Debug restart recompilation stopped working

[basvandriel]

Problem Description

The debug restart recompilation feature (implemented in PR #17923, merged Sep 27, 2024) has stopped working. When pressing the restart button (⟳) while debugging Rust tests, the code is no longer recompiled, causing the debugger to use stale binaries even after code changes.

Root Cause

PR #17923 implemented the feature by listening to the onDidTerminateDebugSession event to detect when a debug session was restarting:

vscode.debug.onDidTerminateDebugSession((session) => {
    if (activeDebugSessionIds.includes(session.id)) {
        // Recompile on restart
    }
});

However, VS Code's debug API behavior has changed (likely in a recent VS Code update). The onDidTerminateDebugSession event no longer fires when the restart button is clicked - it only fires when a session is fully terminated/stopped.

Impact

Users who modify code during a debug session and press restart will see:

• Old variable values in the debugger
• Outdated code behavior
• Breakpoints may not align with the actual source
• Confusing debugging experience requiring manual stop + re-run

Why This Happened

VS Code uses the Debug Adapter Protocol (DAP) for communication between the editor and debug adapters. When restart is triggered:

Before (working):

1. User clicks restart button
2. onDidTerminateDebugSession event fires
3. Extension recompiles
4. Session restarts with new binary

Now (broken):

1. User clicks restart button
2. DAP sends restart command directly to debug adapter
3. onDidTerminateDebugSession does NOT fire
4. Session restarts with cached/stale binary

The debug adapter caches the binary and symbols, so even if we tried to recompile asynchronously, the restart would use the old data.

Attempted Solutions

I tested several approaches:

1. ❌ Wait for onDidTerminateDebugSession - No longer fires on restart
2. ❌ Check binary modification time - Debug adapter caches symbols regardless
3. ❌ Send invalidate request to debug adapter - Not supported by all adapters
4. ✅ Intercept DAP restart command - Working solution (PR Fix debug restart to recompile code before restarting #21081)

Solution (PR #21081)

Use DebugAdapterTracker.onWillReceiveMessage to intercept the DAP restart command before it reaches the debug adapter:

vscode.debug.registerDebugAdapterTrackerFactory("*", {
    createDebugAdapterTracker(session) {
        return {
            onWillReceiveMessage: async (message) => {
                if (message.command === "restart") {
                    // Stop session (clears cache)
                    await vscode.debug.stopDebugging(session);
                    // Recompile with progress notification
                    await recompile(session);
                    // Start fresh session
                    await vscode.debug.startDebugging(session.workspaceFolder, session.configuration);
                }
            }
        };
    }
});

The stop → recompile → fresh start sequence is necessary because debug adapters cache binaries/symbols. A simple restart would continue using stale data.

Related Issues

• Original feature request: Support for building when a debug session is restarted #17901
• Original implementation: Building before a debugging session was restarted #17923 (merged Sep 27, 2024)
• Fix for broken behavior: Fix debug restart to recompile code before restarting #21081 (current PR)

Testing

Reproduced on:

• VS Code 1.95+
• CodeLLDB 1.10+
• rust-analyzer latest

The fix has been tested with:

• ✅ Single restart after code changes
• ✅ Multiple consecutive restarts
• ✅ Error handling for compilation failures
• ✅ Progress notifications during recompilation
• ✅ Unit tests for DAP message interception
• ✅ Integration test documentation