Plugin/RELOAD - Tune usage of var global, add limit to options (#1457)

* tune usage of var global, add limit to options

* update readme for minimal values

* useless change to quick-off codecov

* fix msgs for min values and tune the flag for end of reload usage, with a 'maybe' option

* adding UT for min values, adding MD5 of corefile on the log

1 files changed, 21 insertions, 5 deletions

diff --git a/plugin/reload/setup.go b/plugin/reload/setup.go
index af6fe1334..480bd5b74 100644
--- a/plugin/reload/setup.go
+++ b/plugin/reload/setup.go
@@ -1,6 +1,7 @@
 package reload
 
 import (
+	"fmt"
 	"math/rand"
 	"sync"
 	"time"
@@ -17,7 +18,11 @@ func init() {
 	})
 }
 
-var r *reload
+// the info reload is global to all application, whatever number of reloads.
+// it is used to transmit data between Setup and start of the hook called 'onInstanceStartup'
+// channel for QUIT is never changed in purpose.
+// WARNING: this data may be unsync after an invalid attempt of reload Corefile.
+var r = reload{interval: defaultInterval, usage: unused, quit: make(chan bool)}
 var once sync.Once
 
 func setup(c *caddy.Controller) error {
@@ -32,19 +37,25 @@ func setup(c *caddy.Controller) error {
 	if len(args) > 0 {
 		d, err := time.ParseDuration(args[0])
 		if err != nil {
-			return err
+			return plugin.Error("reload", err)
 		}
 		i = d
 	}
+	if i < minInterval {
+		return plugin.Error("reload", fmt.Errorf("interval value must be greater or equal to %v", minInterval))
+	}
 
 	j := defaultJitter
 	if len(args) > 1 {
 		d, err := time.ParseDuration(args[1])
 		if err != nil {
-			return err
+			return plugin.Error("reload", err)
 		}
 		j = d
 	}
+	if j < minJitter {
+		return plugin.Error("reload", fmt.Errorf("jitter value must be greater or equal to %v", minJitter))
+	}
 
 	if j > i/2 {
 		j = i / 2
@@ -53,20 +64,25 @@ func setup(c *caddy.Controller) error {
 	jitter := time.Duration(rand.Int63n(j.Nanoseconds()) - (j.Nanoseconds() / 2))
 	i = i + jitter
 
-	r = &reload{interval: i, quit: make(chan bool)}
+	// prepare info for next onInstanceStartup event
+	r.interval = i
+	r.usage = used
+
 	once.Do(func() {
 		caddy.RegisterEventHook("reload", hook)
 	})
 
+	// re-register on finalShutDown as the instance most-likely will be changed
 	c.OnFinalShutdown(func() error {
 		r.quit <- true
 		return nil
 	})
-
 	return nil
 }
 
 const (
+	minJitter       = 1 * time.Second
+	minInterval     = 2 * time.Second
 	defaultInterval = 30 * time.Second
 	defaultJitter   = 15 * time.Second
 )