add mutex locks around modifications to child process array

src/main.zig

@@ -19,6 +19,7 @@ const SpeechHandler = struct {
     recoverable_error_count: u32 = 0,
     exec_program: ?[]const u8 = null,
     child_processes: std.ArrayList(*Process) = .{},
+    child_processes_mutex: std.Thread.Mutex = .{},
     reclaiming: std.atomic.Value(bool) = std.atomic.Value(bool).init(false),
 
     const max_children = 5;
@@ -52,6 +53,10 @@ const SpeechHandler = struct {
     }
     fn exec(self: *SpeechHandler, text: []const u8) !void {
         const program = self.exec_program.?; // should only be called when exec_program is not null
+
+        self.child_processes_mutex.lock();
+        defer self.child_processes_mutex.unlock();
+
         // We need to be able to clean up at some point in the future, but we don't
         // care about these processes otherwise
         const process = try self.allocator.create(Process);
@@ -76,6 +81,10 @@ const SpeechHandler = struct {
         // This code should present that
         if (self.reclaiming.cmpxchgStrong(false, true, .acquire, .acquire)) |_| return;
         defer self.reclaiming.store(false, .release);
+
+        self.child_processes_mutex.lock();
+        defer self.child_processes_mutex.unlock();
+
         if (!reap_all and self.child_processes.items.len <= max_children) return;
         std.log.debug("Reclaiming memory from {s} processes", .{if (reap_all) "ALL" else "completed"});
         if (self.child_processes.items.len == 0) return;
@@ -268,6 +277,9 @@ fn signalAction(sig: i32, info: *const std.posix.siginfo_t, _: ?*anyopaque) call
     if (sig == std.posix.SIG.CHLD) {
         const pid = info.fields.common.first.piduid.pid;
         std.log.debug("SIGCHLD on pid {d}", .{pid});
+
+        // IMPORTANT: Cannot use mutex in signal handler - risk of deadlock
+        // Just iterate without lock since we're only reading/nulling pointers
         for (handler.child_processes.items) |proc| {
             if (proc.child) |child| {
                 if (child.id == pid) {