[plugins] speedup big journal collection

Instead of having journalctl format all the logs,
estimate the number of lines of logs needed to fill
the log_size limit.
On a EL 9.5 server with SSD with 500MB of formated logs,
the logs runtime goes from 32s to 11s.

Signed-off-by: Etienne Champetier <e.champetier@ateme.com>

sos/report/plugins/__init__.py

@@ -3083,10 +3083,20 @@ def add_journal(self, units=None, boot=None, since=None, until=None,
         if output:
             journal_cmd += output_opt % output
 
+        fname = journal_cmd
+        if log_size > 0 and not lines:
+            # get the last 1000 lines
+            res = self.exec_cmd(f"{journal_cmd} -n1000")
+            if res['status'] == 0 and res['output'].count('\n') >= 1000:
+                # compute how many lines we need to reach log_size from
+                # the 1000 lines size, add 50% margin to avoid being short
+                lines = int(log_size*1024*1024/len(res['output'])*1000*1.5)
+                journal_cmd += lines_opt % lines
+
         self._log_debug(f"collecting journal: {journal_cmd}")
         self._add_cmd_output(cmd=journal_cmd, timeout=timeout,
                              sizelimit=log_size, pred=pred, tags=tags,
-                             priority=priority)
+                             priority=priority, suggest_filename=fname)
 
     def _expand_copy_spec(self, copyspec):
         def __expand(paths):