logger: output to std::cerr by default

Currently writing the log messages to std::cout is unsafe
and messages can get lost if the program terminates abnormally
on segfault or abort().  Use std::cerr instead, by default,
to prevent that as std::cerr is unbuffered by default.

Signed-off-by: Benny Halevy <[email protected]>

include/seastar/util/log.hh

@@ -77,6 +77,7 @@ class logger_registry;
 class logger {
     sstring _name;
     std::atomic<log_level> _level = { log_level::info };
+    static std::ostream* _out;
     static std::atomic<bool> _stdout;
     static std::atomic<bool> _syslog;
 private:

src/util/log.cc

@@ -147,6 +147,7 @@ std::istream& operator>>(std::istream& in, log_level& level) {
     return in;
 }
 
+std::ostream* logger::_out = &std::cerr;
 std::atomic<bool> logger::_stdout = { true };
 std::atomic<bool> logger::_syslog = { false };
 
@@ -209,7 +210,7 @@ logger::really_do_log(log_level level, const char* fmt, const stringer* stringer
     if (is_stdout_enabled) {
         out << level_map[int(level)] << space_and_current_timestamp();
         print_once(out);
-        std::cout << out.str();
+        *_out << out.str();
     }
     if (is_syslog_enabled) {
         print_once(log);