Fix lsof parsing after Xenial behavior change.

suite_test.go

@@ -27,6 +27,7 @@
 package mgo_test
 
 import (
+	"bytes"
 	"errors"
 	"flag"
 	"fmt"
@@ -150,14 +151,16 @@ func (s *S) Stop(host string) {
 }
 
 func (s *S) pid(host string) int {
-	output, err := exec.Command("lsof", "-iTCP:"+hostPort(host), "-sTCP:LISTEN", "-Fp").CombinedOutput()
+	// Note recent releases of lsof force 'f' to be present in the output (WTF?).
+	cmd := exec.Command("lsof", "-iTCP:"+hostPort(host), "-sTCP:LISTEN", "-Fpf")
+	output, err := cmd.CombinedOutput()
 	if err != nil {
 		panic(err)
 	}
-	pidstr := string(output[1 : len(output)-1])
+	pidstr := string(bytes.Fields(output[1:])[0])
 	pid, err := strconv.Atoi(pidstr)
 	if err != nil {
-		panic("cannot convert pid to int: " + pidstr)
+		panic(fmt.Errorf("cannot convert pid to int: %q, command line: %q", pidstr, cmd.Args))
 	}
 	return pid
 }