Make QNetworkAccessManager threadsafe

[kshegunov]

@Phoenox said in Make QNetworkAccessManager threadsafe:

Question 1: Is this correct?

Yes, that's correct. NAM uses threading internally though.

Question 2: Is this feasible?

It's a good plan. However you can just use plain ol' signal and slots instead of using your own thread-safe wrapper around the (single) NAM instance.

@Buckwheat said in Make QNetworkAccessManager threadsafe:

Typically "re-entrant" implies thread-safe as variables are on the stack and do not interfere with each other.

Not exactly. Reentrant literally means a thread can be interrupted and then the reentry can be made without side effects. This implies that the variables are thread-local (independent of stack or heap), i.e. there are no globals to take care of.

[Buckwheat]

@kshegunov ... correct. but add... there are no side-effects if invoked again before the last invocation finishes. Hence no static (global) members. The safest way is still to use mutexing and protect yourself.

[kshegunov]

@Buckwheat said in Make QNetworkAccessManager threadsafe:

... correct. but add...

No "but". It means safe for re-entry. It doesn't include "if invoked again". A global may be modified from anywhere, and a thread context switch can happen (almost) anywhere, so there's no reason to think that there's need for a condition to be put to the function to be called a second time.

[Buckwheat]

@kshegunov ...

https://en.wikipedia.org/wiki/Reentrancy_(computing)

Reentrancy (computing) In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution, and then be safely called again ("re-entered") before its previous invocations complete execution.

That is the definition I grew up in computing when concurrency was a nifty trick and threads were not part of the OS. So, both are correct. I had already mentioned the exception of globals. So, no need to be angry and snotty.

[kshegunov]

@Buckwheat

So, no need to be angry and snotty.

Not intended as such, sorry if it was perceived this way. I meant the following (real code):

int testFunction()
{
    int x = 0;
    return x + 5;
}

leading to:

            11 [1]	{
0x5555555568f0                   55                    push   %rbp
0x5555555568f1  <+0x0001>        48 89 e5              mov    %rsp,%rbp
        12 [1]	    int x = 0;
0x5555555568f4  <+0x0004>        c7 45 fc 00 00 00 00  movl   $0x0,-0x4(%rbp)
        13 [1]	    return x + 5;
0x5555555568fb  <+0x000b>        8b 45 fc              mov    -0x4(%rbp),%eax
0x5555555568fe  <+0x000e>        83 c0 05              add    $0x5,%eax
        14 [1]	}
0x555555556901  <+0x0011>        5d                    pop    %rbp
0x555555556902  <+0x0012>        c3                    retq

This function can be thought reentrant if a thread breaks after the movl $0x0,-0x4(%rbp) and when re-entering it can execute mov -0x4(%rbp),%eax without side effects, which is the implication that another thread can't (or will not) modify -0x4(%rbp) while the former is sleeping. And since one can't control where context switches are done, we get the implication that globals break reentrancy. So that's what I meant by It doesn't include "if invoked again".

Kind regards.

[Buckwheat]

@kshegunov ... I said globals are bad: "That being said, if there are static methods being called using static members than that can cause problems." I hate to date myself, but I was writing machine guidance and avionics before you were actually born! LOL!

[kshegunov]

@Buckwheat said in Make QNetworkAccessManager threadsafe:

I said globals are bad

Never claimed you hadn't. I was just expanding on my comment, making it clearer what I meant, or at least I hope.

[Buckwheat]

@kshegunov ... I said globals are bad: "That being said, if there are static methods being called using static members than that can cause problems."

One more thing. Your example is always re-entrant and always thread-safe no matter what circumstances because it is all stack/frame pointer based. So it proves the point I was making. Only in languages like FORTRAN where all variables are static would there be a problem. But you can also make some really cool design strategies with it.

[Buckwheat]

@kshegunov ... OK. It is hard to have tech discussions like this. A large cup of coffee and a nice Xtreme session would be ideal :D

[Phoenox]

Okay, I just protected my single instance of the QNetworkAccessManager with mutexes, as @Buckwheat suggested.
Until now, I'm not experiencing any problems.

Thanks everyone for the tips and the clarification on reentrancy/thread-safety.