async: add intrusive queue

all code contributed by @steeve

* add intrusive queue
* change the constructor of Channel with default AsyncThread executor

---------

Co-authored-by: Steeve Morin <steeve@zml.ai>

async/BUILD.bazel

@@ -3,6 +3,7 @@ load("@rules_zig//zig:defs.bzl", "zig_library")
 zig_library(
     name = "async",
     srcs = [
+        "queue.zig",
         "queue_mpsc.zig",
     ],
     import_name = "async",

async/queue.zig

@@ -0,0 +1,101 @@
+const std = @import("std");
+const assert = std.debug.assert;
+
+/// An intrusive queue implementation. The type T must have a field
+/// "next" of type `?*T`.
+///
+/// For those unaware, an intrusive variant of a data structure is one in which
+/// the data type in the list has the pointer to the next element, rather
+/// than a higher level "node" or "container" type. The primary benefit
+/// of this (and the reason we implement this) is that it defers all memory
+/// management to the caller: the data structure implementation doesn't need
+/// to allocate "nodes" to contain each element. Instead, the caller provides
+/// the element and how its allocated is up to them.
+pub fn Intrusive(comptime T: type) type {
+    return struct {
+        const Self = @This();
+
+        /// Head is the front of the queue and tail is the back of the queue.
+        head: ?*T = null,
+        tail: ?*T = null,
+
+        /// Enqueue a new element to the back of the queue.
+        pub fn push(self: *Self, v: *T) void {
+            assert(v.next == null);
+
+            if (self.tail) |tail| {
+                // If we have elements in the queue, then we add a new tail.
+                tail.next = v;
+                self.tail = v;
+            } else {
+                // No elements in the queue we setup the initial state.
+                self.head = v;
+                self.tail = v;
+            }
+        }
+
+        /// Dequeue the next element from the queue.
+        pub fn pop(self: *Self) ?*T {
+            // The next element is in "head".
+            const next = self.head orelse return null;
+
+            // If the head and tail are equal this is the last element
+            // so we also set tail to null so we can now be empty.
+            if (self.head == self.tail) self.tail = null;
+
+            // Head is whatever is next (if we're the last element,
+            // this will be null);
+            self.head = next.next;
+
+            // We set the "next" field to null so that this element
+            // can be inserted again.
+            next.next = null;
+            return next;
+        }
+
+        /// Returns true if the queue is empty.
+        pub fn empty(self: *const Self) bool {
+            return self.head == null;
+        }
+    };
+}
+
+test Intrusive {
+    const testing = std.testing;
+
+    // Types
+    const Elem = struct {
+        const Self = @This();
+        next: ?*Self = null,
+    };
+    const Queue = Intrusive(Elem);
+    var q: Queue = .{};
+    try testing.expect(q.empty());
+
+    // Elems
+    var elems: [10]Elem = .{.{}} ** 10;
+
+    // One
+    try testing.expect(q.pop() == null);
+    q.push(&elems[0]);
+    try testing.expect(!q.empty());
+    try testing.expect(q.pop().? == &elems[0]);
+    try testing.expect(q.pop() == null);
+    try testing.expect(q.empty());
+
+    // Two
+    try testing.expect(q.pop() == null);
+    q.push(&elems[0]);
+    q.push(&elems[1]);
+    try testing.expect(q.pop().? == &elems[0]);
+    try testing.expect(q.pop().? == &elems[1]);
+    try testing.expect(q.pop() == null);
+
+    // Interleaved
+    try testing.expect(q.pop() == null);
+    q.push(&elems[0]);
+    try testing.expect(q.pop().? == &elems[0]);
+    q.push(&elems[1]);
+    try testing.expect(q.pop().? == &elems[1]);
+    try testing.expect(q.pop() == null);
+}

async/zigcoro.zig

@@ -5,6 +5,51 @@ const libcoro = @import("libcoro");
 const aio = libcoro.asyncio;
 const queue_mpsc = @import("queue_mpsc.zig");
 
