http: Change routes to sequential matcher evaluation (#2967)

Previously, all matchers in a route would be evaluated before any
handlers were executed, and a composite route of the matching routes
would be created. This made rewrites especially tricky, since the only
way to defer later matchers' evaluation was to wrap them in a subroute,
or to invoke a "rehandle" which often caused bugs.

Instead, this new sequential design evaluates each route's matchers then
its handlers in lock-step; matcher-handlers-matcher-handlers...

If the first matching route consists of a rewrite, then the second route
will be evaluated against the rewritten request, rather than the original
one, and so on.

This should do away with any need for rehandling.

I've also taken this opportunity to avoid adding new values to the
request context in the handler chain, as this creates a copy of the
Request struct, which may possibly lead to bugs like it has in the past
(see PR #1542, PR #1481, and maybe issue #2463). We now add all the
expected context values in the top-level handler at the server, then
any new values can be added to the variable table via the VarsCtxKey
context key, or just the GetVar/SetVar functions. In particular, we are
using this facility to convey dial information in the reverse proxy.

Had to be careful in one place as the middleware compilation logic has
changed, and moved a bit. We no longer compile a middleware chain per-
request; instead, we can compile it at provision-time, and defer only the
evaluation of matchers to request-time, which should slightly improve
performance. Doing this, however, we take advantage of multiple function
closures, and we also changed the use of HandlerFunc (function pointer)
to Handler (interface)... this led to a situation where, if we aren't
careful, allows one request routed a certain way to permanently change
the "next" handler for all/most other requests! We avoid this by making
a copy of the interface value (which is a lightweight pointer copy) and
using exclusively that within our wrapped handlers. This way, the
original stack frame is preserved in a "read-only" fashion. The comments
in the code describe this phenomenon.

This may very well be a breaking change for some configurations, however
I do not expect it to impact many people. I will make it clear in the
release notes that this change has occurred.

2 files changed, 2 insertions, 22 deletions

diff --git a/modules/caddyhttp/rewrite/caddyfile.go b/modules/caddyhttp/rewrite/caddyfile.go
index 6674313..532df9a 100644
--- a/modules/caddyhttp/rewrite/caddyfile.go
+++ b/modules/caddyhttp/rewrite/caddyfile.go
@@ -44,7 +44,6 @@ func parseCaddyfileRewrite(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler,
 			return nil, h.ArgErr()
 		}
 	}
-	rewr.Rehandle = true
 	return rewr, nil
 }
 
@@ -52,7 +51,7 @@ func parseCaddyfileRewrite(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler,
 //
 //     strip_prefix [<matcher>] <prefix>
 //
-// The request path will be stripped its prefix if it matches <prefix>.
+// The request path will be stripped the given prefix.
 func parseCaddyfileStripPrefix(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
 	var rewr Rewrite
 	for h.Next() {
@@ -71,7 +70,7 @@ func parseCaddyfileStripPrefix(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHand
 //
 //     strip_suffix [<matcher>] <suffix>
 //
-// The request path will be stripped its suffix if it matches <suffix>.
+// The request path will be stripped the given suffix.
 func parseCaddyfileStripSuffix(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
 	var rewr Rewrite
 	for h.Next() {
diff --git a/modules/caddyhttp/rewrite/rewrite.go b/modules/caddyhttp/rewrite/rewrite.go
index b875adb..09478b8 100644
--- a/modules/caddyhttp/rewrite/rewrite.go
+++ b/modules/caddyhttp/rewrite/rewrite.go
@@ -15,7 +15,6 @@
 package rewrite
 
 import (
-	"fmt"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -32,9 +31,6 @@ func init() {
 
 // Rewrite is a middleware which can rewrite HTTP requests.
 //
-// The Rehandle and HTTPRedirect properties are mutually exclusive
-// (you cannot both rehandle and issue a redirect).
-//
 // These rewrite properties are applied to a request in this order:
 // Method, URI, StripPathPrefix, StripPathSuffix, URISubstring.
 //
@@ -61,10 +57,6 @@ type Rewrite struct {
 	// given status code.
 	HTTPRedirect caddyhttp.WeakString `json:"http_redirect,omitempty"`
 
-	// If true, the request will sent for rehandling after rewriting
-	// only if anything about the request was changed.
-	Rehandle bool `json:"rehandle,omitempty"`
-
 	logger *zap.Logger
 }
 
@@ -82,14 +74,6 @@ func (rewr *Rewrite) Provision(ctx caddy.Context) error {
 	return nil
 }
 
-// Validate ensures rewr's configuration is valid.
-func (rewr Rewrite) Validate() error {
-	if rewr.HTTPRedirect != "" && rewr.Rehandle {
-		return fmt.Errorf("cannot be configured to both redirect externally and rehandle internally")
-	}
-	return nil
-}
-
 func (rewr Rewrite) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
 	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
 
@@ -104,9 +88,6 @@ func (rewr Rewrite) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddy
 			zap.String("method", r.Method),
 			zap.String("uri", r.RequestURI),
 		)
-		if rewr.Rehandle {
-			return caddyhttp.ErrRehandle
-		}
 		if rewr.HTTPRedirect != "" {
 			statusCode, err := strconv.Atoi(repl.ReplaceAll(rewr.HTTPRedirect.String(), ""))
 			if err != nil {