wasi: nonblocking I/O for sockets and pipes on Windows

RATIONALE.md

@@ -1273,7 +1273,7 @@ However, if the reader is detected to read from `os.Stdin`,
 a special code path is followed, invoking `platform.Select()`.
 
 `platform.Select()` is a wrapper for `select(2)` on POSIX systems,
-and it is mocked for a handful of cases also on Windows.
+and it is emulated on Windows.
 
 ### Select on POSIX
 
@@ -1303,25 +1303,35 @@ unless data becomes available on `Stdin` itself.
 
 ### Select on Windows
 
-On Windows the `platform.Select()` is much more straightforward,
-and it really just replicates the behavior found in the general cases
-for `FdRead` subscriptions: in other words, the subscription to `Stdin`
-is immediately acknowledged.
+On Windows `platform.Select()` cannot be delegated to a single
+syscall, because there is no single syscall to handle sockets,
+pipes and regular files.
 
-The implementation also support a timeout, but in this case
-it relies on `time.Sleep()`, which notably, as compared to the POSIX
-case, interruptible and compatible with goroutines.
+Instead, we emulate its behavior for the cases that are currently
+of interest. 
 
-However, because `Stdin` subscriptions are always acknowledged
-without wait and because this code path is always followed only
-when at least one `Stdin` subscription is present, then the
-timeout is effectively always handled externally.
+- For regular files, we _always_ report them as ready, as
+[most operating systems do anyway][async-io-windows].
 
-In any case, the behavior of `platform.Select` on Windows
-is sensibly different from the behavior on POSIX platforms;
-we plan to refine and further align it in semantics in the future.
+- For pipes, we iterate on the given `readfds` 
+and we invoke [`PeekNamedPipe`][peeknamedpipe]. We currently ignore
+`writefds` and `exceptfds` for pipes. In particular,
+`Stdin`, when present, is set to the `readfds` FdSet.
+
+- Notably, we include also support for sockets using the [WinSock
+implementation of `select`][winsock-select], but instead
+of relying on the timeout argument of the `select` function,
+we set a 0-duration timeout so that it behaves like a peek.
+
+This way, we can check for regular files all at once,
+at the beginning of the function, then we poll pipes and 
+sockets periodically using a cancellable `time.Tick`,
+which plays nicely with the rest of the Go runtime.
 
 [poll_oneoff]: https://github.com/WebAssembly/wasi-poll#why-is-the-function-called-poll_oneoff
+[async-io-windows]: https://tinyclouds.org/iocp_links
+[peeknamedpipe]: https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-peeknamedpipe
+[winsock-select]: https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-select
 
 ## Signed encoding of integer global constant initializers
 

experimental/sys/syscall_errno_windows.go

@@ -22,6 +22,10 @@ const (
 	// _ERROR_DIRECTORY is a Windows error returned by syscall.Rmdir
 	// instead of syscall.ENOTDIR
 	_ERROR_DIRECTORY = syscall.Errno(0x10B)
+
+	// _ERROR_INVALID_SOCKET is a Windows error returned by winsock_select
+	// when a given handle is not a socket.
+	_ERROR_INVALID_SOCKET = syscall.Errno(0x2736)
 )
 
 func errorToErrno(err error) Errno {
@@ -39,7 +43,7 @@ func errorToErrno(err error) Errno {
 			return ENOTEMPTY
 		case syscall.ERROR_FILE_EXISTS:
 			return EEXIST
-		case _ERROR_INVALID_HANDLE:
+		case _ERROR_INVALID_HANDLE, _ERROR_INVALID_SOCKET:
 			return EBADF
 		case syscall.ERROR_ACCESS_DENIED:
 			// POSIX read and write functions expect EBADF, not EACCES when not

imports/wasi_snapshot_preview1/wasi_stdlib_test.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"os/exec"
 	"path"
-	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -478,13 +477,11 @@ func testSock(t *testing.T, bin []byte) {
 	console := <-ch
 	require.NotEqual(t, 0, n)
 	require.NoError(t, err)
-	require.Equal(t, "wazero\n", console)
+	// Nonblocking connections may contain error logging, we ignore those.
+	require.Equal(t, "wazero\n", console[len(console)-7:])
 }
 
 func Test_HTTP(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("fsapi.Nonblocking() is not supported on wasip1+windows.")
-	}
 	toolchains := map[string][]byte{}
 	if wasmGotip != nil {
 		toolchains["gotip"] = wasmGotip

internal/platform/fdset.go

@@ -1,3 +1,5 @@
+//go:build !windows
+
 package platform
 
 // Set adds the given fd to the set.

