1 package p9p
2 
3 import (
4 	"bufio"
5 	"encoding/binary"
6 	"fmt"
7 	"io"
8 	"io/ioutil"
9 	"log"
10 	"net"
11 	"time"
12 
13 	"golang.org/x/net/context"
14 )
15 
16 // Channel defines the operations necessary to implement a 9p message channel
17 // interface. Typically, message channels do no protocol processing except to
18 // send and receive message frames.
19 type Channel interface {
20 	// ReadFcall reads one fcall frame into the provided fcall structure. The
21 	// Fcall may be cleared whether there is an error or not. If the operation
22 	// is successful, the contents of the fcall will be populated in the
23 	// argument. ReadFcall cannot be called concurrently with other calls to
24 	// ReadFcall. This both to preserve message ordering and to allow lockless
25 	// buffer reusage.
26 	ReadFcall(ctx context.Context, fcall *Fcall) error
27 
28 	// WriteFcall writes the provided fcall to the channel. WriteFcall cannot
29 	// be called concurrently with other calls to WriteFcall.
30 	WriteFcall(ctx context.Context, fcall *Fcall) error
31 
32 	// MSize returns the current msize for the channel.
33 	MSize() int
34 
35 	// SetMSize sets the maximum message size for the channel. This must never
36 	// be called currently with ReadFcall or WriteFcall.
37 	SetMSize(msize int)
38 }
39 
40 func NewChannel(conn net.Conn, msize int) Channel {
41 	return newChannel(conn, codec9p{}, msize)
42 }
43 
44 const (
45 	defaultRWTimeout = 30 * time.Second // default read/write timeout if not set in context
46 )
47 
48 // channel provides bidirectional protocol framing for 9p over net.Conn.
49 // Operations are not thread-safe but reads and writes may be carried out
50 // concurrently, supporting separate read and write loops.
51 //
52 // Lifecyle
53 //
54 // A connection, or message channel abstraction, has a lifecycle delineated by
55 // Tversion/Rversion request response cycles. For now, this is part of the
56 // channel itself but doesn't necessarily influence the channels state, except
57 // the msize. Visually, it might look something like this:
58 //
59 // 	[Established] -> [Version] -> [Session] -> [Version]---+
60 //	                     ^                                 |
61 // 	                     |_________________________________|
62 //
63 // The connection is established, then we negotiate a version, run a session,
64 // then negotiate a version and so on. For most purposes, we are likely going
65 // to terminate the connection after the session but we may want to support
66 // connection pooling. Pooling may result in possible security leaks if the
67 // connections are shared among contexts, since the version is negotiated at
68 // the start of the session. To avoid this, we can actually use a "tombstone"
69 // version message which clears the server's session state without starting a
70 // new session. The next version message would then prepare the session
71 // without leaking any Fid's.
72 type channel struct {
73 	conn   net.Conn
74 	codec  Codec
75 	brd    *bufio.Reader
76 	bwr    *bufio.Writer
77 	closed chan struct{}
78 	msize  int
79 	rdbuf  []byte
80 }
81 
82 func newChannel(conn net.Conn, codec Codec, msize int) *channel {
83 	return &channel{
84 		conn:   conn,
85 		codec:  codec,
86 		brd:    bufio.NewReaderSize(conn, msize), // msize may not be optimal buffer size
87 		bwr:    bufio.NewWriterSize(conn, msize),
88 		closed: make(chan struct{}),
89 		msize:  msize,
90 		rdbuf:  make([]byte, msize),
91 	}
92 }
93 
94 func (ch *channel) MSize() int {
95 	return ch.msize
96 }
97 
98 // setmsize resizes the buffers for use with a separate msize. This call must
99 // be protected by a mutex or made before passing to other goroutines.
100 func (ch *channel) SetMSize(msize int) {
101 	// NOTE(stevvooe): We cannot safely resize the buffered reader and writer.
102 	// Proceed assuming that original size is sufficient.
103 
104 	ch.msize = msize
105 	if msize < len(ch.rdbuf) {
106 		// just change the cap
107 		ch.rdbuf = ch.rdbuf[:msize]
108 		return
109 	}
110 
111 	ch.rdbuf = make([]byte, msize)
112 }
113 
114 // ReadFcall reads the next message from the channel into fcall.
