From: ruby-lang.org@...
Date: 2014-12-11T18:01:56+00:00
Subject: [ruby-core:66785] [ruby-trunk - Bug #10583] Process.spawn stalls forever opening named pipes (fifo)

Issue #10583 has been updated by Justin Greer.


While the reduced testcase I attached previously sounds like a rare usage, it's actually pretty common any time you're tying together multiple programs and one of them only knows how to work with a file/fifo instead of stdin/stdout.  Our actual usage is more along the lines of this pattern:

~~~
fifo_path = '/path/to/data_fifo'
`mkfifo '#{fifo_path}'`

decoder_pid = Process.spawn("decode_data", "/path/to/input_file", fifo_path, :close_others => true)
filter_pid = Process.spawn("filter_data", :STDIN => fifo_path, :STDOUT => "/path/to/filtered_file", :close_others => true)

result = Process.wait2(decoder_pid)
# Handle result...
result = Process.waitpid(filter_pid)
# Handle result...
~~~

The above seems to be expected/common usage when you want to be able to monitor process status better than if it's being run by a shell.  (Possibly also reading its stderr as it runs.)

I'm not sure that ignore_error really conveys the usage correctly here.  Generally speaking, I would expect that the default behavior of Process.spawn matches the description of posix_spawn which it's based on.  Having a version that checks for specific issues with exec seems like it should be an alternate form, maybe Process.safe_spawn or Process.spawn(..., :synchronous => true).

Either that, or it should use a non-blocking open when it tries to open files, so that the stall doesn't happen.  (Then it would need to switch back to non-blocking, so it doesn't change the standard behavior for the exec'ed process.)

----------------------------------------
Bug #10583: Process.spawn stalls forever opening named pipes (fifo)
https://bugs.ruby-lang.org/issues/10583#change-50364

* Author: Justin Greer
* Status: Feedback
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-darwin14.0]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
Ruby's implementation of Process.spawn seems to attempt to send error/success information from the spawned process back to the parent, and the parent won't continue until it gets this information.  However, a named pipe (fifo) is mapped to the spawned process' IO, it will stall opening the IO stream, and never be able to send the error/success status back to the parent.

While stalled, the parent process is sitting here: https://github.com/ruby/ruby/blob/ruby_2_1/process.c#L3403  This prevents spawning multiple commands that communicate through a named pipe.

Example testcase is attached.

---Files--------------------------------
spawn_bug_example.rb (486 Bytes)


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