internal/platform/fdset_test.go

@@ -1,17 +1,14 @@
+//go:build !windows
+
 package platform
 
 import (
-	"runtime"
 	"testing"
 
 	"github.com/tetratelabs/wazero/internal/testing/require"
 )
 
 func TestFdSet(t *testing.T) {
-	if runtime.GOOS != "linux" && runtime.GOOS != "darwin" {
-		t.Skip("not supported")
-	}
-
 	allBitsSetAtIndex0 := FdSet{}
 	allBitsSetAtIndex0.Bits[0] = -1
 

internal/platform/fdset_unsupported.go

@@ -1,4 +1,4 @@
-//go:build !darwin && !linux
+//go:build !darwin && !linux && !windows
 
 package platform
 

internal/platform/fdset_windows.go

@@ -0,0 +1,239 @@
+package platform
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var procGetNamedPipeInfo = kernel32.NewProc("GetNamedPipeInfo")
+
+// Maximum number of fds in a WinSockFdSet.
+const _FD_SETSIZE = 64
+
+// WinSockFdSet implements the FdSet representation that is used internally by WinSock.
+//
+// Note: this representation is quite different from the one used in most POSIX implementations
+// where a bitfield is usually implemented; instead on Windows we have a simpler array+count pair.
+// Notice that because it keeps a count of the inserted handles, the first argument of select
+// in WinSock is actually ignored.
+//
+// The implementation of the Set, Clear, IsSet, Zero, methods follows exactly
+// the real implementation found in WinSock2.h, e.g. see:
+// https://github.com/microsoft/win32metadata/blob/ef7725c75c6b39adfdc13ba26fb1d89ac954449a/generation/WinSDK/RecompiledIdlHeaders/um/WinSock2.h#L124-L175
+type WinSockFdSet struct {
+	// count is the number of used slots used in the handles slice.
+	count uint64
+	// handles is the array of handles. This is called "array" in the WinSock implementation
+	// and it has a fixed length of _FD_SETSIZE.
+	handles [_FD_SETSIZE]syscall.Handle
+}
+
+// FdSet implements the same methods provided on other plaforms.
+//
+// Note: the implementation is very different from POSIX; Windows provides
+// POSIX select only for sockets. We emulate a select for other APIs in the sysfs
+// package, but we still want to use the "real" select in the case of sockets.
+// So, we keep separate FdSets for sockets, pipes and regular files, so that we can
+// handle them separately. For instance sockets can be used directly in winsock select.
+type FdSet struct {
+	sockets WinSockFdSet
+	pipes   WinSockFdSet
+	regular WinSockFdSet
+}
+
+// SetAll overwrites all the fields in f with the fields in g.
+func (f *FdSet) SetAll(g *FdSet) {
+	if f == nil {
+		return
+	}
+	f.sockets = g.sockets
+	f.pipes = g.pipes
+	f.regular = g.regular
+}
+
+// Sockets returns a WinSockFdSet containing the handles in this FdSet that are sockets.
+func (f *FdSet) Sockets() *WinSockFdSet {
+	if f == nil {
+		return nil
+	}
+	return &f.sockets
+}
+
+// Regular returns a WinSockFdSet containing the handles in this FdSet that are regular files.
+func (f *FdSet) Regular() *WinSockFdSet {
+	if f == nil {
+		return nil
+	}
+	return &f.regular
+}
+
+// Pipes returns a WinSockFdSet containing the handles in this FdSet that are pipes.
+func (f *FdSet) Pipes() *WinSockFdSet {
+	if f == nil {
+		return nil
+	}
+	return &f.pipes
+}
+
+// getFdSetFor returns a pointer to the right fd set for the given fd.
+// It checks the type for fd and returns the field for pipes, regular or sockets
