Add netstat sample application

[dtrugman]

An alternative suggestion to #38 and #39.

[rm5248]

Note: this currently fails to build for me with g++ 12 / clang++ 14(using Debian 12). It's not immediately obvious to me why it fails on these types and it doesn't for the other types. Here's the clang error(g++ errors on the same lines):

[ 48%] Building CXX object CMakeFiles/sample.dir/sample/enum_block.cpp.o
In file included from /home/robert/pfs/sample/enum_block.cpp:18:
/home/robert/pfs/sample/log.hpp:71:5: error: call to function 'operator<<' that is neither visible in the template definition nor found by argument-dependent lookup
    LOG(value);
    ^
/home/robert/pfs/sample/log.hpp:25:28: note: expanded from macro 'LOG'
#define LOG(...) std::cout << __VA_ARGS__ << ENDL
                           ^
/home/robert/pfs/sample/enum_block.cpp:38:9: note: in instantiation of function template specialization '__print<pfs::block_stat>' requested here
        print(stat);
        ^
/home/robert/pfs/sample/log.hpp:84:18: note: expanded from macro 'print'
#define print(x) __print(#x, x)
                 ^
/home/robert/pfs/sample/format.hpp:731:22: note: 'operator<<' should be declared prior to the call site or in namespace 'pfs'
inline std::ostream& operator<<(std::ostream& out,
                     ^
In file included from /home/robert/pfs/sample/enum_block.cpp:18:
/home/robert/pfs/sample/log.hpp:71:5: error: call to function 'operator<<' that is neither visible in the template definition nor found by argument-dependent lookup
    LOG(value);
    ^
/home/robert/pfs/sample/log.hpp:25:28: note: expanded from macro 'LOG'
#define LOG(...) std::cout << __VA_ARGS__ << ENDL
                           ^
/home/robert/pfs/sample/enum_block.cpp:41:9: note: in instantiation of function template specialization '__print<pfs::block_queue>' requested here
        print(queue);
        ^
/home/robert/pfs/sample/log.hpp:84:18: note: expanded from macro 'print'
#define print(x) __print(#x, x)
                 ^
/home/robert/pfs/sample/format.hpp:753:22: note: 'operator<<' should be declared prior to the call site or in namespace 'pfs'
inline std::ostream& operator<<(std::ostream& out,
                     ^
2 errors generated.

One possibly confusing thing: it seems that the filter returns true if the socket should be discarded. To me, true should be that we want to keep this, and false means we don't. Perhaps turn this into an enum?

enum class FilterResponse{
  Accept,
  Reject
};

[dtrugman]

Thanks for the useful comments @rm5248.
The compilation was indeed broken with newer compilers. Reordering two includes fixed that.
I also agree with your comment. I was sure that I used "true" as filter in, but it turns out I didn't. Used an enum to make it explicit.

[rm5248]

The return value here is backwards, it should be:

Suggested change

	return inodes.find(sock.inode) == inodes.end()
	? pfs::filter::action::keep
	: pfs::filter::action::drop;
	return inodes.find(sock.inode) == inodes.end()
	? pfs::filter::action::drop
	: pfs::filter::action::keep;

[dtrugman]

You are totally right. Thanks for noticing. Fixed.

[rm5248]

This looks pretty good to me; I think the ability to add in the custom filters is a good trade-off to make it easy to filter things from only one process while still making it be "just parse /proc" and simple in that regard.

[dtrugman]

Thank you! I'm happy you find this resolution suitable!
Will push the code, bump the version, and release it.