syscall: implement wasip1 SetNonblock and IsNonblock

Allows for the NONBLOCK file descriptor flag to be set and queried
on wasip1.

syscall.SetNonblock uses the fd_fdstat_set_flags WASI system call
and unix.IsNonblock uses the fd_fdstat_get system call.

This is a prerequisite for non-blocking I/O support.

Change-Id: I2bf79fd57142b2ec53eed3977d9aac8c6337eb80
Reviewed-on: https://go-review.googlesource.com/c/go/+/493356
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Julien Fabre <ju.pryz@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>

src/internal/syscall/unix/nonblocking_wasip1.go

@@ -6,6 +6,22 @@
 
 package unix
 
+import (
+	"syscall"
+	_ "unsafe" // for go:linkname
+)
+
 func IsNonblock(fd int) (nonblocking bool, err error) {
-	return false, nil
+	flags, e1 := fd_fdstat_get_flags(fd)
+	if e1 != nil {
+		return false, e1
+	}
+	return flags&syscall.FDFLAG_NONBLOCK != 0, nil
 }
+
+// This helper is implemented in the syscall package. It means we don't have
+// to redefine the fd_fdstat_get host import or the fdstat struct it
+// populates.
+//
+//go:linkname fd_fdstat_get_flags syscall.fd_fdstat_get_flags
+func fd_fdstat_get_flags(fd int) (uint32, error)

src/syscall/fs_wasip1.go

@@ -255,6 +255,30 @@ func path_open(rootFD int32, dirflags lookupflags, path unsafe.Pointer, pathLen
 //go:noescape
 func random_get(buf unsafe.Pointer, bufLen size) Errno
 
+// https://github.com/WebAssembly/WASI/blob/a2b96e81c0586125cc4dc79a5be0b78d9a059925/legacy/preview1/docs.md#-fdstat-record
+// fdflags must be at offset 2, hence the uint16 type rather than the
+// fdflags (uint32) type.
+type fdstat struct {
+	filetype         filetype
+	fdflags          uint16
+	rightsBase       rights
+	rightsInheriting rights
+}
+
+//go:wasmimport wasi_snapshot_preview1 fd_fdstat_get
+//go:noescape
+func fd_fdstat_get(fd int32, buf unsafe.Pointer) Errno
+
+//go:wasmimport wasi_snapshot_preview1 fd_fdstat_set_flags
+//go:noescape
+func fd_fdstat_set_flags(fd int32, flags fdflags) Errno
+
+func fd_fdstat_get_flags(fd int) (uint32, error) {
+	var stat fdstat
+	errno := fd_fdstat_get(int32(fd), unsafe.Pointer(&stat))
+	return uint32(stat.fdflags), errnoErr(errno)
+}
+
 type preopentype = uint8
 
 const (

src/syscall/net_wasip1.go

@@ -122,7 +122,3 @@ func SetWriteDeadline(fd int, t int64) error {
 func Shutdown(fd int, how int) error {
 	return ENOSYS
 }
-
-func SetNonblock(fd int, nonblocking bool) error {
-	return ENOSYS
-}

src/syscall/syscall_wasip1.go

@@ -464,3 +464,17 @@ const (
 //go:wasmimport wasi_snapshot_preview1 clock_time_get
 //go:noescape
 func clock_time_get(id clockid, precision timestamp, time unsafe.Pointer) Errno
+
+func SetNonblock(fd int, nonblocking bool) error {
+	flags, err := fd_fdstat_get_flags(fd)
+	if err != nil {
+		return err
+	}
+	if nonblocking {
+		flags |= FDFLAG_NONBLOCK
+	} else {
+		flags &^= FDFLAG_NONBLOCK
+	}
+	errno := fd_fdstat_set_flags(int32(fd), flags)
+	return errnoErr(errno)
+}