115 func (ch *channel) ReadFcall(ctx context.Context, fcall *Fcall) error {
116 	select {
117 	case <-ctx.Done():
118 		return ctx.Err()
119 	case <-ch.closed:
120 		return ErrClosed
121 	default:
122 	}
123 
124 	deadline, ok := ctx.Deadline()
125 	if !ok {
126 		deadline = time.Now().Add(defaultRWTimeout)
127 	}
128 
129 	if err := ch.conn.SetReadDeadline(deadline); err != nil {
130 		log.Printf("transport: error setting read deadline on %v: %v", ch.conn.RemoteAddr(), err)
131 	}
132 
133 	n, err := readmsg(ch.brd, ch.rdbuf)
134 	if err != nil {
135 		// TODO(stevvooe): There may be more we can do here to detect partial
136 		// reads. For now, we just propagate the error untouched.
137 		return err
138 	}
139 
140 	if n > len(ch.rdbuf) {
141 		// TODO(stevvooe): Make this error detectable and respond with error
142 		// message.
143 		return fmt.Errorf("message large than buffer:", n)
144 	}
145 
146 	// clear out the fcall
147 	*fcall = Fcall{}
148 	if err := ch.codec.Unmarshal(ch.rdbuf[:n], fcall); err != nil {
149 		return err
150 	}
151 
152 	return nil
153 }
154 
155 func (ch *channel) WriteFcall(ctx context.Context, fcall *Fcall) error {
156 	select {
157 	case <-ctx.Done():
158 		return ctx.Err()
159 	case <-ch.closed:
160 		return ErrClosed
161 	default:
162 	}
163 
164 	deadline, ok := ctx.Deadline()
165 	if !ok {
166 		deadline = time.Now().Add(defaultRWTimeout)
167 	}
168 
169 	if err := ch.conn.SetWriteDeadline(deadline); err != nil {
170 		log.Printf("transport: error setting read deadline on %v: %v", ch.conn.RemoteAddr(), err)
171 	}
172 
173 	p, err := ch.codec.Marshal(fcall)
174 	if err != nil {
175 		return err
176 	}
177 
178 	if err := sendmsg(ch.bwr, p); err != nil {
179 		return err
180 	}
181 
182 	return ch.bwr.Flush()
183 }
184 
185 // readmsg reads a 9p message into p from rd, ensuring that all bytes are
186 // consumed from the size header. If the size header indicates the message is
187 // larger than p, the entire message will be discarded, leaving a truncated
188 // portion in p. Any error should be treated as a framing error unless n is
189 // zero. The caller must check that n is less than or equal to len(p) to
190 // ensure that a valid message has been read.
191 func readmsg(rd io.Reader, p []byte) (n int, err error) {
192 	var msize uint32
193 
194 	if err := binary.Read(rd, binary.LittleEndian, &msize); err != nil {
195 		return 0, err
196 	}
197 
198 	n += binary.Size(msize)
199 	mbody := int(msize) - 4
200 
201 	if mbody < len(p) {
202 		p = p[:mbody]
203 	}
204 
205 	np, err := io.ReadFull(rd, p)
206 	if err != nil {
207 		return np + n, err
208 	}
209 	n += np
210 
211 	if mbody > len(p) {
212 		// message has been read up to len(p) but we must consume the entire
213 		// message. This is an error condition but is non-fatal if we can
214 		// consume msize bytes.
215 		nn, err := io.CopyN(ioutil.Discard, rd, int64(mbody-len(p)))
216 		n += int(nn)
217 		if err != nil {
218 			return n, err
219 		}
220 	}
221 
222 	return n, nil
223 }
224 
225 // sendmsg writes a message of len(p) to wr with a 9p size header. All errors
226 // should be considered terminal.
227 func sendmsg(wr io.Writer, p []byte) error {
228 	size := uint32(len(p) + 4) // message size plus 4-bytes for size.
229 	if err := binary.Write(wr, binary.LittleEndian, size); err != nil {
230 		return err
231 	}
232 
233 	// This assume partial writes to wr aren't possible. Not sure if this
234 	// valid. Matters during timeout retries.
235 	if n, err := wr.Write(p); err != nil {
236 		return err
237 	} else if n < len(p) {
238 		return io.ErrShortWrite
239 	}
240 
241 	return nil
242 }