Cherry-pick: Implement notifies for transfer plugin (#3972) (#4142)

* Implement notifies for transfer plugin (#3972)

* Fix notifies in transfer plugin

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

* Make it compile

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

* Port more plugins

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

* golint

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

* Fix tests

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

* Fix notifies in transfer plugin

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

* Make it compile

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

* Port more plugins

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

* golint

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

* Fix tests

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

* Fix tests

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

* really fix test

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

* Implement ixfr fallback and unify file and auto for transfering

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

* Add transfer tests

copied and modified from #3452

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

* Test correct selection of plugin

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

* add upstream back in

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

* Implement ixfr fallback and unify file and auto for transfering

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

* fix test

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

* properly merge

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

* Remove plugin/kubernetes/setup_transfer_test.go

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Co-authored-by: Miek Gieben <miek@miek.nl>

plugin/transfer/README.md

@@ -2,34 +2,38 @@
 
 ## Name
 
-*transfer* - perform zone transfers for other plugins.
+*transfer* - perform (outgoing) zone transfers for other plugins.
 
 ## Description
 
-This plugin answers zone transfers for authoritative plugins that implement
-`transfer.Transferer`.  Currently, no internal plugins implement this interface.
+This plugin answers zone transfers for authoritative plugins that implement `transfer.Transferer`.
 
-Transfer answers full zone transfer (AXFR) requests and incremental zone transfer (IXFR) requests
+*transfer* answers full zone transfer (AXFR) requests and incremental zone transfer (IXFR) requests
 with AXFR fallback if the zone has changed.
 
-Notifies are not currently supported.
+When a plugin wants to notify it's secondaries it will call back into the *transfer* plugin.
+
+The following plugins implement zone transfers using this plugin: *file*, *auto*, *secondary*, and
+*kubernetes*. See `transfer.go` for implementation details if you are a plugin author that wants to
+use this plugin.
 
 ## Syntax
 
 ~~~
 transfer [ZONE...] {
-  to HOST...
+  to ADDRESS...
 }
 ~~~
 
-* **ZONES** The zones *transfer* will answer zone requests for. If left blank,
-  the zones are inherited from the enclosing server block. To answer zone
-  transfers for a given zone, there must be another plugin in the same server
-  block that serves the same zone, and implements `transfer.Transferer`.
+ *  **ZONE** The zones *transfer* will answer zone transfer requests for. If left blank, the zones
+    are inherited from the enclosing server block. To answer zone transfers for a given zone,
+    there must be another plugin in the same server block that serves the same zone, and implements
+    `transfer.Transferer`.
 
-* `to ` **HOST...** The hosts *transfer* will transfer to. Use `*` to permit
-  transfers to all hosts.
+ *  `to` **ADDRESS...** The hosts *transfer* will transfer to. Use `*` to permit transfers to all
+    addresses. **ADDRESS** must be denoted in CIDR notation (e.g., 127.0.0.1/32) or just as plain
+    addresses. `to` may be specified multiple times.
 
 ## Examples
 
-TODO
+See the specific plugins using this plugin for examples on it's usage.