-opt/ysh
+  local ysh=_bin/cxx-dbg/ysh
+  #local ysh=_bin/cxx-asan/ysh
+  ninja $ysh
+
+  # gc_mylib.cc sets threshold to 50K
+  # it doesn't seem to matter!
+  #
+  # Oh does this have to do with recursively calling back into the interpreter?
+
+  #GDB='gdb --args '
+  GDB=''
+
+  OIL_GC_STATS=1 OILS_GC_VERBOSE=1 OILS_GC_THRESHOLD=100 $GDB $ysh -c '
+var x = "z"
+for i in (1 .. 1000) { 
+  echo $i
+  # z -> zz creates exponential data
+  setvar x = x=>replace("z", "y")
+  setvar x = x=>replace("y", "z")
+}
+echo $x
+'
+}
+
+replace-exp() {
+  local osh=_bin/cxx-opt/osh
+  ninja $osh
+
+  # 2.791 seconds for 19 iterations, up to 1 MB
+  SH=$osh
+
+  SH=bash
+  for sh in bash $osh; do
+    echo "=== $sh ==="
+    echo
+
+    time $sh -c '
+x=z
+for i in {1..19}; do
+  x=${x//z/zz}
+  echo len=${#x}
+done
+  '
+  done
+}
+
+# Reduce this bug
+# https://github.com/oilshell/oil/issues/1986
+
+bug-1986() {
+  local ysh=_bin/cxx-dbg/ysh
+  #local ysh=_bin/cxx-asan/ysh
+  ninja $ysh
+
+  GDB="gdb --args $PWD/$ysh"
+  cd bug/code
+
+  set -x
+  $GDB src/amd-scripts/amd-test --select imperfect aomp-amd-staging.cfg 14
+}
+
+"$@"