Add TCP Middlewares support

pkg/tcp/chain.go

@@ -0,0 +1,86 @@
+package tcp
+
+import (
+	"fmt"
+)
+
+// Constructor A constructor for a piece of TCP middleware.
+// Some TCP middleware use this constructor out of the box,
+// so in most cases you can just pass somepackage.New.
+type Constructor func(Handler) (Handler, error)
+
+// Chain is a chain for TCP handlers.
+// Chain acts as a list of tcp.Handler constructors.
+// Chain is effectively immutable:
+// once created, it will always hold
+// the same set of constructors in the same order.
+type Chain struct {
+	constructors []Constructor
+}
+
+// NewChain creates a new TCP chain,
+// memorizing the given list of TCP middleware constructors.
+// New serves no other function,
+// constructors are only called upon a call to Then().
+func NewChain(constructors ...Constructor) Chain {
+	return Chain{constructors: constructors}
+}
+
+// Then adds an handler at the end of the chain.
+func (c Chain) Then(h Handler) (Handler, error) {
+	if h == nil {
+		return nil, fmt.Errorf("cannot add a nil handler to the chain")
+	}
+
+	for i := range c.constructors {
+		handler, err := c.constructors[len(c.constructors)-1-i](h)
+		if err != nil {
+			return nil, err
+		}
+		h = handler
+	}
+
+	return h, nil
+}
+
+// Append extends a chain, adding the specified constructors
+// as the last ones in the request flow.
+//
+// Append returns a new chain, leaving the original one untouched.
+//
+//     stdChain := tcp.NewChain(m1, m2)
+//     extChain := stdChain.Append(m3, m4)
+//     // requests in stdChain go m1 -> m2
+//     // requests in extChain go m1 -> m2 -> m3 -> m4
+func (c Chain) Append(constructors ...Constructor) Chain {
+	newCons := make([]Constructor, 0, len(c.constructors)+len(constructors))
+	newCons = append(newCons, c.constructors...)
+	newCons = append(newCons, constructors...)
+
+	return Chain{newCons}
+}
+
+// Extend extends a chain by adding the specified chain
+// as the last one in the request flow.
+//
+// Extend returns a new chain, leaving the original one untouched.
+//
+//     stdChain := tcp.NewChain(m1, m2)
+//     ext1Chain := tcp.NewChain(m3, m4)
+//     ext2Chain := stdChain.Extend(ext1Chain)
+//     // requests in stdChain go  m1 -> m2
+//     // requests in ext1Chain go m3 -> m4
+//     // requests in ext2Chain go m1 -> m2 -> m3 -> m4
+//
+// Another example:
+//  aHtmlAfterNosurf := tcp.NewChain(m2)
+// 	aHtml := tcp.NewChain(m1, func(h tcp.Handler) tcp.Handler {
+// 		csrf := nosurf.New(h)
+// 		csrf.SetFailureHandler(aHtmlAfterNosurf.ThenFunc(csrfFail))
+// 		return csrf
+// 	}).Extend(aHtmlAfterNosurf)
+//		// requests to aHtml hitting nosurfs success handler go m1 -> nosurf -> m2 -> target-handler
+//		// requests to aHtml hitting nosurfs failure handler go m1 -> nosurf -> m2 -> csrfFail
+func (c Chain) Extend(chain Chain) Chain {
+	return c.Append(chain.constructors...)
+}