KSyCoca, Thread safety, and Cache invalidation

[Thiago Macieira]

On Saturday 14 November 2015 14:19:18 David Faure wrote:
> indeed.
> > > This code:
> > >     qCDebug(SYCOCA) << "checking file timestamps";
> > >     const QDateTime stamp = QDateTime::fromMSecsSinceEpoch(timestamp);
> > 
> > You're using the function that creates a LocalTime timestamp and yet:
> QDateTime::fromMSecsSinceEpoch() indeed creates a localtime timestamp,
> which means it calls qt_localtime() which calls tzset(), so it's not
> threadsafe.
> 
> #1  0x00007ffff7118d1f in qt_tzset () at tools/qdatetime.cpp:2117
> #2  0x00007ffff7119194 in qt_localtime (msecsSinceEpoch=1447506886000,
> localDate=0x7fffffffbed0, localTime=0x7fffffffbec0,
> daylightStatus=0x7fffffffbebc) at tools/qdatetime.cpp:2 333

We can easily add a mutex around this. An uncontended mutex on Linux is very, 
very fast. It might cause some false sharing and cache thrashing, but it 
shouldn't be too noticeable.

> Yeah, I'm 100% sure most code out there doesn't convert to localtime,
> hence my suggestion of QFileInfo::lastModifiedUtc(), to avoid breaking code.
> 
> But then we also need QDateTime::fromMSecsSinceEpoch(time_t, UTC) ?

Yeah... 

Another way, a little more difficult, would be to be lazy: keep the UTC date 
and wait for the user to decide which timezone to use. The problem with lazy 
is that QDateTime is shared and I removed the laziness in 5.5 due to threading 
problems. It's possible to do it if we do it right: we'd have to make sure we 
always write to the same members and in the same order. We'd need some 
atomics.

> Is there really no way to do localtime/UTC conversions in a threadsafe way?

The problem is finding out what timezone it is in the first place. We have 
QTimeZone and we could read the system file, bypassing localtime_r and tzset. 
John would know more.

> This tzset() issue is really awful, we'll never manage to make sure that
> 100% of the code that needs to be threadsafe uses UTC everywhere.

Hence the mutex.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358