Merge pull request #1645 from matthewrmshin/fix-ssh-tail-follow-becom…

…es-zombie

cylc.gui.tailer: fix SSH zombie tail

lib/cylc/gui/tailer.py

@@ -38,12 +38,24 @@ class Tailer(threading.Thread):
     logview -- A GUI view to display the content of the log file.
     filename -- The name of the log file.
     cmd_tmpl -- The command template use to follow the log file.
-                (default=Tailer.CMD_TMPL)
+                (default=Tailer.L_CMD_TMPL for local files, Tailer.R_CMD_TMPL
+                for remote files)
     pollable -- If specified, it must implement a pollable.poll() method,
                 which is called at regular intervals.
     """
 
-    CMD_TMPL = "tail -n +1 -F %(filename)s"
+    # Template for tail commands on local files.
+    L_CMD_TMPL = "tail -n +1 -F %(filename)s"
+    # Template for tail commands on remote files.
+    # On signal to "ssh" client, a signal is sent to "sshd" on server.
+    # However, "sshd" cannot send a signal to the "tail" command, because it is
+    # not a terminal. Apparently, we can use "ssh -t" or "ssh -tt", but that
+    # just causes the command to hang here for some reason. The easiest
+    # solution is to use the "--pid=PID" option of the "tail" command, so it
+    # dies as soon as PID dies. Note: if remote login shell is bash/ksh, we can
+    # use $PPID instead of `ps ...` command, but we do have to support users
+    # whose login shell is "tcsh".
+    R_CMD_TMPL = "tail --pid=`ps h -o ppid $$` -n +1 -F %(filename)s"
     READ_SIZE = 4096
     TAGS = {
         "CRITICAL": [re.compile(r"\b(?:CRITICAL|ERROR)\b"), "red"],
@@ -83,10 +95,11 @@ def run(self):
             ssh = str(GLOBAL_CFG.get_host_item(
                 "remote shell template", host, owner)).replace(" %s", "")
             command = shlex.split(ssh) + ["-n", user_at_host]
+            cmd_tmpl = self.R_CMD_TMPL
         else:
             filename = self.filename
+            cmd_tmpl = self.L_CMD_TMPL
 
-        cmd_tmpl = self.CMD_TMPL
         if self.cmd_tmpl:
             cmd_tmpl = self.cmd_tmpl
         command += shlex.split(cmd_tmpl % {"filename": filename})