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..84d85a05e36414b5a6c5ef559cb5ac10be3179ff
--- /dev/null
+++ b/server/router/handler.go
@@ -0,0 +1,64 @@
+// 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
+
+import "golang.org/x/net/context"
+
+// TODO(nishanths): Base and TestingBase need better names now that they are in
+// a package named router. Previously, it was fine because they were in the
+// middleware package.
+
+type (
+	// Handler is the type for all request handlers.
+	Handler func(*Context)
+
+	// Middleware is a function that returns a Handler.
+	Middleware func() Handler

[nodir, 2016/06/13 20:20:52]

I think this type is unnecessary

+
+	// Base returns a list of handlers. It usually is
+	// the start of the middleware chain.
+	// TODO(nishanths): It may not be necessary to have a specific type for

[nodir, 2016/06/13 20:20:52]

"//\n" before TODO

+	// this after issue 617774.
+	Base func() []Handler

[nodir, 2016/06/13 20:20:52]

why Base is a function that returns []Handler rather than []Handler? This
function does not accept arguments, so either it returns same handlers all the
time, or it depends on global state (which is bad)

+)
+
+// TestingBase is Base that passes given context to the handler. Useful in
+// tests.
+func TestingBase(c context.Context) Base {
+	return func() (h []Handler) {
+		return []Handler{func(ctx *Context) {
+			ctx.Context = c
+		}}
+	}
+}
+
+// HandlerWithContext creates a handler that assigns the given context to
+// the Context field.
+func HandlerWithContext(c context.Context) Handler {
+	return func(ctx *Context) {
+		ctx.Context = c
+	}
+}
+
+// ChainHandlers combines handlers into a single executable function.
+// Useful to create a single executable function for tests.
+// The Writer, Request, and Params fields in the Context available to the
+// handlers are not set and should be set before use.
+func ChainHandlers(handlers ...Handler) func() {
+	return func() {
+		if len(handlers) > 0 {
+			c := &Context{handlers: handlers, index: -1}
+			c.Next()
+		}
+	}
+}
+
+// HandlerWithContextValue is a middleware that adds a value to the context before
+// calling the handler.
+func HandlerWithContextValue(key, val interface{}) Handler {
+	return func(c *Context) {
+		c.Context = context.WithValue(c.Context, key, val)
+	}
+}