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probe.go
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probe.go
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package io
import (
"fmt"
"log"
"net"
"time"
)
type TokenSet [][]byte
type tokenCounter struct {
expected int
collected [][]byte
}
// Receives and parses packets read as a response to the probe. Tries to
// group packets based on their sequence uint16. Once it observers a compelte
// set of tokens, it writes it to the results channel. Finally, this function
// closes the results channel.
func responseParser(packetChan chan []byte, resultsChan chan TokenSet) {
defer close(resultsChan)
results := make(map[uint16]tokenCounter)
for p := range packetChan {
if p == nil {
break
} else if seq, total, token, err := FromPacket(p); err != nil {
log.Printf("could not parse token packet: %s", err)
} else {
log.Printf("received token [seq: %d, total: %d, len: %d]", seq, total, len(token))
counter, ok := results[seq]
if !ok {
counter = tokenCounter{expected: total, collected: make([][]byte, 0, total)}
} else if counter.expected != total {
log.Printf("packets with same seq differ in total tokens")
continue
}
counter.collected = append(counter.collected, token)
if len(counter.collected) == counter.expected {
resultsChan <- counter.collected
delete(results, seq)
} else {
results[seq] = counter
}
}
}
}
// Sends a zero-sized packet to the probe address and listens for a response on
// the given port. Any complete set of tokens is written to the result channel.
// Result channel will be closed by this method.
// Listens for timeout many seconds, even if it received one set of tokens
// already.
func UDPProbe(listenPort int, probeAddr *net.UDPAddr, timeout int64, resultsChan chan TokenSet) {
laddr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%d", listenPort))
if err != nil {
log.Fatalf("could not resolve local address: %s", err)
}
conn, err := net.DialUDP("udp", laddr, probeAddr)
if conn != nil {
defer conn.Close()
}
if err != nil {
log.Fatalf("could not dial: %s", err)
}
if _, err = conn.Write([]byte{}); err != nil {
log.Fatalf("could not send probe: %s", err)
} else {
packetChan := make(chan []byte)
defer close(packetChan)
go responseParser(packetChan, resultsChan)
conn.SetReadDeadline(time.Now().Add(time.Second * time.Duration(timeout)))
for {
buff := make([]byte, 4096)
n, _, err := conn.ReadFromUDP(buff)
if err != nil {
log.Printf("connection closed: %s", err)
break
} else {
packetChan <- buff[:n]
}
}
}
}