Rotation counter reaches only 0 when rotating in the same directory

[livingsilver94]

I want to rotate log files at startup, not when the application closes.
The idea is to have runtime.log as the current file, and runtime%N.log as previous log files, in the same directory, e.g. ./log. When the application starts, runtime.log is renamed into runtime%N.log and a new runtime.log is created. Obviously, %N is replaced with whatever value the counter holds.

I couldn't figure out a way to do that with pure Boost.Log API, so I added a manual rotate_file() call if the previous log file (created in a previous execution) exists.
Now, I said the current and old log files must reside in the same directory. Using the code below as a reproducible example, you can see that the maximum value the counter reaches is... zero. If you instead replace keywords::target = directory with, say, keywords::target = ".", the counter is able to go beyond zero, but of course the old log files are placed in a different directory which is not what I want to do. I'm not sure whether this is a bug or a misuse of Boost.Log.

You can compile the code below by running g++ -lboost_filesystem -lboost_log -lboost_date_time -lboost_thread -DBOOST_LOG_DYN_LINK=1 test.cpp

#include <boost/log/attributes.hpp>
#include <boost/log/common.hpp>
#include <boost/log/core.hpp>
#include <boost/log/exceptions.hpp>
#include <boost/log/utility/setup/file.hpp>
#include <boost/log/expressions.hpp>
#include <boost/log/sinks.hpp>
#include <boost/log/support/date_time.hpp>
#include <boost/log/sources/severity_feature.hpp>
#include <boost/log/sources/severity_logger.hpp>
#include <filesystem>

using namespace boost::log;

enum class Severity {
    Verbose = 1,
    Info = 2,
    Error = 255,
};

std::ostream& operator<<(std::ostream& s, Severity sev) {
    s << (int)sev;
    return s;
}

BOOST_LOG_ATTRIBUTE_KEYWORD(timestamp, "Timestamp", attributes::local_clock)
BOOST_LOG_ATTRIBUTE_KEYWORD(severity, "Severity", Severity)
BOOST_LOG_ATTRIBUTE_KEYWORD(context, "Context", std::string_view)

BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT(logger, sources::severity_logger_mt<Severity>)

auto initFile(Severity minSev, const std::filesystem::path& directory) -> void
{
    namespace exp = expressions;

    // Register the sink backend that writes on files.
    auto sink = add_file_log(
        keywords::auto_flush = true,
        keywords::file_name = directory / "runtime.log",
        keywords::target_file_name = directory / "runtime%N.log",
        keywords::enable_final_rotation = false,
        keywords::open_mode = std::ios_base::app,
        keywords::target = directory,
        keywords::format =
            (exp::stream << "[" << exp::format_date_time<boost::posix_time::ptime>("Timestamp", "%Y-%m-%dT%H:%M:%S")
                         << "] " << severity << ": "

                         << exp::if_(exp::has_attr(context))[exp::stream << "<" << context << ">: "] << exp::smessage));

    auto core = core::get();
    core->add_global_attribute("Timestamp", attributes::local_clock());
    core->set_filter(severity <= minSev);

    sink->locked_backend()->scan_for_files();
    if (std::filesystem::exists(directory / "runtime.log")) {
        BOOST_LOG_SEV(logger::get(), Severity::Error) << L"ROTATING AT STARTUP";
        sink->locked_backend()->rotate_file();
    }
}


int main(int argc, char** argv) {
   static const std::filesystem::path directory = "log";
   initFile(Severity::Error, directory);
   BOOST_LOG_SEV(logger::get(), Severity::Error) << "TERMINATING";
}

This was performed on Fedora using GCC with Boost 1.83.0.

[Lastique]

I'm not sure what exactly you're doing, but if you want to take previously written log files into account (including to start counting the new log files from the last written one), you need to call scan_for_files on the sink backend. Try that, and if it doesn't help, please provide a short reproducer that shows the problem.

[livingsilver94]

I'm sorry, I filed this issue in a moment of despair. I've updated the first message, including your suggestion that doesn't seem to help unfortunately.

[Lastique]

Thanks for the report and the reproducer. This should be fixed now in develop and hopefully in Boost 1.88.0.

A few comments:

• You actually don't need to explicitly call scan_for_files as it is called implicitly by add_file_log if it creates a file collector (i.e. keywords::target parameter is specified). I initially forgot about this, sorry.
• In the older Boost releases, before the fix, the issue can be mitigated by writing log files to a location outside the target storage specified in keywords::target. That is, both keywords::file_name and keywords::target_file_name should specify a location outside that in keywords::target.