d/libn/i/nftables: add fluent chain builder API

With most of the dynamism of nftables rulesets being powered by named
maps and sets, the rules of a chain are often initialized when the chain
is added, and never touched again. Add a fluent API for adding the
creation of a chain and all its rules to a modifier without having to
repeat the chain name for each rule.

Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/libnetwork/internal/nftables/fluentapi_linux.go

@@ -0,0 +1,30 @@
+package nftables
+
+type chainBuilder struct {
+	chain string
+	tm    *Modifier
+}
+
+func (b BaseChain) Builder() chainBuilder {
+	tm := &Modifier{}
+	tm.create(b, 1)
+	return chainBuilder{chain: b.Name, tm: tm}
+}
+
+func (c Chain) Builder() chainBuilder {
+	tm := &Modifier{}
+	tm.create(c, 1)
+	return chainBuilder{chain: c.Name, tm: tm}
+}
+
+func (b chainBuilder) Rule(rule ...string) chainBuilder {
+	b.tm.create(Rule{
+		Chain: b.chain,
+		Rule:  rule,
+	}, 1)
+	return b
+}
+
+func (b chainBuilder) Create(tm *Modifier) {
+	tm.cmds = append(tm.cmds, b.tm.cmds...)
+}

daemon/libnetwork/internal/nftables/nftables_linux.go

@@ -490,7 +490,11 @@ type Modifier struct {
 
 // Create enqueues creation of object o, to be applied by tm.Apply.
 func (tm *Modifier) Create(o Obj) {
-	_, f, l, _ := runtime.Caller(1)
+	tm.create(o, 1)
+}
+
+func (tm *Modifier) create(o Obj, skipFrames int) {
+	_, f, l, _ := runtime.Caller(skipFrames + 1)
 	tm.cmds = append(tm.cmds, command{
 		obj:        o,
 		callerFile: f,