我有一台服务器通过TCP向我发送消息，其中前4个字节确定消息其余部分的长度。所以我需要
1）将4个字节读入UInt32（正常）并将其存储到 bytes_expected
2）将 bytes_expected 字节读入消息
现在我的代码看起来像这样：
private let inputStreamAccessQueue = DispatchQueue(label: "SynchronizedInputStreamAccess")
func inputStreamHandler(_ event: Stream.Event) {
switch event {
case Stream.Event.hasBytesAvailable:
self.handleInput()
...
}
}
func handleInput() {
// **QUESTION: Do I use this barrier wrong?**
self.inputStreamAccessQueue.sync(flags: .barrier) {
guard let istr = self.inputStream else {
log.error(self.buildLogMessage("InputStream is nil"))
return
}
guard istr.hasBytesAvailable else {
log.error(self.buildLogMessage("handleInput() called when inputstream has no bytes available"))
return
}
let lengthbuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: 4)
defer { lengthbuffer.deallocate(capacity: 4) }
let lenbytes_read = istr.read(lengthbuffer, maxLength: 4)
guard lenbytes_read == 4 else {
self.errorHandler(NetworkingError.InputError("Input Stream received \(lenbytes_read) (!=4) bytes"))
return
}
let bytes_expected = Int(UnsafeRawPointer(lengthbuffer).load(as: UInt32.self).bigEndian)
log.info(self.buildLogMessage("expect \(bytes_expected) bytes"))
print("::DEBUG", call, "bytes_expected", bytes_expected)
var message = ""
var bytes_missing = bytes_expected
while bytes_missing > 0 {
//print("::DEBUG", call, "bytes_missing", bytes_missing)
let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: bytes_missing)
let bytes_read = istr.read(buffer, maxLength: bytes_missing)
print("::DEBUG", call, "bytes_read", bytes_read)
guard bytes_read > 0 else {
print("bytes_read not > 0: \(bytes_read)")
return
}
guard bytes_read <= bytes_missing else {
print("Read more bytes than expected. missing=\(bytes_missing), read=\(bytes_read)")
return
}
guard let partial_message = String(bytesNoCopy: buffer, length: bytes_read, encoding: .utf8, freeWhenDone: true) else {
log.error("ERROR WHEN READING")
return
}
message = message + partial_message
bytes_missing -= bytes_read
}
self.handleMessage(message)
}
}
我的问题是istr.read（buffer，maxLength：bytes_missing）有时不能一次读取所有消息，所以我循环直到我读完了我想要的全部内容。但我仍然看到我的应用程序崩溃（很少）因为再次调用handleInput（）而另一个对该方法的调用仍在运行。在这种情况下，bytes_expected包含随机值，并且应用程序因非法内存分配而崩溃。
我以为我可以通过使用屏障来避免这种情况。但似乎这不起作用......我使用屏障是错误的吗？
答案 0 :(得分：1)
我的建议不是要对抗网络I / O的异步性质。
每当Stream.Event.hasBytesAvailable事件时，在缓冲区中读取和收集数据
发出信号。如果缓冲区包含足够的数据（4个长度字节加上
预期的消息长度）然后处理数据并将其删除。否则什么都不做
并等待更多数据。
以下（未经测试的）代码用作演示。 它仅显示与此特定问题相关的部分。 为简洁起见，省略了初始化，事件处理程序等。
class MessageReader {
var buffer = Data(count: 1024) // Must be large enough for largest message + 4
var bytesRead = 0 // Number of bytes read so far
// Called from `handleInput` with a complete message.
func processMessage(message: Data) {
// ...
}
// Called from event handler if `Stream.Event.hasBytesAvailable` is signalled.
func handleInput(istr: InputStream) {
assert(bytesRead < buffer.count)
// Read from input stream, appending to previously read data:
let maxRead = buffer.count - bytesRead
let amount = buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
istr.read(p + bytesRead, maxLength: maxRead)
}
guard amount > 0 else {
// handle EOF or read error ...
fatalError()
}
bytesRead += amount
while bytesRead >= 4 {
// Read message size:
let messageSize = buffer.withUnsafeBytes { (p: UnsafePointer<UInt32>) in
Int(UInt32(bigEndian: p.pointee))
}
let totalSize = 4 + messageSize
guard totalSize <= buffer.count else {
// Handle buffer too small for message situation ...
fatalError()
}
if bytesRead < totalSize {
break // Not enough data to read message.
}
// Buffer contains complete message now. Process it ...
processMessage(message: buffer[4 ..< totalSize])
// ... and remove it from the buffer:
if totalSize < bytesRead {
// Move remaining data to the front:
buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
_ = memmove(p, p + totalSize, bytesRead - totalSize)
}
}
bytesRead -= totalSize
}
}
}
答案 1 :(得分：1)
受到Martin R的启发（https://stackoverflow.com/a/48344040/3827381 - 非常感谢！）我想出了这个解决方案：
var buffer = Data(count: 4096)
var offset = 0 // the index of the first byte that can be overridden
var readState = 0
var missingMsgBytes = 0
var msg = ""
func handleInput(_ istr: InputStream) {
assert(buffer.count >= 5, "buffer must be large enough to contain length info (4 bytes) and at least one payload byte => min 5 bytes buffer required")
assert(offset < buffer.count, "offset \(offset) is not smaller than \(buffer.count)")
let toRead = buffer.count - offset
let read = buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in istr.read(p + offset, maxLength: toRead) }
guard read > 0 else {
self.errorHandler(NetworkingError.InputError("Input Stream received \(read) bytes which is smaller than 0 => Network error"))
return
}
offset += read
var msgStart = 0
var msgEnd = 0
if readState == 0 {
if offset < 4 {
return
}
missingMsgBytes = buffer[0...3].withUnsafeBytes { (p: UnsafePointer<UInt32>) in Int(UInt32(bigEndian: p.pointee)) }
msgStart = 4
msgEnd = offset
readState = 1
} else {
msgStart = 0
msgEnd = offset
}
var fullMessageRead = false
if readState == 1 {
let payloadRead = msgEnd - msgStart
if payloadRead <= missingMsgBytes {
assert(msgEnd > msgStart, "msgEnd (\(msgEnd) <= msgStart \(msgStart). This should not happen")
if msgEnd > msgStart {
msg += String(data: buffer[msgStart..<msgEnd], encoding: .utf8)!
missingMsgBytes -= payloadRead
offset = 0
}
fullMessageRead = (missingMsgBytes == 0)
} else { // read more than was missing
msg += String(data: buffer[msgStart..<msgStart+missingMsgBytes], encoding: .utf8)!
fullMessageRead = true
buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
_ = memmove(p, p + missingMsgBytes, read - missingMsgBytes) // dst, src, number
}
offset = read-missingMsgBytes
}
}
if fullMessageRead {
handleMessage(msg)
readState = 0
msg = ""
missingMsgBytes = 0
}
}
此解决方案能够读取任意大小的消息。缓冲区大小仅确定一次可读取的数量=＆gt;缓冲区越大，应用程序就越快。
我现在测试了大约一个小时的代码并且它没有崩溃。旧代码在1-2分钟后崩溃。它现在似乎终于奏效了。
但是，当我想提高我的编程知识时，我想问一下我的代码中是否存在一些不必要的复杂内容，或者是否有人发现可能仍然导致应用程序崩溃或读取错误数据的错误？