coro: make it possible to make a task uncanceable

src/coro.zig

@@ -29,6 +29,7 @@ pub const ResetEvent = Sync.ResetEvent;
 // Aliases
 pub const current = Task.current;
 pub const yield = Task.yield;
+pub const setCancelable = Task.setCancelable;
 
 /// Nonblocking IO (on a task)
 pub const io = struct {

src/coro/Frame.zig

@@ -30,6 +30,7 @@ stack: ?Fiber.Stack = null,
 result: *anyopaque,
 scheduler: *Scheduler,
 canceled: bool = false,
+cancelable: bool = true,
 status: Status = .active,
 yield_state: u8 = 0,
 link: List.Node = .{ .data = .{} },
@@ -158,7 +159,7 @@ pub fn complete(self: *@This(), mode: CompleteMode, comptime Result: type) Resul
         var timer: ?std.time.Timer = std.time.Timer.start() catch null;
         while (self.status != .completed) {
             if (mode == .cancel and tryCancel(self)) break;
-            if (timer != null and self.status != .io_cancel) {
+            if (timer != null and self.status != .io_cancel and self.cancelable) {
                 if (timer.?.read() >= 5 * std.time.ns_per_s) {
                     log.err("deadlock detected, tearing down the scheduler by a force", .{});
                     self.scheduler.state = .tear_down;
@@ -192,8 +193,15 @@ pub fn complete(self: *@This(), mode: CompleteMode, comptime Result: type) Resul
     }
 }
 
+pub const SetCancelableError = error{Canceled};
+
+pub inline fn setCancelable(self: *@This(), cancelable: bool) SetCancelableError!void {
+    if (self.canceled) return error.Canceled;
+    self.cancelable = cancelable;
+}
+
 pub fn tryCancel(self: *@This()) bool {
-    if (self.status != .completed) {
+    if (self.status != .completed and self.cancelable) {
         self.canceled = true;
         switch (self.status) {
             .active => {},

src/coro/Task.zig

@@ -38,6 +38,14 @@ pub inline fn state(self: @This(), T: type) T {
 
 pub const YieldError = error{Canceled};
 
+pub inline fn setCancelable(cancelable: bool) YieldError!void {
+    if (Frame.current()) |frame| {
+        try frame.setCancelable(cancelable);
+    } else {
+        unreachable;
+    }
+}
+
 pub inline fn yield(yield_state: anytype) YieldError!void {
     if (Frame.current()) |frame| {
         frame.yield_state = @intFromEnum(yield_state);