traefik/pkg/tcp/chain.go

package tcp

import (
	"errors"
)

// 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, errors.New("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...)
}