Update crash handler behavior to opt-in (#484)

README.md

@@ -66,3 +66,6 @@ Please note that on Mac, once a private key is used with a certificate, that cer
 ```
 static: certificate has an existing certificate-key pair that was previously imported into the Keychain. Using key from Keychain instead of the one provided.
 ```
+
+## Crash Handler
+You can enable the crash handler by setting the environment variable `AWS_CRT_CRASH_HANDLER=1`. This will print the callstack to `stderr` in the event of a fatal error.

source/module.c

@@ -804,6 +804,7 @@ static PyMethodDef s_module_methods[] = {
 
 static const char s_module_name[] = "_awscrt";
 PyDoc_STRVAR(s_module_doc, "C extension for binding AWS implementations of MQTT, HTTP, and friends");
+AWS_STATIC_STRING_FROM_LITERAL(s_crash_handler_env_var, "AWS_CRT_CRASH_HANDLER");
 
 /*******************************************************************************
  * Module Init
@@ -828,11 +829,17 @@ PyMODINIT_FUNC PyInit__awscrt(void) {
     }
 
     s_init_allocator();
-    s_install_crash_handler();
 
     /* Don't report this memory when dumping possible leaks. */
     struct aws_allocator *nontracing_allocator = aws_default_allocator();
 
+    struct aws_string *crash_handler_env = NULL;
+    aws_get_environment_value(nontracing_allocator, s_crash_handler_env_var, &crash_handler_env);
+    if (aws_string_eq_c_str(crash_handler_env, "1")) {
+        s_install_crash_handler();
+    }
+    aws_string_destroy(crash_handler_env);
+
     aws_http_library_init(nontracing_allocator);
     aws_auth_library_init(nontracing_allocator);
     aws_mqtt_library_init(nontracing_allocator);

test/__init__.py

@@ -6,6 +6,7 @@
 # the "noqa" comment prevents the autoformatter from moving this line below other imports
 import os
 os.environ['AWS_CRT_MEMORY_TRACING'] = '2'  # noqa
+os.environ['AWS_CRT_CRASH_HANDLER'] = '1'   # noqa
 
 from awscrt import NativeResource
 from awscrt._test import check_for_leaks