+// to simplify code.
+//
+// For instance, if fd is a socket and it must be set if f.pipes, instead
+// of writing:
+//
+//	 if isSocket(fd) {
+//			f.sockets.Set(fd)
+//		}
+//
+// It is possible to write:
+//
+//	f.getFdSetFor(fd).Set(fd)
+func (f *FdSet) getFdSetFor(fd int) *WinSockFdSet {
+	h := syscall.Handle(fd)
+	t, err := syscall.GetFileType(h)
+	if err != nil {
+		return nil
+	}
+	switch t {
+	case syscall.FILE_TYPE_CHAR, syscall.FILE_TYPE_DISK:
+		return &f.regular
+	case syscall.FILE_TYPE_PIPE:
+		if isSocket(h) {
+			return &f.sockets
+		} else {
+			return &f.pipes
+		}
+	default:
+		return nil
+	}
+}
+
+// Set adds the given fd to the set.
+func (f *FdSet) Set(fd int) {
+	if s := f.getFdSetFor(fd); s != nil {
+		s.Set(fd)
+	}
+}
+
+// Clear removes the given fd from the set.
+func (f *FdSet) Clear(fd int) {
+	if s := f.getFdSetFor(fd); s != nil {
+		s.Clear(fd)
+	}
+}
+
+// IsSet returns true when fd is in the set.
+func (f *FdSet) IsSet(fd int) bool {
+	if s := f.getFdSetFor(fd); s != nil {
+		return s.IsSet(fd)
+	}
+	return false
+}
+
+// Copy returns a copy of this FdSet. It returns nil, if the FdSet is nil.
+func (f *FdSet) Copy() *FdSet {
+	if f == nil {
+		return nil
+	}
+	return &FdSet{
+		sockets: f.sockets,
+		pipes:   f.pipes,
+		regular: f.regular,
+	}
+}
+
+// Zero clears the set. It returns 0 if the FdSet is nil.
+func (f *FdSet) Count() int {
+	if f == nil {
+		return 0
+	}
+	return f.sockets.Count() + f.regular.Count() + f.pipes.Count()
+}
+
+// Zero clears the set.
+func (f *FdSet) Zero() {
+	if f == nil {
+		return
+	}
+	f.sockets.Zero()
+	f.regular.Zero()
+	f.pipes.Zero()
+}
+
+// Set adds the given fd to the set.
+func (f *WinSockFdSet) Set(fd int) {
+	if f.count < _FD_SETSIZE {
+		f.handles[f.count] = syscall.Handle(fd)
+		f.count++
+	}
+}
+
+// Clear removes the given fd from the set.
+func (f *WinSockFdSet) Clear(fd int) {
+	h := syscall.Handle(fd)
+	for i := uint64(0); i < f.count; i++ {
+		if f.handles[i] == h {
+			for ; i < f.count-1; i++ {
+				f.handles[i] = f.handles[i+1]
+			}
+			f.count--
+			break
+		}
+	}
+}
+
+// IsSet returns true when fd is in the set.
+func (f *WinSockFdSet) IsSet(fd int) bool {
+	h := syscall.Handle(fd)
+	for i := uint64(0); i < f.count; i++ {
+		if f.handles[i] == h {
+			return true
+		}
+	}
+	return false
+}
+
+// Zero clears the set.
+func (f *WinSockFdSet) Zero() {
+	if f == nil {
+		return
+	}
+	f.handles = [64]syscall.Handle{}
+	f.count = 0
+}
+
+// Count returns the number of values that are set in the fd set.
+func (f *WinSockFdSet) Count() int {
+	if f == nil {
+		return 0
+	}
+	return int(f.count)
+}
+
+// Copy returns a copy of the fd set or nil if it is nil.
+func (f *WinSockFdSet) Copy() *WinSockFdSet {
+	if f == nil {
+		return nil
+	}
+	copy := *f
+	return &copy
+}
+
+// Get returns the handle at the given index.
+func (f *WinSockFdSet) Get(index int) syscall.Handle {
+	return f.handles[index]
+}
+
+// isSocket returns true if the given file handle
+// is a pipe.
+func isSocket(fd syscall.Handle) bool {
+	r, _, errno := syscall.SyscallN(
+		procGetNamedPipeInfo.Addr(),
+		uintptr(fd),
+		uintptr(unsafe.Pointer(nil)),
+		uintptr(unsafe.Pointer(nil)),
+		uintptr(unsafe.Pointer(nil)),
+		uintptr(unsafe.Pointer(nil)))
+	return r == 0 || errno != 0
+}