3 files changed, 99 insertions, 1 deletions

diff --git a/std/event.zig b/std/event.zig
index 516defebf8..f3913a432b 100644
--- a/std/event.zig
+++ b/std/event.zig
@@ -4,6 +4,7 @@ pub const Lock = @import("event/lock.zig").Lock;
 pub const tcp = @import("event/tcp.zig");
 pub const Channel = @import("event/channel.zig").Channel;
 pub const Group = @import("event/group.zig").Group;
+pub const Future = @import("event/future.zig").Group;
 
 test "import event tests" {
     _ = @import("event/locked.zig");
@@ -12,4 +13,5 @@ test "import event tests" {
     _ = @import("event/tcp.zig");
     _ = @import("event/channel.zig");
     _ = @import("event/group.zig");
+    _ = @import("event/future.zig");
 }
diff --git a/std/event/future.zig b/std/event/future.zig
new file mode 100644
index 0000000000..8001f675a2
--- /dev/null
+++ b/std/event/future.zig
@@ -0,0 +1,87 @@
+const std = @import("../index.zig");
+const assert = std.debug.assert;
+const builtin = @import("builtin");
+const AtomicRmwOp = builtin.AtomicRmwOp;
+const AtomicOrder = builtin.AtomicOrder;
+const Lock = std.event.Lock;
+const Loop = std.event.Loop;
+
+/// This is a value that starts out unavailable, until a value is put().
+/// While it is unavailable, coroutines suspend when they try to get() it,
+/// and then are resumed when the value is put().
+/// At this point the value remains forever available, and another put() is not allowed.
+pub fn Future(comptime T: type) type {
+    return struct {
+        lock: Lock,
+        data: T,
+        available: u8, // TODO make this a bool
+
+        const Self = this;
+        const Queue = std.atomic.QueueMpsc(promise);
+
+        pub fn init(loop: *Loop) Self {
+            return Self{
+                .lock = Lock.initLocked(loop),
+                .available = 0,
+                .data = undefined,
+            };
+        }
+
+        /// Obtain the value. If it's not available, wait until it becomes
+        /// available.
+        /// Thread-safe.
+        pub async fn get(self: *Self) T {
+            if (@atomicLoad(u8, &self.available, AtomicOrder.SeqCst) == 1) {
+                return self.data;
+            }
+            const held = await (async self.lock.acquire() catch unreachable);
+            defer held.release();
+
+            return self.data;
+        }
+
+        /// Make the data become available. May be called only once.
+        pub fn put(self: *Self, value: T) void {
+            self.data = value;
+            const prev = @atomicRmw(u8, &self.available, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+            assert(prev == 0); // put() called twice
+            Lock.Held.release(Lock.Held{ .lock = &self.lock });
+        }
+    };
+}
+
+test "std.event.Future" {
+    var da = std.heap.DirectAllocator.init();
+    defer da.deinit();
+
+    const allocator = &da.allocator;
+
+    var loop: Loop = undefined;
+    try loop.initMultiThreaded(allocator);
+    defer loop.deinit();
+
+    const handle = try async<allocator> testFuture(&loop);
+    defer cancel handle;
+
+    loop.run();
+}
+
+async fn testFuture(loop: *Loop) void {
+    var future = Future(i32).init(loop);
+
+    const a = async waitOnFuture(&future) catch @panic("memory");
+    const b = async waitOnFuture(&future) catch @panic("memory");
+    const c = async resolveFuture(&future) catch @panic("memory");
+
+    const result = (await a) + (await b);
+    cancel c;
+    assert(result == 12);
+}
+
+async fn waitOnFuture(future: *Future(i32)) i32 {
+    return await (async future.get() catch @panic("memory"));
+}
+
+async fn resolveFuture(future: *Future(i32)) void {
+    future.put(6);
+}
diff --git a/std/event/lock.zig b/std/event/lock.zig
index 2a8d5ada77..cba3594b50 100644
--- a/std/event/lock.zig
+++ b/std/event/lock.zig
@@ -73,6 +73,15 @@ pub const Lock = struct {
         };
     }
 
+    pub fn initLocked(loop: *Loop) Lock {
+        return Lock{
+            .loop = loop,
+            .shared_bit = 1,
+            .queue = Queue.init(),
+            .queue_empty_bit = 1,
+        };
+    }
+
     /// Must be called when not locked. Not thread safe.
     /// All calls to acquire() and release() must complete before calling deinit().
     pub fn deinit(self: *Lock) void {
@@ -81,7 +90,7 @@ pub const Lock = struct {
     }
 
     pub async fn acquire(self: *Lock) Held {
-        s: suspend |handle| {
+        suspend |handle| {
             // TODO explicitly put this memory in the coroutine frame #1194
             var my_tick_node = Loop.NextTickNode{
                 .data = handle,