osproc: writes can crash process with SIGPIPE, instead of raising IOError on EPIPE

[timotheecour]

Example

when true:
  when defined caseSub:
    for i in 0..<3:
      let a = stdin.readLine()
      echo (a, i)
    quit 3
  else:
    import std/[osproc,os,strformat,streams]

    proc main()=
      let output = "/tmp/z01"
      const nim = getCurrentCompilerExe()
      const file = currentSourcePath()
      let a = execShellCmd(fmt"{nim} c -o:{output} -d:caseSub {file}")
      doAssert a == 0
      var p = startProcess(command=output, options={poStdErrToStdOut})
      let outp = p.outputStream
      let inp = p.inputStream
      for i in 0..<100000:
        echo i
        inp.writeLine "ok" # SIGPIPE: Pipe closed => crash
        # inp.flush() # ditto, can also give SIGPIPE
      for ai in outp.lines: echo ai
      let status = waitForExit(p)
      p.close # bug D20210416T212230:here docs + code often mention using close without first calling waitForExit, causing zombies
      echo status
    main()

Current Output

...
27303
27304
27305
27306
Traceback (most recent call last)
/Users/timothee/git_clone/nim/timn/tests/nim/all/t12180.nim(30) t12180
/Users/timothee/git_clone/nim/timn/tests/nim/all/t12180.nim(24) main
/Users/timothee/git_clone/nim/Nim_devel/lib/pure/streams.nim(390) writeLine
/Users/timothee/git_clone/nim/Nim_devel/lib/pure/streams.nim(364) write
/Users/timothee/git_clone/nim/Nim_devel/lib/pure/streams.nim(326) writeData
/Users/timothee/git_clone/nim/Nim_devel/lib/pure/streams.nim(1319) fsWriteData
SIGPIPE: Pipe closed.

Expected Output

raise IOError showing the signal EPIPE

Possible Solution

ignore SIGPIPE. It's the most sensible approach and that's what's commonly used.

• in a multithreaded environment (especially in library code), it's tricky but doable, see http://www.microhowto.info/howto/ignore_sigpipe_without_affecting_other_threads_in_a_process.html
• see also blockSigpipe in lib/pure/net.nim which probably implements this already (but requires a bit of refactoring so osproc in particular can use it)

Additional Information

• 1.5.1 957478c

related

• nim CI fails on i386 SIGPIPE nimsuggest/tester #17748
• ignore signal SIGPIPE on Darwin #1795 used (for a different context, lib/pure/rawsockets.nim):

# We ignore signal SIGPIPE on Darwin
when defined(macosx):
  signal(SIGPIPE, SIG_IGN)

but IIUC signal isn't recommended nowdays

• No verbose exit on SIGPIPE #1168 (comment)

I think that SIGPIPE should be ignored by default (as in Python), because exception from write will anyway kill program and do it at least in safe manner.

• for sockets, we have other better options in some OS, eg:
  SO_SIGNOPIPE, MSG_NOSIGNAL
• http://www.pixelbeat.org/programming/sigpipe_handling.html