[GHC] #9058: System.IO.openTempFile does not scale

[GHC]

#9058: System.IO.openTempFile does not scale
------------------------------------+-------------------------------------
       Reporter:  slyfox            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 In search of a bug in darcs http://bugs.darcs.net/issue2364
 i've notice very bad property of openTempFile:
 it's pattern is very predictable and has O(n^2) of already created temp
 files.

 Predictability allows very fun bugs survive in buggy programs, like:
 {{{
   thread1:
     (fn, fh) <- openTempFile "." "hello"
     renameFile fn "something"
     -- some time after
     when (some_rare_buggy_condition) $
         -- oops, reused temp name, but too late, other thread killed it
         writeFileFile fn
   thread2:
     (fn, fh) <- openTempFile "." "hello"
     workWithFn fn -- nobody should touch it, right?
 }}}
 It's _very_ hard to debug data corruption when
 all temp files are named "foo${pid}" and sometimes "foo${pid+1}".

 And more serious bug:
 the more threads you have trying to create similar temps
 performance drops significantly:

 Attached program shows the following numbers:
 {{{
 $ time ./bench-temps same 2000

 real    0m2.795s
 user    0m1.516s
 sys     0m1.190s

 $ time ./bench-temps diff 2000

 real    0m0.161s
 user    0m0.043s
 sys     0m0.115s
 }}}

 It's O(N^2) growing open() storm.

 https://github.com/ghc/ghc/blob/master/libraries/base/System/IO.hs#L465

 {{{
     FileExists -> findTempName (x + 1)
 }}}
 This is the source of the problem. I'd suggest always using
 random name for it. For portability reasons I suggest adding
 at least insecure random '''rand()''' value from C library.

 That way we will succeed in opening temp file at the first attempt.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9058>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler