Merge pull request #6412 from kristoff-it/generalize-event-loop

Make os.zig not depend on the event loop

4 files changed, 425 insertions, 229 deletions

diff --git a/lib/std/event/loop.zig b/lib/std/event/loop.zig
index 13e704a8d3..2ed9f938d8 100644
--- a/lib/std/event/loop.zig
+++ b/lib/std/event/loop.zig
@@ -721,6 +721,50 @@ pub const Loop = struct {
         }
     }
 
+    /// ------- I/0 APIs -------
+    pub fn accept(
+        self: *Loop,
+        /// This argument is a socket that has been created with `socket`, bound to a local address
+        /// with `bind`, and is listening for connections after a `listen`.
+        sockfd: os.fd_t,
+        /// This argument is a pointer to a sockaddr structure.  This structure is filled in with  the
+        /// address  of  the  peer  socket, as known to the communications layer.  The exact format of the
+        /// address returned addr is determined by the socket's address  family  (see  `socket`  and  the
+        /// respective  protocol  man  pages).
+        addr: *os.sockaddr,
+        /// This argument is a value-result argument: the caller must initialize it to contain  the
+        /// size (in bytes) of the structure pointed to by addr; on return it will contain the actual size
+        /// of the peer address.
+        ///
+        /// The returned address is truncated if the buffer provided is too small; in this  case,  `addr_size`
+        /// will return a value greater than was supplied to the call.
+        addr_size: *os.socklen_t,
+        /// The following values can be bitwise ORed in flags to obtain different behavior:
+        /// * `SOCK_CLOEXEC`  - Set the close-on-exec (`FD_CLOEXEC`) flag on the new file descriptor.   See  the
+        ///   description  of the `O_CLOEXEC` flag in `open` for reasons why this may be useful.
+        flags: u32,
+    ) os.AcceptError!os.fd_t {
+        while (true) {
+            return os.accept(sockfd, addr, addr_size, flags | os.SOCK_NONBLOCK) catch |err| switch (err) {
+                error.WouldBlock => {
+                    self.waitUntilFdReadable(sockfd);
+                    continue;
+                },
+                else => return err,
+            };
+        }
+    }
+
+    pub fn connect(self: *Loop, sockfd: os.socket_t, sock_addr: *const os.sockaddr, len: os.socklen_t) os.ConnectError!void {
+        os.connect(sockfd, sock_addr, len) catch |err| switch (err) {
+            error.WouldBlock => {
+                self.waitUntilFdWritable(sockfd);
+                return os.getsockoptError(sockfd);
+            },
+            else => return err,
+        };
+    }
+
     /// Performs an async `os.open` using a separate thread.
     pub fn openZ(self: *Loop, file_path: [*:0]const u8, flags: u32, mode: os.mode_t) os.OpenError!os.fd_t {
         var req_node = Request.Node{
@@ -779,152 +823,309 @@ pub const Loop = struct {
 
     /// Performs an async `os.read` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn read(self: *Loop, fd: os.fd_t, buf: []u8) os.ReadError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .read = .{
-                        .fd = fd,
-                        .buf = buf,
-                        .result = undefined,
+    pub fn read(self: *Loop, fd: os.fd_t, buf: []u8, simulate_evented: bool) os.ReadError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .read = .{
+                            .fd = fd,
+                            .buf = buf,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.read.result;
+        } else {
+            while (true) {
+                return os.read(fd, buf) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdReadable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.read.result;
     }
 
     /// Performs an async `os.readv` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn readv(self: *Loop, fd: os.fd_t, iov: []const os.iovec) os.ReadError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .readv = .{
-                        .fd = fd,
-                        .iov = iov,
-                        .result = undefined,
+    pub fn readv(self: *Loop, fd: os.fd_t, iov: []const os.iovec, simulate_evented: bool) os.ReadError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .readv = .{
+                            .fd = fd,
+                            .iov = iov,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.readv.result;
+        } else {
+            while (true) {
+                return os.readv(fd, iov) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdReadable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.readv.result;
     }
 
     /// Performs an async `os.pread` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn pread(self: *Loop, fd: os.fd_t, buf: []u8, offset: u64) os.PReadError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .pread = .{
-                        .fd = fd,
-                        .buf = buf,
-                        .offset = offset,
-                        .result = undefined,
+    pub fn pread(self: *Loop, fd: os.fd_t, buf: []u8, offset: u64, simulate_evented: bool) os.PReadError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .pread = .{
+                            .fd = fd,
+                            .buf = buf,
+                            .offset = offset,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.pread.result;
+        } else {
+            while (true) {
+                return os.pread(fd, buf, offset) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdReadable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.pread.result;
     }
 
     /// Performs an async `os.preadv` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn preadv(self: *Loop, fd: os.fd_t, iov: []const os.iovec, offset: u64) os.ReadError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .preadv = .{
-                        .fd = fd,
-                        .iov = iov,
-                        .offset = offset,
-                        .result = undefined,
+    pub fn preadv(self: *Loop, fd: os.fd_t, iov: []const os.iovec, offset: u64, simulate_evented: bool) os.ReadError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .preadv = .{
+                            .fd = fd,
+                            .iov = iov,
+                            .offset = offset,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.preadv.result;
+        } else {
+            while (true) {
+                return os.preadv(fd, iov, offset) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdReadable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.preadv.result;
     }
 
     /// Performs an async `os.write` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn write(self: *Loop, fd: os.fd_t, bytes: []const u8) os.WriteError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .write = .{
-                        .fd = fd,
-                        .bytes = bytes,
-                        .result = undefined,
+    pub fn write(self: *Loop, fd: os.fd_t, bytes: []const u8, simulate_evented: bool) os.WriteError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .write = .{
+                            .fd = fd,
+                            .bytes = bytes,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.write.result;
+        } else {
+            while (true) {
+                return os.write(fd, bytes) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdWritable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.write.result;
     }
 
     /// Performs an async `os.writev` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn writev(self: *Loop, fd: os.fd_t, iov: []const os.iovec_const) os.WriteError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .writev = .{
-                        .fd = fd,
-                        .iov = iov,
-                        .result = undefined,
+    pub fn writev(self: *Loop, fd: os.fd_t, iov: []const os.iovec_const, simulate_evented: bool) os.WriteError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .writev = .{
+                            .fd = fd,
+                            .iov = iov,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.writev.result;
+        } else {
+            while (true) {
+                return os.writev(fd, iov) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdWritable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
+        }
+    }
+
+    /// Performs an async `os.pwrite` using a separate thread.
+    /// `fd` must block and not return EAGAIN.
+    pub fn pwrite(self: *Loop, fd: os.fd_t, bytes: []const u8, offset: u64, simulate_evented: bool) os.PerformsWriteError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .pwrite = .{
+                            .fd = fd,
+                            .bytes = bytes,
+                            .offset = offset,
+                            .result = undefined,
+                        },
+                    },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
+                },
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.pwrite.result;
+        } else {
+            while (true) {
+                return os.pwrite(fd, bytes, offset) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdWritable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
         }
-        return req_node.data.msg.writev.result;
     }
 
