log: do not repeat errors to stderr

If we get an error writing to the log, log it only once to stderr.
This avoids generating, say, 72 GB of ENOSPC errors in
teuthology.log when /var/log fills up.

Fixes: #14616
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit d9ac047)

Conflicts:
	src/log/Log.cc (drop m_uid and m_gid which are not used in hammer;
	order of do_stderr, do_syslog, do_fd conditional blocks is reversed in
	hammer; drop irrelevant speed optimization code from
	5bfe05a)

src/log/Log.cc

@@ -41,6 +41,7 @@ Log::Log(SubsystemMap *s)
     m_flush_mutex_holder(0),
     m_new(), m_recent(),
     m_fd(-1),
+    m_fd_last_error(0),
     m_syslog_log(-2), m_syslog_crash(-2),
     m_stderr_log(1), m_stderr_crash(-1),
     m_stop(false),
@@ -236,8 +237,13 @@ void Log::_flush(EntryQueue *t, EntryQueue *requeue, bool crash)
 	  r = safe_write(m_fd, s.data(), s.size());
 	if (r >= 0)
 	  r = write(m_fd, "\n", 1);
-	if (r < 0)
-	  cerr << "problem writing to " << m_log_file << ": " << cpp_strerror(r) << std::endl;
+	if (r != m_fd_last_error) {
+	  if (r < 0)
+	    cerr << "problem writing to " << m_log_file
+		 << ": " << cpp_strerror(r)
+		 << std::endl;
+	  m_fd_last_error = r;
+	}
       }
 
       if (do_syslog) {

src/log/Log.h

@@ -35,6 +35,8 @@ class Log : private Thread
   std::string m_log_file;
   int m_fd;
 
+  int m_fd_last_error;  ///< last error we say writing to fd (if any)
+
   int m_syslog_log, m_syslog_crash;
   int m_stderr_log, m_stderr_crash;