Fix concurrency errors with ConcurrentDictionary

[zyzhu]

Attempt to fix issue 340 in pull 341. I simply replaced dictionary with concurrent dictionary. Ping @tpetricek for review.
#341

[kblohm]

Hi,
you might want to have a look at ConcurrentDictionary.GetOrAdd(key, Func) .

[zyzhu]

@kblohm Thanks for the tip. Code is simplified a lot.
BTW, @dsyme and I initiated some discussions about general data science in f# in the following thread. Any inputs would be more than welcome.
https://github.com/fslaborg/FsLab/issues/137

[kblohm]

@zyzhu
Since you are using the ConcurrentDictionary.GetOrAdd(TKey, TValue) overload and not ConcurrentDictionary.GetOrAdd(TKey, Func<TKey,TValue>), you are compiling your factory-lambda every single time you call the function, so your caching is not really doing anything now.
This should probably be something along the lines of:

let createTypedVector : _ -> seq<OptionalValue<obj>> -> _ =
    let cache = ConcurrentDictionary<_, _ -> _>()
    fun typ ->
        let valueFactory t =   
            let par = Expression.Parameter(typeof<seq<OptionalValue<obj>>>)
            let body = Expression.Call(createTypedVectorMi.MakeGenericMethod([| t |]), par)
            Expression.Lambda<Func<seq<OptionalValue<obj>>, IVector>>(body, par).Compile().Invoke      
        cache.GetOrAdd(typ, Func<_,_>(valueFactory))

Also, while having a quick look at the module, there is a getCachedCompileProjection aswell that is using a Dictionary to cache.

[zyzhu]

@kblohm Thanks for pointing it out. I added back trygetvalue so that cache shall work. I also changed getCachedCompileProjection

[kblohm]

@zyzhu now you are not really solving the concurrency problem. Right now checking for the value and adding the value is not atomic, that is what GetOrAdd is for.

[zyzhu]

@kblohm Clearly I'm a newbie on concurrent dictionary. I learned about valuefactory today and fixed it along your lines.
A dumb question. Why the cache stays inside createTypedVector function? Isn't the cache re-initiated everytime a thread calls this function?

[dsyme]

Change this to:

   let valueFactory = 
       Func<_,_> (fun typ -> 
         let par = Expression.Parameter(typeof<seq<OptionalValue<obj>>>)
         let body = Expression.Call(createTypedVectorMi.MakeGenericMethod([| typ |]), par)
         Expression.Lambda<Func<seq<OptionalValue<obj>>, _>>(body, par).Compile().Invoke)
   fun typ ->
     cache.GetOrAdd(typ, valueFactory)    

So the Func<_,_> call to create the delegate is only executed once

[dsyme]

Similar change requested here

[dsyme]

Two changes requested, otherwise great

[zyzhu]

@dsyme Thanks for review. I've made above changes.