     /// Performs an async `os.pwritev` using a separate thread.
     /// `fd` must block and not return EAGAIN.
-    pub fn pwritev(self: *Loop, fd: os.fd_t, iov: []const os.iovec_const, offset: u64) os.WriteError!usize {
-        var req_node = Request.Node{
-            .data = .{
-                .msg = .{
-                    .pwritev = .{
-                        .fd = fd,
-                        .iov = iov,
-                        .offset = offset,
-                        .result = undefined,
+    pub fn pwritev(self: *Loop, fd: os.fd_t, iov: []const os.iovec_const, offset: u64, simulate_evented: bool) os.PWriteError!usize {
+        if (simulate_evented) {
+            var req_node = Request.Node{
+                .data = .{
+                    .msg = .{
+                        .pwritev = .{
+                            .fd = fd,
+                            .iov = iov,
+                            .offset = offset,
+                            .result = undefined,
+                        },
                     },
+                    .finish = .{ .TickNode = .{ .data = @frame() } },
                 },
-                .finish = .{ .TickNode = .{ .data = @frame() } },
-            },
-        };
-        suspend {
-            self.posixFsRequest(&req_node);
+            };
+            suspend {
+                self.posixFsRequest(&req_node);
+            }
+            return req_node.data.msg.pwritev.result;
+        } else {
+            while (true) {
+                return os.pwritev(fd, iov, offset) catch |err| switch (err) {
+                    error.WouldBlock => {
+                        self.waitUntilFdWritable(fd);
+                        continue;
+                    },
+                    else => return err,
+                };
+            }
+        }
+    }
+
+    pub fn sendto(
+        self: *Loop,
+        /// The file descriptor of the sending socket.
+        sockfd: os.fd_t,
+        /// Message to send.
+        buf: []const u8,
+        flags: u32,
+        dest_addr: ?*const os.sockaddr,
+        addrlen: os.socklen_t,
+    ) os.SendError!usize {
+        while (true) {
+            return os.sendto(sockfd, buf, flags, dest_addr, addrlen) catch |err| switch (err) {
+                error.WouldBlock => {
+                    self.waitUntilFdWritable(sockfd);
+                    continue;
+                },
+                else => return err,
+            };
+        }
+    }
+
+    pub fn recvfrom(
+        sockfd: os.fd_t,
+        buf: []u8,
+        flags: u32,
+        src_addr: ?*os.sockaddr,
+        addrlen: ?*os.socklen_t,
+    ) os.RecvFromError!usize {
+        while (true) {
+            return os.recvfrom(sockfd, buf, flags, src_addr, addrlen) catch |err| switch (err) {
+                error.WouldBlock => {
+                    self.waitUntilFdReadable(sockfd);
+                    continue;
+                },
+                else => return err,
+            };
         }
-        return req_node.data.msg.pwritev.result;
     }
 
     /// Performs an async `os.faccessatZ` using a separate thread.
@@ -1079,6 +1280,9 @@ pub const Loop = struct {
                     .writev => |*msg| {
                         msg.result = os.writev(msg.fd, msg.iov);
                     },
+                    .pwrite => |*msg| {
+                        msg.result = os.pwrite(msg.fd, msg.bytes, msg.offset);
+                    },
                     .pwritev => |*msg| {
                         msg.result = os.pwritev(msg.fd, msg.iov, msg.offset);
                     },
@@ -1148,6 +1352,7 @@ pub const Loop = struct {
             readv: ReadV,
             write: Write,
             writev: WriteV,
+            pwrite: PWrite,
             pwritev: PWriteV,
             pread: PRead,
             preadv: PReadV,
@@ -1191,6 +1396,15 @@ pub const Loop = struct {
                 pub const Error = os.WriteError;
             };
 
+            pub const PWrite = struct {
+                fd: os.fd_t,
+                bytes: []const u8,
+                offset: usize,
+                result: Error!usize,
+
+                pub const Error = os.PWriteError;
+            };
+
             pub const PWriteV = struct {
                 fd: os.fd_t,
                 iov: []const os.iovec_const,
diff --git a/lib/std/fs/file.zig b/lib/std/fs/file.zig
index 73babf5fa2..8d4f5df2e8 100644
--- a/lib/std/fs/file.zig
+++ b/lib/std/fs/file.zig
@@ -414,10 +414,12 @@ pub const File = struct {
     pub fn read(self: File, buffer: []u8) ReadError!usize {
         if (is_windows) {
             return windows.ReadFile(self.handle, buffer, null, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.read(self.handle, buffer);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.read(self.handle, buffer);
+        } else {
+            return std.event.Loop.instance.?.read(self.handle, buffer, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -436,10 +438,12 @@ pub const File = struct {
     pub fn pread(self: File, buffer: []u8, offset: u64) PReadError!usize {
         if (is_windows) {
             return windows.ReadFile(self.handle, buffer, offset, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.pread(self.handle, buffer, offset);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.pread(self.handle, buffer, offset);
+        } else {
+            return std.event.Loop.instance.?.pread(self.handle, buffer, offset, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -461,10 +465,12 @@ pub const File = struct {
             if (iovecs.len == 0) return @as(usize, 0);
             const first = iovecs[0];
             return windows.ReadFile(self.handle, first.iov_base[0..first.iov_len], null, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.readv(self.handle, iovecs);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.readv(self.handle, iovecs);
+        } else {
+            return std.event.Loop.instance.?.readv(self.handle, iovecs, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -500,10 +506,12 @@ pub const File = struct {
             if (iovecs.len == 0) return @as(usize, 0);
             const first = iovecs[0];
             return windows.ReadFile(self.handle, first.iov_base[0..first.iov_len], offset, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.preadv(self.handle, iovecs, offset);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.preadv(self.handle, iovecs, offset);
+        } else {
+            return std.event.Loop.instance.?.preadv(self.handle, iovecs, offset, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -539,10 +547,12 @@ pub const File = struct {
     pub fn write(self: File, bytes: []const u8) WriteError!usize {
         if (is_windows) {
             return windows.WriteFile(self.handle, bytes, null, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.write(self.handle, bytes);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.write(self.handle, bytes);
+        } else {
+            return std.event.Loop.instance.?.write(self.handle, bytes, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -556,10 +566,12 @@ pub const File = struct {
     pub fn pwrite(self: File, bytes: []const u8, offset: u64) PWriteError!usize {
         if (is_windows) {
             return windows.WriteFile(self.handle, bytes, offset, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.pwrite(self.handle, bytes, offset);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.pwrite(self.handle, bytes, offset);
+        } else {
+            return std.event.Loop.instance.?.pwrite(self.handle, bytes, offset, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -576,10 +588,12 @@ pub const File = struct {
             if (iovecs.len == 0) return @as(usize, 0);
             const first = iovecs[0];
             return windows.WriteFile(self.handle, first.iov_base[0..first.iov_len], null, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.writev(self.handle, iovecs);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.writev(self.handle, iovecs);
+        } else {
+            return std.event.Loop.instance.?.writev(self.handle, iovecs, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
@@ -607,10 +621,12 @@ pub const File = struct {
             if (iovecs.len == 0) return @as(usize, 0);
             const first = iovecs[0];
             return windows.WriteFile(self.handle, first.iov_base[0..first.iov_len], offset, self.intended_io_mode);
-        } else if (self.capable_io_mode != self.intended_io_mode) {
-            return std.event.Loop.instance.?.pwritev(self.handle, iovecs, offset);
-        } else {
+        }
+
+        if (self.intended_io_mode == .blocking) {
             return os.pwritev(self.handle, iovecs, offset);
+        } else {
+            return std.event.Loop.instance.?.pwritev(self.handle, iovecs, offset, self.capable_io_mode != self.intended_io_mode);
         }
     }
 
diff --git a/lib/std/net.zig b/lib/std/net.zig
index 45d8f07f04..fe7d0fafe6 100644
--- a/lib/std/net.zig
+++ b/lib/std/net.zig
@@ -614,11 +614,11 @@ pub fn connectUnixSocket(path: []const u8) !fs.File {
 
     var addr = try std.net.Address.initUnix(path);
 
-    try os.connect(
-        sockfd,
-        &addr.any,
-        addr.getOsSockLen(),
-    );
+    if (std.io.is_async) {
+        try loop.connect(sockfd, &addr.any, addr.getOsSockLen());
+    } else {
+        try os.connect(sockfd, &addr.any, addr.getOsSockLen());
+    }
 
     return fs.File{
         .handle = sockfd,
@@ -677,7 +677,13 @@ pub fn tcpConnectToAddress(address: Address) !fs.File {
         (if (builtin.os.tag == .windows) 0 else os.SOCK_CLOEXEC);
     const sockfd = try os.socket(address.any.family, sock_flags, os.IPPROTO_TCP);
     errdefer os.close(sockfd);
-    try os.connect(sockfd, &address.any, address.getOsSockLen());
+
+    if (std.io.is_async) {
+        const loop = std.event.Loop.instance orelse return error.WouldBlock;
+        try loop.connect(sockfd, &address.any, address.getOsSockLen());
+    } else {
+        try os.connect(sockfd, &address.any, address.getOsSockLen());
+    }
 
     return fs.File{ .handle = sockfd };
 }
@@ -1429,7 +1435,11 @@ fn resMSendRc(
                 if (answers[i].len == 0) {
                     var j: usize = 0;
                     while (j < ns.len) : (j += 1) {
-                        _ = os.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                        if (std.io.is_async) {
+                            _ = std.event.Loop.instance.?.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                        } else {
+                            _ = os.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                        }
                     }
                 }
             }
@@ -1444,7 +1454,10 @@ fn resMSendRc(
 
         while (true) {
             var sl_copy = sl;
-            const rlen = os.recvfrom(fd, answer_bufs[next], 0, &sa.any, &sl_copy) catch break;
+            const rlen = if (std.io.is_async)
+                std.event.Loop.instance.?.recvfrom(fd, answer_bufs[next], 0, &sa.any, &sl_copy) catch break
+            else
+                os.recvfrom(fd, answer_bufs[next], 0, &sa.any, &sl_copy) catch break;
 
             // Ignore non-identifiable packets
             if (rlen < 4) continue;
@@ -1470,7 +1483,11 @@ fn resMSendRc(
                 0, 3 => {},
                 2 => if (servfail_retry != 0) {
                     servfail_retry -= 1;
-                    _ = os.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                    if (std.io.is_async) {
+                        _ = std.event.Loop.instance.?.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                    } else {
+                        _ = os.sendto(fd, queries[i], os.MSG_NOSIGNAL, &ns[j].any, sl) catch undefined;
+                    }
                 },
                 else => continue,
             }
@@ -1661,18 +1678,23 @@ pub const StreamServer = struct {
 
     /// If this function succeeds, the returned `Connection` is a caller-managed resource.
     pub fn accept(self: *StreamServer) AcceptError!Connection {
-        const nonblock = if (std.io.is_async) os.SOCK_NONBLOCK else 0;
-        const accept_flags = nonblock | os.SOCK_CLOEXEC;
         var accepted_addr: Address = undefined;
         var adr_len: os.socklen_t = @sizeOf(Address);
-        if (os.accept(self.sockfd.?, &accepted_addr.any, &adr_len, accept_flags)) |fd| {
+        const accept_result = blk: {
+            if (std.io.is_async) {
+                const loop = std.event.Loop.instance orelse return error.UnexpectedError;
+                break :blk loop.accept(self.sockfd.?, &accepted_addr.any, &adr_len, os.SOCK_CLOEXEC);
+            } else {
+                break :blk os.accept(self.sockfd.?, &accepted_addr.any, &adr_len, os.SOCK_CLOEXEC);
+            }
+        };
+
+        if (accept_result) |fd| {
             return Connection{
                 .file = fs.File{ .handle = fd },
                 .address = accepted_addr,
             };
         } else |err| switch (err) {
-            // We only give SOCK_NONBLOCK when I/O mode is async, in which case this error
-            // is handled by os.accept4.
             error.WouldBlock => unreachable,
             else => |e| return e,
         }
diff --git a/lib/std/os.zig b/lib/std/os.zig
index 0b09b1f82a..c06ce4ed00 100644
--- a/lib/std/os.zig
+++ b/lib/std/os.zig
@@ -314,8 +314,8 @@ pub const ReadError = error{
 
