gui: exec.tcl: cache local IP address across calls to getMyIP

Modify getMyIP to cache the local machine's IP address, and
return it without further accessing of the underlying resolver
libraries during subsequent invocations.

getMyIP is called roughly once a second from within monitor_loop
when refreshing CPU utilization in the bottom-right corner of
the GUI window. Other, dedicated CPU usage windows might also
call getMyIP at regular intervals.

With systemd commit cda458a54 (between v232 and v233), the
implementation of gethostbyname4_r() was updated to a more
intransigently standard-compliant error reporting convention,
which in turn causes glibc to retry in a more labor intensive
way (see https://github.com/systemd/systemd/pull/5359).

Under certain circumstances depending on the local hostname
and IP configuration, the glibc/systemd back-end resolver
routines triggered by getMyIP's call to [socket ...] and
[fconfigure ...] end up taking long enough to noticeably slow
down refreshing the main core-gui window, to the point where
interaction with the GUI becomes difficult.

Signed-off-by: Gabriel Somlo <glsomlo@cert.org>

gui/exec.tcl

@@ -782,14 +782,17 @@ proc manageCPUwindow {xpos ypos start} {
 }
 
 proc getMyIP { } {
-    if { [catch {set theServer [socket -server none -myaddr \
-                                [info hostname] 0]} ] } {
-        return "127.0.0.1"
-    }
+    variable myIP
+    if { ![info exists myIP] } {
+	if { [catch {set theServer [socket -server none \
+					   -myaddr [info hostname] 0]} ] } {
+	    set myIP "127.0.0.1"
+	} else {
 	    set myIP [lindex [fconfigure $theServer -sockname] 0]
 	    close $theServer
+	}
+    }
     return $myIP
-	
 }
 
 # display all values stored in cpu usage history for each server