System.IO.openTempFile does not scale

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.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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