 /// Returns the number of bytes that were read, which can be less than
 /// buf.len. If 0 bytes were read, that means EOF.
-/// If the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in error.WouldBlock.
+/// If `fd` is opened in non blocking mode, the function will return error.WouldBlock
+/// when EAGAIN is received.
 ///
 /// Linux has a limit on how many bytes may be transferred in one `read` call, which is `0x7ffff000`
 /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
@@ -366,12 +366,7 @@ pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForReading, // Can be a race condition.
             EIO => return error.InputOutput,
             EISDIR => return error.IsDir,
@@ -387,8 +382,8 @@ pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
 
 /// Number of bytes read is returned. Upon reading end-of-file, zero is returned.
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 ///
@@ -428,12 +423,7 @@ pub fn readv(fd: fd_t, iov: []const iovec) ReadError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForReading, // can be a race condition
             EIO => return error.InputOutput,
             EISDIR => return error.IsDir,
@@ -450,8 +440,8 @@ pub const PReadError = ReadError || error{Unseekable};
 ///
 /// Retries when interrupted by a signal.
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 pub fn pread(fd: fd_t, buf: []u8, offset: u64) PReadError!usize {
@@ -492,12 +482,7 @@ pub fn pread(fd: fd_t, buf: []u8, offset: u64) PReadError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForReading, // Can be a race condition.
             EIO => return error.InputOutput,
             EISDIR => return error.IsDir,
@@ -586,8 +571,8 @@ pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
 ///
 /// Retries when interrupted by a signal.
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 ///
@@ -637,12 +622,7 @@ pub fn preadv(fd: fd_t, iov: []const iovec, offset: u64) PReadError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForReading, // can be a race condition
             EIO => return error.InputOutput,
             EISDIR => return error.IsDir,
@@ -687,8 +667,8 @@ pub const WriteError = error{
 /// another  write() call to transfer the remaining bytes.  The subsequent call will either
 /// transfer further bytes or may result in an error (e.g., if the disk is now full).
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 ///
@@ -741,12 +721,7 @@ pub fn write(fd: fd_t, bytes: []const u8) WriteError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdWritable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForWriting, // can be a race condition.
             EDESTADDRREQ => unreachable, // `connect` was never called.
             EDQUOT => return error.DiskQuota,
@@ -772,8 +747,8 @@ pub fn write(fd: fd_t, bytes: []const u8) WriteError!usize {
 /// another  write() call to transfer the remaining bytes.  The subsequent call will either
 /// transfer further bytes or may result in an error (e.g., if the disk is now full).
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.k`.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 ///
@@ -814,12 +789,7 @@ pub fn writev(fd: fd_t, iov: []const iovec_const) WriteError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdWritable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForWriting, // Can be a race condition.
             EDESTADDRREQ => unreachable, // `connect` was never called.
             EDQUOT => return error.DiskQuota,
@@ -847,8 +817,8 @@ pub const PWriteError = WriteError || error{Unseekable};
 /// another  write() call to transfer the remaining bytes.  The subsequent call will either
 /// transfer further bytes or may result in an error (e.g., if the disk is now full).
 ///
-/// For POSIX systems, if the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// For POSIX systems, if `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
 /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
 ///
@@ -905,12 +875,7 @@ pub fn pwrite(fd: fd_t, bytes: []const u8, offset: u64) PWriteError!usize {
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdWritable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForWriting, // Can be a race condition.
             EDESTADDRREQ => unreachable, // `connect` was never called.
             EDQUOT => return error.DiskQuota,
@@ -939,8 +904,8 @@ pub fn pwrite(fd: fd_t, bytes: []const u8, offset: u64) PWriteError!usize {
 /// another  write() call to transfer the remaining bytes.  The subsequent call will either
 /// transfer further bytes or may result in an error (e.g., if the disk is now full).
 ///
-/// If the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in `error.WouldBlock`.
+/// If `fd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 ///
 /// The following systems do not have this syscall, and will return partial writes if more than one
 /// vector is provided:
@@ -993,12 +958,7 @@ pub fn pwritev(fd: fd_t, iov: []const iovec_const, offset: u64) PWriteError!usiz
             EINTR => continue,
             EINVAL => unreachable,
             EFAULT => unreachable,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdWritable(fd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => return error.NotOpenForWriting, // Can be a race condition.
             EDESTADDRREQ => unreachable, // `connect` was never called.
             EDQUOT => return error.DiskQuota,
@@ -2846,8 +2806,8 @@ pub const AcceptError = error{
 } || UnexpectedError;
 
 /// Accept a connection on a socket.
-/// If the application has a global event loop enabled, EAGAIN is handled
-/// via the event loop. Otherwise EAGAIN results in error.WouldBlock.
+/// If `sockfd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 pub fn accept(
     /// This argument is a socket that has been created with `socket`, bound to a local address
     /// with `bind`, and is listening for connections after a `listen`.
@@ -2890,12 +2850,7 @@ pub fn accept(
                 return fd;
             },
             EINTR => continue,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(sockfd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EBADF => unreachable, // always a race condition
             ECONNABORTED => return error.ConnectionAborted,
             EFAULT => unreachable,
@@ -3081,6 +3036,8 @@ pub const ConnectError = error{
 } || UnexpectedError;
 
 /// Initiate a connection on a socket.
+/// If `sockfd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN or EINPROGRESS is received.
 pub fn connect(sockfd: socket_t, sock_addr: *const sockaddr, len: socklen_t) ConnectError!void {
     if (builtin.os.tag == .windows) {
         const rc = windows.ws2_32.connect(sockfd, sock_addr, len);
@@ -3113,11 +3070,7 @@ pub fn connect(sockfd: socket_t, sock_addr: *const sockaddr, len: socklen_t) Con
             EADDRINUSE => return error.AddressInUse,
             EADDRNOTAVAIL => return error.AddressNotAvailable,
             EAFNOSUPPORT => return error.AddressFamilyNotSupported,
-            EAGAIN, EINPROGRESS => {
-                const loop = std.event.Loop.instance orelse return error.WouldBlock;
-                loop.waitUntilFdWritable(sockfd);
-                return getsockoptError(sockfd);
-            },
+            EAGAIN, EINPROGRESS => return error.WouldBlock,
             EALREADY => unreachable, // The socket is nonblocking and a previous connection attempt has not yet been completed.
             EBADF => unreachable, // sockfd is not a valid open file descriptor.
             ECONNREFUSED => return error.ConnectionRefused,
@@ -4620,14 +4573,8 @@ pub fn sendto(
         const rc = system.sendto(sockfd, buf.ptr, buf.len, flags, dest_addr, addrlen);
         switch (errno(rc)) {
             0 => return @intCast(usize, rc),
-
             EACCES => return error.AccessDenied,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdWritable(sockfd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             EALREADY => return error.FastOpenAlreadyInProgress,
             EBADF => unreachable, // always a race condition
             ECONNRESET => return error.ConnectionResetByPeer,
@@ -5106,6 +5053,8 @@ pub const RecvFromError = error{
     SystemResources,
 } || UnexpectedError;
 
+/// If `sockfd` is opened in non blocking mode, the function will
+/// return error.WouldBlock when EAGAIN is received.
 pub fn recvfrom(
     sockfd: fd_t,
     buf: []u8,
@@ -5123,12 +5072,7 @@ pub fn recvfrom(
             ENOTCONN => unreachable,
             ENOTSOCK => unreachable,
             EINTR => continue,
-            EAGAIN => if (std.event.Loop.instance) |loop| {
-                loop.waitUntilFdReadable(sockfd);
-                continue;
-            } else {
-                return error.WouldBlock;
-            },
+            EAGAIN => return error.WouldBlock,
             ENOMEM => return error.SystemResources,
             ECONNREFUSED => return error.ConnectionRefused,
             else => |err| return unexpectedErrno(err),