std.Io: adjust concurrent error set

Now std.Io.Threaded can return error.ConcurrencyUnavailable rather than
asserting. This is handy for logic that wants to try a concurrent
implementation but then fall back to a synchronous one.

lib/std/Io.zig

@@ -552,6 +552,8 @@ test {
     _ = Reader;
     _ = Writer;
     _ = tty;
+    _ = Evented;
+    _ = Threaded;
     _ = @import("Io/test.zig");
 }
 
@@ -596,7 +598,7 @@ pub const VTable = struct {
         context: []const u8,
         context_alignment: std.mem.Alignment,
         start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-    ) error{OutOfMemory}!*AnyFuture,
+    ) ConcurrentError!*AnyFuture,
     /// This function is only called when `async` returns a non-null value.
     ///
     /// Thread-safe.
@@ -1557,6 +1559,12 @@ pub fn async(
     return future;
 }
 
+pub const ConcurrentError = error{
+    /// May occur due to a temporary condition such as resource exhaustion, or
+    /// to the Io implementation not supporting concurrency.
+    ConcurrencyUnavailable,
+};
+
 /// Calls `function` with `args`, such that the return value of the function is
 /// not guaranteed to be available until `await` is called, allowing the caller
 /// to progress while waiting for any `Io` operations.
@@ -1568,7 +1576,7 @@ pub fn concurrent(
     io: Io,
     function: anytype,
     args: std.meta.ArgsTuple(@TypeOf(function)),
-) error{OutOfMemory}!Future(@typeInfo(@TypeOf(function)).@"fn".return_type.?) {
+) ConcurrentError!Future(@typeInfo(@TypeOf(function)).@"fn".return_type.?) {
     const Result = @typeInfo(@TypeOf(function)).@"fn".return_type.?;
     const Args = @TypeOf(args);
     const TypeErased = struct {

lib/std/Io/IoUring.zig

@@ -866,7 +866,7 @@ fn concurrent(
     context: []const u8,
     context_alignment: Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) error{OutOfMemory}!*std.Io.AnyFuture {
+) Io.ConcurrentError!*std.Io.AnyFuture {
     assert(result_alignment.compare(.lte, Fiber.max_result_align)); // TODO
     assert(context_alignment.compare(.lte, Fiber.max_context_align)); // TODO
     assert(result_len <= Fiber.max_result_size); // TODO

lib/std/Io/Kqueue.zig

@@ -933,14 +933,14 @@ fn concurrent(
     context: []const u8,
     context_alignment: Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) error{OutOfMemory}!*Io.AnyFuture {
+) Io.ConcurrentError!*Io.AnyFuture {
     const k: *Kqueue = @ptrCast(@alignCast(userdata));
     assert(result_alignment.compare(.lte, Fiber.max_result_align)); // TODO
     assert(context_alignment.compare(.lte, Fiber.max_context_align)); // TODO
     assert(result_len <= Fiber.max_result_size); // TODO
     assert(context.len <= Fiber.max_context_size); // TODO
 
-    const fiber = try Fiber.allocate(k);
+    const fiber = Fiber.allocate(k) catch return error.ConcurrencyUnavailable;
     std.log.debug("allocated {*}", .{fiber});
 
     const closure: *AsyncClosure = .fromFiber(fiber);

lib/std/Io/Threaded.zig

@@ -91,9 +91,9 @@ pub fn init(
     return t;
 }
 
-/// Statically initialize such that any call to the following functions will
+/// Statically initialize such that calls to `Io.VTable.concurrent` will fail
-/// fail with `error.OutOfMemory`:
+/// with `error.ConcurrencyUnavailable`.
-/// * `Io.VTable.concurrent`
+///
 /// When initialized this way, `deinit` is safe, but unnecessary to call.
 pub const init_single_threaded: Threaded = .{
     .allocator = .failing,
@@ -481,8 +481,8 @@ fn concurrent(
     context: []const u8,
     context_alignment: std.mem.Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) error{OutOfMemory}!*Io.AnyFuture {
+) Io.ConcurrentError!*Io.AnyFuture {
-    if (builtin.single_threaded) unreachable;
+    if (builtin.single_threaded) return error.ConcurrencyUnavailable;
 
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const cpu_count = t.cpu_count catch 1;
@@ -490,7 +490,9 @@ fn concurrent(
     const context_offset = context_alignment.forward(@sizeOf(AsyncClosure));
     const result_offset = result_alignment.forward(context_offset + context.len);
     const n = result_offset + result_len;
-    const ac: *AsyncClosure = @ptrCast(@alignCast(try gpa.alignedAlloc(u8, .of(AsyncClosure), n)));
+    const ac_bytes = gpa.alignedAlloc(u8, .of(AsyncClosure), n) catch
+        return error.ConcurrencyUnavailable;
+    const ac: *AsyncClosure = @ptrCast(@alignCast(ac_bytes));
 
     ac.* = .{
         .closure = .{
@@ -515,7 +517,7 @@ fn concurrent(
     t.threads.ensureTotalCapacity(gpa, thread_capacity) catch {
         t.mutex.unlock();
         ac.free(gpa, result_len);
-        return error.OutOfMemory;
+        return error.ConcurrencyUnavailable;
     };
 
     t.run_queue.prepend(&ac.closure.node);
@@ -525,7 +527,7 @@ fn concurrent(
             assert(t.run_queue.popFirst() == &ac.closure.node);
             t.mutex.unlock();
             ac.free(gpa, result_len);
-            return error.OutOfMemory;
+            return error.ConcurrencyUnavailable;
         };
         t.threads.appendAssumeCapacity(thread);
     }