ktest.pl: Add MAIL_MAX_SIZE to limit the amount of log emailed

Add the ktest config option MAIL_MAX_SIZE that will limit the size of the
log file that is placed into the email on failure.

Link: https://lore.kernel.org/r/20200701231756.790637968@goodmis.org

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tools/testing/ktest/ktest.pl

@@ -227,6 +227,7 @@
 my $mailto;
 my $mailer;
 my $mail_path;
+my $mail_max_size;
 my $mail_command;
 my $email_on_error;
 my $email_when_finished;
@@ -263,6 +264,7 @@
     "MAILTO"			=> \$mailto,
     "MAILER"			=> \$mailer,
     "MAIL_PATH"			=> \$mail_path,
+    "MAIL_MAX_SIZE"		=> \$mail_max_size,
     "MAIL_COMMAND"		=> \$mail_command,
     "EMAIL_ON_ERROR"		=> \$email_on_error,
     "EMAIL_WHEN_FINISHED"	=> \$email_when_finished,
@@ -1497,10 +1499,18 @@ sub dodie {
 	my $log_file;
 
 	if (defined($opt{"LOG_FILE"})) {
+	    my $size = 0;
+	    if (defined($mail_max_size)) {
+		my $log_size = tell LOG;
+		$log_size -= $test_log_start;
+		if ($log_size > $mail_max_size) {
+		    $size = $log_size - $mail_max_size;
+		}
+	    }
 	    $log_file = "$tmpdir/log";
 	    open (L, "$opt{LOG_FILE}") or die "Can't open $opt{LOG_FILE} to read)";
 	    open (O, "> $tmpdir/log") or die "Can't open $tmpdir/log\n";
-	    seek(L, $test_log_start, 0);
+	    seek(L, $test_log_start + $size, 0);
 	    while (<L>) {
 		print O;
 	    }

tools/testing/ktest/sample.conf

@@ -442,6 +442,19 @@
 # Users can cancel the test by Ctrl^C
 # (default 0)
 #EMAIL_WHEN_CANCELED = 1
+#
+# If a test ends with an error and EMAIL_ON_ERROR is set as well
+# as a LOG_FILE is defined, then the log of the failing test will
+# be included in the email that is sent.
+# It is possible that the log may be very large, in which case,
+# only the last amount of the log should be sent. To limit how
+# much of the log is sent, set MAIL_MAX_SIZE. This will be the
+# size in bytes of the last portion of the log of the failed
+# test file. That is, if this is set to 100000, then only the
+# last 100 thousand bytes of the log file will be included in
+# the email.
+# (default undef)
+#MAIL_MAX_SIZE = 1000000
 
 # Start a test setup. If you leave this off, all options
 # will be default and the test will run once.