Make HTTP middleware easier to use

server/router/handler.go

Index: server/router/handler.go
diff --git a/server/router/handler.go b/server/router/handler.go
new file mode 100644
index 0000000000000000000000000000000000000000..1995436b910e4f401be501ed820886fbd1b0e041
--- /dev/null
+++ b/server/router/handler.go
@@ -0,0 +1,48 @@
+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package router
+
+type (

[Vadim Sh., 2016/06/16 22:47:14]

nit: it's the first chunk of code that uses type ( .... ) construct. I'd prefer
each type has it's own "type" keyword. That makes them standout visually more
when glancing over the file.

type Handler ...

type Middleware ...

....

[nishanths, 2016/06/16 23:33:00]

Done.

+	// Handler is the type for all request handlers.
+	Handler func(*Context)
+
+	// Middleware is a function that accepts a shared context and the next
+	// function. Since Middleware is typically part of a chain of functions
+	// that handles an HTTP request, it must obey the following rules.
+	//
+	//     - Middleware must call next if it has not written to the Context
+	//       by the end of the function.
+	//     - Middleware must not call next if it has written to the Context.
+	//     - Middleware must not write to the Context after next is called and
+	//       c.Written()==true.
+	//     - Middleware may modify the embedded context before calling next.
+	Middleware func(c *Context, next Handler)
+
+	// MiddlewareChain is a list of Middleware.
+	MiddlewareChain []Middleware
+)
+
+// Run executes the middleware chain and final handler with the given context as
+// the initial context.
+func (mc MiddlewareChain) Run(c *Context, h Handler) {
+	run(c, mc, nil, h)
+}
+
+// run executes the middleware chains m and n, and the handler h using the
+// Context c as the initial context.
+func run(c *Context, m, n MiddlewareChain, h Handler) {
+	switch {
+	case len(m) > 0:
+		m[0](c, func(ctx *Context) {
+			run(ctx, m[1:], n, h)
+		})
+	case len(n) > 0:
+		n[0](c, func(ctx *Context) {
+			run(ctx, m, n[1:], h)

[Vadim Sh., 2016/06/16 22:47:14]

nit: run(ctx, nil, n[1:], h)

(to make it clear that we are done with 'm')

[nishanths, 2016/06/16 23:33:00]

Done.

+		})
+	default:
+		h(c)
+	}
+}