From 7e10ebe1010898c48e3f1cfc12ad42d0bb5c0fa1 Mon Sep 17 00:00:00 2001
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Mon, 14 May 2018 00:28:30 +0200
Subject: Introduce rwcancel

---
 rwcancel/rwcancel_unix.go | 132 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 132 insertions(+)
 create mode 100644 rwcancel/rwcancel_unix.go

(limited to 'rwcancel')

diff --git a/rwcancel/rwcancel_unix.go b/rwcancel/rwcancel_unix.go
new file mode 100644
index 0000000..cd3661f
--- /dev/null
+++ b/rwcancel/rwcancel_unix.go
@@ -0,0 +1,132 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2017-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+package rwcancel
+
+import (
+	"errors"
+	"golang.org/x/sys/unix"
+	"os"
+	"runtime"
+	"syscall"
+)
+
+type RWCancel struct {
+	fd            int
+	closingReader *os.File
+	closingWriter *os.File
+}
+
+type fdSet struct {
+	fdset unix.FdSet
+}
+
+func (fdset *fdSet) set(i int) {
+	bits := 32 << (^uint(0) >> 63)
+	fdset.fdset.Bits[i/bits] |= 1 << uint(i%bits)
+}
+
+func (fdset *fdSet) check(i int) bool {
+	bits := 32 << (^uint(0) >> 63)
+	return (fdset.fdset.Bits[i/bits] & (1 << uint(i%bits))) != 0
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func NewRWCancel(fd int) (*RWCancel, error) {
+	err := unix.SetNonblock(fd, true)
+	if err != nil {
+		return nil, err
+	}
+	rwcancel := RWCancel{fd: fd}
+
+	rwcancel.closingReader, rwcancel.closingWriter, err = os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+
+	runtime.SetFinalizer(&rwcancel, func(rw *RWCancel) {
+		rw.Cancel()
+	})
+
+	return &rwcancel, nil
+}
+
+/* https://golang.org/src/crypto/rand/eagain.go */
+func ErrorIsEAGAIN(err error) bool {
+	if pe, ok := err.(*os.PathError); ok {
+		if errno, ok := pe.Err.(syscall.Errno); ok && errno == syscall.EAGAIN {
+			return true
+		}
+	}
+	if errno, ok := err.(syscall.Errno); ok && errno == syscall.EAGAIN {
+		return true
+	}
+	return false
+}
+
+func (rw *RWCancel) ReadyRead() bool {
+	closeFd := int(rw.closingReader.Fd())
+	fdset := fdSet{}
+	fdset.set(rw.fd)
+	fdset.set(closeFd)
+	_, err := unix.Select(max(rw.fd, closeFd)+1, &fdset.fdset, nil, nil, nil)
+	if err != nil {
+		return false
+	}
+	if fdset.check(closeFd) {
+		return false
+	}
+	return fdset.check(rw.fd)
+}
+
+func (rw *RWCancel) ReadyWrite() bool {
+	closeFd := int(rw.closingReader.Fd())
+	fdset := fdSet{}
+	fdset.set(rw.fd)
+	fdset.set(closeFd)
+	_, err := unix.Select(max(rw.fd, closeFd)+1, nil, &fdset.fdset, nil, nil)
+	if err != nil {
+		return false
+	}
+	if fdset.check(closeFd) {
+		return false
+	}
+	return fdset.check(rw.fd)
+}
+
+func (rw *RWCancel) Read(p []byte) (n int, err error) {
+	for {
+		n, err := unix.Read(rw.fd, p)
+		if err == nil || !ErrorIsEAGAIN(err) {
+			return n, err
+		}
+		if !rw.ReadyRead() {
+			return 0, errors.New("fd closed")
+		}
+	}
+}
+
+func (rw *RWCancel) Write(p []byte) (n int, err error) {
+	for {
+		n, err := unix.Write(rw.fd, p)
+		if err == nil || !ErrorIsEAGAIN(err) {
+			return n, err
+		}
+		if !rw.ReadyWrite() {
+			return 0, errors.New("fd closed")
+		}
+	}
+}
+
+func (rw *RWCancel) Cancel() (err error) {
+	_, err = rw.closingWriter.Write([]byte{0})
+	return
+}
-- 
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