Fix g++6 compile error

[cryptokat]

this error would be emitted when compiling with g++6

/pebblesdb/src/db/db_test.cc: In function ‘void leveldb::print_timer_info(std::__cxx11::string, uint64_t, uint64_t)’:                                                             
/pebblesdb/src/db/db_test.cc:2237:25: error: call of overloaded ‘abs(uint64_t)’ is ambiguous
  uint64_t diff = abs(a-b);  

[vijay03]

Hi @cryptokat thanks for the PR! If abs() doesn't link properly in g++ 6, I think the right move is to find the g++6 specific version of the function, and include it with ifdefs. Removing abs causes tests to fail: we do want the absolute value there.

[cryptokat]

@vijay03 How does it make sense to use abs here when both a and b are unsigned?

[vijay03]

I might be missing something here, but even with unsigned, if b is greater than a, a - b results in a large number instead of the difference which we want.

A hacky way to do this would be:

uint64_t a, b, diff;
a = 1; b = 3;
if (a > b) { diff = a - b; } else diff = b - a;

[cryptokat]

It turns out that DBTest.MultiThreaded crashes occasionally because VersionSet::RemoveFileLevelBloomFilterInfo isn't thread-safe.

Here's a stack trace:

==== Test DBTest.MultiThreaded
[New Thread 0x7fff4cfd9700 (LWP 23220)]
[New Thread 0x7fff4c7d8700 (LWP 23221)]
[New Thread 0x7fff4bfd7700 (LWP 23222)]
... starting thread 0
[New Thread 0x7fff4b7d6700 (LWP 23223)]
... starting thread 1
[New Thread 0x7fff4afd5700 (LWP 23224)]
... starting thread 2
[New Thread 0x7fff4a7d4700 (LWP 23225)]
... starting thread 3

Thread 307 "db_test" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fff4c7d8700 (LWP 23221)]
0x00007ffff7afe106 in std::_Rb_tree_rebalance_for_erase(std::_Rb_tree_node_base*, std::_Rb_tree_node_base&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) where
#0  0x00007ffff7afe106 in std::_Rb_tree_rebalance_for_erase(std::_Rb_tree_node_base*, std::_Rb_tree_node_base&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1  0x00005555555d46e3 in std::_Rb_tree<unsigned long, std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*>, std::_Select1st<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::_M_erase_aux(std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >) ()
#2  0x00005555555d2919 in std::_Rb_tree<unsigned long, std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*>, std::_Select1st<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::erase[abi:cxx11](std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >) ()
#3  0x00005555555d0a64 in std::_Rb_tree<unsigned long, std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*>, std::_Select1st<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::_M_erase_aux(std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >) ()
#4  0x00005555555cd5e9 in std::_Rb_tree<unsigned long, std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*>, std::_Select1st<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::erase[abi:cxx11](std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::_Rb_tree_const_iterator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >) ()
#5  0x00005555555c8e3c in std::_Rb_tree<unsigned long, std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*>, std::_Select1st<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> >, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::erase(unsigned long const&) ()
#6  0x00005555555c6273 in std::map<unsigned long, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*> > >::erase(unsigned long const&) ()
#7  0x00005555555b9358 in leveldb::VersionSet::RemoveFileLevelBloomFilterInfo(unsigned long) ()
#8  0x0000555555591e7f in leveldb::DBImpl::DeleteObsoleteFiles() ()
#9  0x0000555555595600 in leveldb::DBImpl::BackgroundCompactionGuards(leveldb::FileLevelFilterBuilder*) ()
#10 0x0000555555594dc3 in leveldb::DBImpl::CompactLevelThread() ()
#11 0x000055555559e395 in leveldb::DBImpl::CompactLevelWrapper(void*) ()
#12 0x00005555555e7403 in leveldb::(anonymous namespace)::StartThreadWrapper(void*) ()
#13 0x00007ffff7326494 in start_thread (arg=0x7fff4c7d8700) at pthread_create.c:333
#14 0x00007ffff7068acf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97

[vijay03]

Thanks for catching this! Could you report this as an issue?