+pub const Queue = @import("queue.zig").Intrusive;
+
+pub const Condition = struct {
+    const CoroResume = struct {
+        coro: libcoro.Frame,
+
+        fn init() CoroResume {
+            return .{ .coro = libcoro.xframe() };
+        }
+
+        fn func(self: *CoroResume) libcoro.Executor.Func {
+            return .{ .func = CoroResume.cb, .userdata = self };
+        }
+
+        fn cb(ud: ?*anyopaque) void {
+            const self: *CoroResume = @ptrCast(@alignCast(ud));
+            libcoro.xresume(self.coro);
+        }
+    };
+
+    exec: *libcoro.Executor,
+    waiters: Queue(libcoro.Executor.Func) = .{},
+
+    pub fn init() Condition {
+        return .{ .exec = &AsyncThread.current.executor.exec };
+    }
+
+    pub fn broadcast(self: *Condition) void {
+        while (self.waiters.pop()) |waiter| {
+            self.exec.runSoon(waiter);
+        }
+    }
+
+    pub fn signal(self: *Condition) void {
+        if (self.waiters.pop()) |waiter| self.exec.runSoon(waiter);
+    }
+
+    pub fn wait(self: *Condition) void {
+        var res = CoroResume.init();
+        var cb = res.func();
+        self.waiters.push(&cb);
+        libcoro.xsuspend();
+    }
+};
+
 pub fn Frame(comptime func: anytype) type {
     const Signature = stdx.meta.FnSignature(func, null);
     return FrameExx(func, Signature.ArgsT, Signature.ReturnT);
@@ -482,6 +527,31 @@ pub const Socket = struct {
     };
 };
 
+pub fn Channel(comptime T: type, capacity: usize) type {
+    return struct {
+        const Self = @This();
+        const Inner = libcoro.Channel(T, .{ .capacity = capacity });
+
+        inner: Inner,
+
+        pub fn init() Self {
+            return .{ .inner = Inner.init(&AsyncThread.current.executor.exec) };
+        }
+
+        pub fn close(self: *Self) void {
+            self.inner.close();
+        }
+
+        pub fn send(self: *Self, val: T) void {
+            self.inner.send(val) catch unreachable;
+        }
+
+        pub fn recv(self: *Self) ?T {
+            return self.inner.recv();
+        }
+    };
+}
+
 pub const Mutex = struct {
     const VoidChannel = libcoro.Channel(void, .{ .capacity = 1 });
 

stdx/signature.zig

@@ -47,37 +47,28 @@ pub fn ArgsTuple(comptime funcT: anytype, comptime ArgsT: ?type) type {
     });
 }
 
-pub fn FnSignature(comptime func: anytype, comptime ArgsT: ?type) type {
-    const n_params = switch (@typeInfo(@TypeOf(func))) {
-        .Fn => |fn_info| fn_info.params.len,
-        else => compileError("FnSignature expects a function as first argument got: {}", .{@TypeOf(func)}),
+pub const Signature = struct {
+    FuncT: type,
+    ArgsT: type,
+    ReturnT: type,
+    ReturnPayloadT: type,
+    ReturnErrorSet: ?type,
 };
-    if (ArgsT != null) {
-        const n_args = switch (@typeInfo(ArgsT.?)) {
-            .Struct => |struct_info| struct_info.fields.len,
-            else => compileError("function {} need to be called with a tuple of args", .{@TypeOf(func)}),
-        };
-        if (n_params != n_args) {
-            compileError("function {} expected {} args, got {}", .{ @TypeOf(func), n_params, n_args });
-        }
-    }
-    return FnSignatureX(func, ArgsTuple(@TypeOf(func), ArgsT));
-}
 
-// TODO: I think this should return a struct instead of returing at type
-// this gives a better error stacktrace because here the error is delayed to when the fields are read.
-fn FnSignatureX(comptime func: anytype, comptime ArgsT_: type) type {
-    return struct {
-        pub const FuncT = @TypeOf(func);
-        pub const ArgsT = ArgsT_;
-        pub const ReturnT = @TypeOf(@call(.auto, func, @as(ArgsT_, undefined)));
-        pub const ReturnPayloadT = switch (@typeInfo(ReturnT)) {
+pub fn FnSignature(comptime func: anytype, comptime argsT_: ?type) Signature {
+    const argsT = ArgsTuple(@TypeOf(func), argsT_);
+    const return_type = @TypeOf(@call(.auto, func, @as(argsT, undefined)));
+    return Signature{
+        .FuncT = @TypeOf(func),
+        .ArgsT = argsT,
+        .ReturnT = return_type,
+        .ReturnPayloadT = switch (@typeInfo(return_type)) {
             .ErrorUnion => |u| u.payload,
-            else => ReturnT,
-        };
-        pub const ReturnErrorSet: ?type = switch (@typeInfo(ReturnT)) {
+            else => return_type,
+        },
+        .ReturnErrorSet = switch (@typeInfo(return_type)) {
             .ErrorUnion => |u| u.error_set,
             else => null,
-        };
+        },
     };
 }