1 files changed, 239 insertions, 0 deletions
diff --git a/src/internal/poll/fd_wasip1.go b/src/internal/poll/fd_wasip1.go
new file mode 100644
index 0000000..aecd896
--- /dev/null
+++ b/src/internal/poll/fd_wasip1.go
@@ -0,0 +1,239 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+type SysFile struct {
+	// RefCountPtr is a pointer to the reference count of Sysfd.
+	//
+	// WASI preview 1 lacks a dup(2) system call. When the os and net packages
+	// need to share a file/socket, instead of duplicating the underlying file
+	// descriptor, we instead provide a way to copy FD instances and manage the
+	// underlying file descriptor with reference counting.
+	RefCountPtr *int32
+
+	// RefCount is the reference count of Sysfd. When a copy of an FD is made,
+	// it points to the reference count of the original FD instance.
+	RefCount int32
+
+	// Cache for the file type, lazily initialized when Seek is called.
+	Filetype uint32
+
+	// If the file represents a directory, this field contains the current
+	// readdir position. It is reset to zero if the program calls Seek(0, 0).
+	Dircookie uint64
+
+	// Absolute path of the file, as returned by syscall.PathOpen;
+	// this is used by Fchdir to emulate setting the current directory
+	// to an open file descriptor.
+	Path string
+
+	// TODO(achille): it could be meaningful to move isFile from FD to a method
+	// on this struct type, and expose it as `IsFile() bool` which derives the
+	// result from the Filetype field. We would need to ensure that Filetype is
+	// always set instead of being lazily initialized.
+}
+
+func (s *SysFile) init() {
+	if s.RefCountPtr == nil {
+		s.RefCount = 1
+		s.RefCountPtr = &s.RefCount
+	}
+}
+
+func (s *SysFile) ref() SysFile {
+	atomic.AddInt32(s.RefCountPtr, +1)
+	return SysFile{RefCountPtr: s.RefCountPtr}
+}
+
+func (s *SysFile) destroy(fd int) error {
+	if s.RefCountPtr != nil && atomic.AddInt32(s.RefCountPtr, -1) > 0 {
+		return nil
+	}
+
+	// We don't use ignoringEINTR here because POSIX does not define
+	// whether the descriptor is closed if close returns EINTR.
+	// If the descriptor is indeed closed, using a loop would race
+	// with some other goroutine opening a new descriptor.
+	// (The Linux kernel guarantees that it is closed on an EINTR error.)
+	return CloseFunc(fd)
+}
+
+// Copy creates a copy of the FD.
+//
+// The FD instance points to the same underlying file descriptor. The file
+// descriptor isn't closed until all FD instances that refer to it have been
+// closed/destroyed.
+func (fd *FD) Copy() FD {
+	return FD{
+		Sysfd:         fd.Sysfd,
+		SysFile:       fd.SysFile.ref(),
+		IsStream:      fd.IsStream,
+		ZeroReadIsEOF: fd.ZeroReadIsEOF,
+		isBlocking:    fd.isBlocking,
+		isFile:        fd.isFile,
+	}
+}
+
+// dupCloseOnExecOld always errors on wasip1 because there is no mechanism to
+// duplicate file descriptors.
+func dupCloseOnExecOld(fd int) (int, string, error) {
+	return -1, "dup", syscall.ENOSYS
+}
+
+// Fchdir wraps syscall.Fchdir.
+func (fd *FD) Fchdir() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Chdir(fd.Path)
+}
+
+// ReadDir wraps syscall.ReadDir.
+// We treat this like an ordinary system call rather than a call
+// that tries to fill the buffer.
+func (fd *FD) ReadDir(buf []byte, cookie syscall.Dircookie) (int, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	for {
+		n, err := syscall.ReadDir(fd.Sysfd, buf, cookie)
+		if err != nil {
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		// Do not call eofError; caller does not expect to see io.EOF.
+		return n, err
+	}
+}
+
+func (fd *FD) ReadDirent(buf []byte) (int, error) {
+	n, err := fd.ReadDir(buf, fd.Dircookie)
+	if err != nil {
+		return 0, err
+	}
+	if n <= 0 {
+		return n, nil // EOF
+	}
+
+	// We assume that the caller of ReadDirent will consume the entire buffer
+	// up to the last full entry, so we scan through the buffer looking for the
+	// value of the last next cookie.
+	b := buf[:n]
+
+	for len(b) > 0 {
+		next, ok := direntNext(b)
+		if !ok {
+			break
+		}
+		size, ok := direntReclen(b)
+		if !ok {
+			break
+		}
+		if size > uint64(len(b)) {
+			break
+		}
+		fd.Dircookie = syscall.Dircookie(next)
+		b = b[size:]
+	}
+
+	// Trim a potentially incomplete trailing entry; this is necessary because
+	// the code in src/os/dir_unix.go does not deal well with partial values in
+	// calls to direntReclen, etc... and ends up causing an early EOF before all
+	// directory entries were consumed. ReadDirent is called with a large enough
+	// buffer (8 KiB) that at least one entry should always fit, tho this seems
+	// a bit brittle but cannot be addressed without a large change of the
+	// algorithm in the os.(*File).readdir method.
+	return n - len(b), nil
+}
+
+// Seek wraps syscall.Seek.
+func (fd *FD) Seek(offset int64, whence int) (int64, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	// syscall.Filetype is a uint8 but we store it as a uint32 in SysFile in
+	// order to use atomic load/store on the field, which is why we have to
+	// perform this type conversion.
+	fileType := syscall.Filetype(atomic.LoadUint32(&fd.Filetype))
+
+	if fileType == syscall.FILETYPE_UNKNOWN {
+		var stat syscall.Stat_t
+		if err := fd.Fstat(&stat); err != nil {
+			return 0, err
+		}
+		fileType = stat.Filetype
+		atomic.StoreUint32(&fd.Filetype, uint32(fileType))
+	}
+
+	if fileType == syscall.FILETYPE_DIRECTORY {
+		// If the file descriptor is opened on a directory, we reset the readdir
+		// cookie when seeking back to the beginning to allow reusing the file
+		// descriptor to scan the directory again.
+		if offset == 0 && whence == 0 {
+			fd.Dircookie = 0
+			return 0, nil
+		} else {
+			return 0, syscall.EINVAL
+		}
+	}
+
+	return syscall.Seek(fd.Sysfd, offset, whence)
+}
+
+// https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#-dirent-record
+const sizeOfDirent = 24
+
+func direntReclen(buf []byte) (uint64, bool) {
+	namelen, ok := direntNamlen(buf)
+	return sizeOfDirent + namelen, ok
+}
+
+func direntNamlen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Namlen), unsafe.Sizeof(syscall.Dirent{}.Namlen))
+}
+
+func direntNext(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Next), unsafe.Sizeof(syscall.Dirent{}.Next))
+}
+
+// readInt returns the size-bytes unsigned integer in native byte order at offset off.
+func readInt(b []byte, off, size uintptr) (u uint64, ok bool) {
+	if len(b) < int(off+size) {
+		return 0, false
+	}
+	return readIntLE(b[off:], size), true
+}
+
+func readIntLE(b []byte, size uintptr) uint64 {
+	switch size {
+	case 1:
+		return uint64(b[0])
+	case 2:
+		_ = b[1] // bounds check hint to compiler; see golang.org/issue/14808
+		return uint64(b[0]) | uint64(b[1])<<8
+	case 4:
+		_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
+		return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24
+	case 8:
+		_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
+		return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
+			uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
+	default:
+		panic("internal/poll: readInt with unsupported size")
+	}
+}