new async exercise

build.zig

@@ -1161,13 +1161,20 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "092_async9.zig",
-        .output = "",
-        .skip = true,
+        .output =
+        \\Main thread continues...
+        \\Computing on a separate thread!
+        \\Main thread done waiting.
+        \\Result: 123
+        , // pay attention to the comma
     },
     .{
         .main_file = "093_async10.zig",
-        .output = "",
-        .skip = true,
+        .output =
+        \\Starting critical section...
+        \\Critical section completed safely.
+        \\Task result: All data saved.
+        , // pay attention to the comma
     },
     .{
         .main_file = "094_async_quiz.zig",

exercises/093_async10.zig

@@ -0,0 +1,67 @@
+//
+// In exercise 088, we learned that cancellation happens at
+// "cancellation points" — any Io function that can return
+// error.Canceled.
+//
+// But sometimes a task has a critical section that MUST NOT
+// be interrupted — for example, writing a consistent state
+// to disk, or completing a transaction.
+//
+// Io provides CancelProtection for this:
+//
+//     const old = io.swapCancelProtection(.blocked);
+//     defer _ = io.swapCancelProtection(old);
+//
+//     // In this block, NO Io function will return error.Canceled.
+//     // The cancel request is held until protection is restored.
+//
+// There are two states:
+//   .unblocked — normal: cancellation points can fire (default)
+//   .blocked   — protected: error.Canceled is never returned
+//
+// There's also io.checkCancel() — a pure cancellation point
+// that does nothing except return error.Canceled if a cancel
+// request is pending. Useful in long CPU-bound loops.
+//
+// And io.recancel() — re-arms a consumed cancel request so
+// the NEXT cancellation point will fire again.
+//
+// Fix this program so the critical section completes even
+// when the task is canceled.
+//
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main(init: std.process.Init) !void {
+    const io = init.io;
+
+    var future = io.async(importantTask, .{io});
+
+    // Give the task time to start and enter its critical section.
+    io.sleep(std.Io.Duration.fromMilliseconds(300), .awake) catch {};
+
+    // Cancel while the task is in its protected section.
+    const result = future.cancel(io);
+    print("Task result: {s}\n", .{result});
+}
+
+fn importantTask(io: std.Io) []const u8 {
+    print("Starting critical section...\n", .{});
+
+    // Protect this section from cancellation.
+    // What method swaps the cancel protection state?
+    const old = io.???(. blocked);
+    defer _ = io.???(old);
+
+    // This sleep will NOT return error.Canceled even though
+    // we get canceled during it — protection is active!
+    io.sleep(std.Io.Duration.fromMilliseconds(600), .awake) catch |err| switch (err) {
+        error.Canceled => {
+            // This should never happen while protected!
+            return "ERROR: canceled during critical section!";
+        },
+    };
+
+    print("Critical section completed safely.\n", .{});
+    return "All data saved.";
+}

patches/patches/093_async10.patch

@@ -0,0 +1,13 @@
+--- exercises/093_async10.zig	2026-04-03 14:25:16.600025924 +0200
++++ answers/093_async10.zig	2026-04-03 14:24:56.192615893 +0200
+@@ -50,8 +50,8 @@
+ 
+     // Protect this section from cancellation.
+     // What method swaps the cancel protection state?
+-    const old = io.???(. blocked);
+-    defer _ = io.???(old);
++    const old = io.swapCancelProtection(.blocked);
++    defer _ = io.swapCancelProtection(old);
+ 
+     // This sleep will NOT return error.Canceled even though
+     // we get canceled during it — protection is active!