etcdctl: add etcdctl snapshot pipe command

To improve the security of etcdctl. Added the ability to write snapshots to stdout without writing data to disk.

Signed-off-by: Ais8Ooz8 <47941654+Ais8Ooz8@users.noreply.github.com>

CHANGELOG/CHANGELOG-3.6.md

@@ -28,6 +28,7 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.5.0...v3.6.0).
 ### etcdctl v3
 
 - Add command to generate [shell completion](https://github.com/etcd-io/etcd/pull/13133).
+- Add command [snapshot pipe](https://github.com/etcd-io/etcd/pull/16243).
 - When print endpoint status, [show db size in use](https://github.com/etcd-io/etcd/pull/13639)
 - [Always print the raft_term in decimal](https://github.com/etcd-io/etcd/pull/13711) when displaying member list in json.
 - [Add one more field `storageVersion`](https://github.com/etcd-io/etcd/pull/13773) into the response of command `etcdctl endpoint status`.

client/v3/snapshot/v3_snapshot.go

@@ -44,7 +44,7 @@ func hasChecksum(n int64) bool {
 // context.DeadlineExceeded). Make sure to specify only one endpoint
 // in client configuration. Snapshot API must be requested to a
 // selected node, and saved snapshot is the point-in-time state of
-// the selected node. Etcd < v3.6 will return "" as version.
+// the selected node. Nota bene: etcd < v3.6 will return "" as version.
 func WriteSnapshotWithVersion(ctx context.Context, lg *zap.Logger, cfg clientv3.Config, f *os.File) (string, error) {
 	cfg.Logger = lg.Named("client")
 	if len(cfg.Endpoints) != 1 {

etcdctl/README.md

@@ -1024,7 +1024,7 @@ The backend snapshot is written to stdout.
 
 Write a snapshot to stdout:
 ```
-./etcdctl snapshot pipe
+./etcdctl snapshot pipe > snapshot.db
 ```
 
 ### SNAPSHOT SAVE \<filename\>