plugin/file: make non-existent file non-fatal (#2955)

* plugin/file: make non-existent file non-fatal

If the zone file being loaded doesn't exist *and* reload is enabled,
just wait the file to pop up in the normal Reload routine.

If reload is set to 0s; we keep this a fatal error on startup. Aslo fix
the ticker in z.Reload(): remove the per second ticks and just use the
reload interval for the ticker.

Brush up the documentation a bit as well.

Fixes: #2951

Signed-off-by: Miek Gieben <miek@miek.nl>

* Stickler and test compile

Signed-off-by: Miek Gieben <miek@miek.nl>

* Remove there too

Signed-off-by: Miek Gieben <miek@miek.nl>

* Cant README test these because zone files dont exist

Signed-off-by: Miek Gieben <miek@miek.nl>

1 files changed, 6 insertions, 3 deletions

diff --git a/plugin/file/file.go b/plugin/file/file.go
index 2fe4b1644..bc582cfaa 100644
--- a/plugin/file/file.go
+++ b/plugin/file/file.go
@@ -129,12 +129,15 @@ func Parse(f io.Reader, origin, fileName string, serial int64) (*Zone, error) {
 			return nil, err
 		}
 
-		if !seenSOA && serial >= 0 {
+		if !seenSOA {
 			if s, ok := rr.(*dns.SOA); ok {
-				if s.Serial == uint32(serial) { // same serial
+				seenSOA = true
+
+				// -1 is valid serial is we failed to load the file on startup.
+
+				if serial >= 0 && s.Serial == uint32(serial) { // same serial
 					return nil, &serialErr{err: "no change in SOA serial", origin: origin, zone: fileName, serial: serial}
 				}
-				seenSOA = true
 			}
 		}