Haskell 写入命名管道意外失败，并显示“openFile: 不存在（没有此类设备或地址）”

Question

如果我使用

mkfifo /tmp2/example

创建了一个命名管道并使用以下函数：

writeToFifo :: FilePath -> IO ()
writeToFifo path = do
  handle <- openFile path WriteMode
  let loop n = do
        let msg = "Message " ++ show n
        hPutStrLn handle msg
        hFlush handle
        putStrLn $ "Wrote: " ++ msg
        threadDelay 1000000  -- Sleep for 1 second
        loop (n + 1)
  loop (1 :: Int)

但这会导致

* Exception: /tmp2/example: openFile: does not exist (No such device or address)

即使它存在

ls -alrth /tmp2/example
prw------- 1 chris users 0 Mar 30 12:54 /tmp2/example

为什么会出现这种情况？我该如何解决它？

Answer 1

这是由于

openFile

是通过文件阻塞实现的，如此处所述https://gitlab.haskell.org/ghc/ghc/-/issues/15715.

可以通过以下方式修复：

import GHC.IO.Handle.FD (openFileBlocking)

并将

openFile

替换为

openFileBlocking

。

或者：

---

writeToFifo :: FilePath -> IO ()
writeToFifo path = do
  -- Open the FIFO for writing, without creating it (assumes it exists)
  fd <- openFd path WriteOnly Nothing defaultFileFlags
  handle <- fdToHandle fd
  let loop n = do
        let msg = "Message " ++ show n
        hPutStrLn handle msg
        hFlush handle
        putStrLn $ "Wrote: " ++ msg
        threadDelay 1000000  -- Sleep for 1 second
        loop (n + 1)
  loop (1 :: Int)