Crash in get_global_event() when accessing non-existent global

[shakfu]

Description

Chuck_Globals_Manager::get_global_event() (and similar functions get_global_ugen(), get_global_object()) can crash with a NULL pointer dereference when the requested global doesn't exist in the map.

Root Cause

The accessor functions use std::map::operator[] without checking if the key exists:

Chuck_Event * Chuck_Globals_Manager::get_global_event( const std::string & name )
{
    return m_global_events[name]->val;
}

If name doesn't exist in the map, operator[] creates a default-initialized entry (NULL pointer), then ->val dereferences NULL, causing a crash.

Reproduction Scenario

This can occur when CK_MSG_CLEARVM is processed after a file is compiled but before its shred runs:

1. Queue CK_MSG_CLEARVM via execute_chuck_msg_with_globals()
2. Compile a file with global CustomEvent e; - this calls init_global_event(), adding the event to the map
3. When audio processing starts, handle_global_queue_messages() processes in FIFO order:
   • CK_MSG_CLEARVM runs first, calling cleanup_global_variables() which clears the map
   • The shred is then sporked and runs
4. Shred executes Chuck_Instr_Alloc_Word_Global which calls get_global_event() - key not found, crash

Minimal Test Case

class CustomEvent extends Event {}
global CustomEvent e;
while (true) { 1::second => now; }

The crash occurs when this file is loaded after a VM clear but before audio processing begins.

Crash Trace

Thread Crashed:: com.apple.audio.IOThread.client
0   Chuck_Globals_Manager::get_global_event(...) + 44 (chuck_globals.cpp:1320)
1   Chuck_Instr_Alloc_Word_Global::execute(...) + 476 (chuck_instr.cpp:4162)

Proposed Fix

Add defensive null checks in the accessor functions:

chuck_globals.cpp:

Chuck_Event * Chuck_Globals_Manager::get_global_event( const std::string & name )
{
    auto it = m_global_events.find(name);
    if( it == m_global_events.end() || it->second == nullptr )
        return nullptr;
    return it->second->val;
}

(Same pattern for get_global_ugen() and get_global_object())

chuck_instr.cpp (in Chuck_Instr_Alloc_Word_Global::execute()):

case te_globalEvent:
    addr = (t_CKUINT) vm->globals_manager()->get_global_event( m_name );
    if( addr == 0 ) {
        EM_error2( 0, "global event '%s' not found", m_name.c_str() );
        goto error;
    }
    break;

This converts crashes into recoverable errors with helpful messages.

Environment

• ChucK version: 1.5.5.x
• Discovered in chuck-max (Max/MSP external embedding ChucK)
• Issue tracking: loading large chuck files before turning on audio causes Max to crash shakfu/chuck-max#11