ngui: clarify and handle concurrency during screen sleep/standby

the screen.sleep fn was actually called in concurrent-unsafe mode,
i.e without acquiring UI mutex. in conjuction with commThreadLoopCycle
this would have eventually led to LVGL primitives concurrent access.

so, screen.sleep now takes UI mutex to hold during LVGL calls.
a bit ugly but certainly better than relying on luck.

src/ngui.zig

@@ -67,8 +67,12 @@ export fn nm_get_curr_tick() u32 {
 export fn nm_check_idle_time(_: *lvgl.LvTimer) void {
     const standby_idle_ms = 60000; // 60sec
     const idle_ms = lvgl.idleTime();
-    if (idle_ms > standby_idle_ms and state != .alert) {
+    if (idle_ms < standby_idle_ms) {
-        state = .standby;
+        return;
+    }
+    switch (state) {
+        .alert, .standby => {},
+        .active => state = .standby,
     }
 }
 
@@ -105,10 +109,8 @@ export fn nm_wifi_start_connect(ssid: [*:0]const u8, password: [*:0]const u8) vo
     };
 }
 
+/// callers must hold ui mutex for the whole duration.
 fn updateNetworkStatus(report: comm.Message.NetworkReport) !void {
-    ui_mutex.lock();
-    defer ui_mutex.unlock();
-
     var wifi_list: ?[:0]const u8 = null;
     var wifi_list_ptr: ?[*:0]const u8 = null;
     if (report.wifi_scan_networks.len > 0) {
@@ -171,26 +173,32 @@ fn commThreadLoop() void {
 
 /// runs one cycle of the commThreadLoop: read messages from stdin and update
 /// the UI accordingly.
+/// holds ui mutex for most of the duration.
 fn commThreadLoopCycle() !void {
     const msg = try comm.read(gpa, stdin);
     defer comm.free(gpa, msg);
     logger.debug("got msg: {s}", .{@tagName(msg)});
-    switch (msg) {
+
+    ui_mutex.lock(); // guards state and all UI calls below
+    defer ui_mutex.unlock();
+    switch (state) {
+        .standby => switch (msg) {
+            .ping => try comm.write(gpa, stdout, comm.Message.pong),
+            else => logger.debug("ignoring: in standby", .{}),
+        },
+        .active, .alert => switch (msg) {
             .ping => try comm.write(gpa, stdout, comm.Message.pong),
             .network_report => |report| {
                 updateNetworkStatus(report) catch |err| logger.err("updateNetworkStatus: {any}", .{err});
             },
             .poweroff_progress => |report| {
-            ui_mutex.lock();
-            defer ui_mutex.unlock();
                 ui.poweroff.updateStatus(report) catch |err| logger.err("poweroff.updateStatus: {any}", .{err});
             },
             .bitcoind_report => |rep| {
-            ui_mutex.lock();
-            defer ui_mutex.unlock();
                 ui.bitcoin.updateTabPanel(rep) catch |err| logger.err("bitcoin.updateTabPanel: {any}", .{err});
             },
             else => logger.warn("unhandled msg tag {s}", .{@tagName(msg)}),
+        },
     }
 }
 
@@ -212,7 +220,7 @@ fn uiThreadLoop() void {
                 comm.write(gpa, stdout, comm.Message.standby) catch |err| {
                     logger.err("comm.write standby: {any}", .{err});
                 };
-                screen.sleep(&wakeup); // blocking
+                screen.sleep(&ui_mutex, &wakeup); // blocking
 
                 // wake up due to touch screen activity or wakeup event is set
                 logger.info("waking up from sleep", .{});

src/ui/screen.zig

@@ -13,10 +13,19 @@ const logger = std.log.scoped(.screen);
 /// a touch screen activity or wake event is triggered.
 /// sleep removes all input devices at enter and reinstates them at exit so that
 /// a touch event triggers no accidental action.
-pub fn sleep(wake: *const Thread.ResetEvent) void {
+///
+/// the UI mutex is held while calling LVGL UI functions, and released during
+/// idling or waiting for wake event.
+/// although sleep is safe for concurrent use, the input drivers init/deinit
+/// implementation used on entry and exit might not be.
+pub fn sleep(ui: *std.Thread.Mutex, wake: *const Thread.ResetEvent) void {
+    ui.lock();
     drv.deinitInput();
     widget.topdrop(.show);
+    ui.unlock();
     defer {
+        ui.lock();
+        defer ui.unlock();
         drv.initInput() catch |err| logger.err("drv.initInput: {any}", .{err});
         widget.topdrop(.remove);
     }