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.

6 files changed, 20 insertions, 14 deletions

diff --git a/modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go b/modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go
index 5317048..a9b02a9 100644
--- a/modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go
+++ b/modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go
@@ -149,8 +149,7 @@ func parsePHPFastCGI(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error
 		}),
 	}
 	rewriteHandler := rewrite.Rewrite{
-		URI:      "{http.matchers.file.relative}{http.request.uri.query_string}",
-		Rehandle: true,
+		URI: "{http.matchers.file.relative}{http.request.uri.query_string}",
 	}
 	rewriteRoute := caddyhttp.Route{
 		MatcherSetsRaw: []caddy.ModuleMap{rewriteMatcherSet},
diff --git a/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go b/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go
index 9a424b0..aa0d1cd 100644
--- a/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go
+++ b/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go
@@ -102,11 +102,9 @@ func (t Transport) RoundTrip(r *http.Request) (*http.Response, error) {
 		defer cancel()
 	}
 
-	// extract dial information from request (this
-	// should embedded by the reverse proxy)
+	// extract dial information from request (should have been embedded by the reverse proxy)
 	network, address := "tcp", r.URL.Host
-	if dialInfoVal := ctx.Value(reverseproxy.DialInfoCtxKey); dialInfoVal != nil {
-		dialInfo := dialInfoVal.(reverseproxy.DialInfo)
+	if dialInfo, ok := reverseproxy.GetDialInfo(ctx); ok {
 		network = dialInfo.Network
 		address = dialInfo.Address
 	}
diff --git a/modules/caddyhttp/reverseproxy/healthchecks.go b/modules/caddyhttp/reverseproxy/healthchecks.go
index 76ee945..a3b57e1 100644
--- a/modules/caddyhttp/reverseproxy/healthchecks.go
+++ b/modules/caddyhttp/reverseproxy/healthchecks.go
@@ -212,7 +212,9 @@ func (h *Handler) doActiveHealthCheck(dialInfo DialInfo, hostAddr string, host H
 	// attach dialing information to this request
 	ctx := context.Background()
 	ctx = context.WithValue(ctx, caddy.ReplacerCtxKey, caddy.NewReplacer())
-	ctx = context.WithValue(ctx, DialInfoCtxKey, dialInfo)
+	ctx = context.WithValue(ctx, caddyhttp.VarsCtxKey, map[string]interface{}{
+		dialInfoVarKey: dialInfo,
+	})
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
 	if err != nil {
 		return fmt.Errorf("making request: %v", err)
diff --git a/modules/caddyhttp/reverseproxy/hosts.go b/modules/caddyhttp/reverseproxy/hosts.go
index 6e25c66..54de5a8 100644
--- a/modules/caddyhttp/reverseproxy/hosts.go
+++ b/modules/caddyhttp/reverseproxy/hosts.go
@@ -15,11 +15,13 @@
 package reverseproxy
 
 import (
+	"context"
 	"fmt"
 	"strconv"
 	"sync/atomic"
 
 	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
 )
 
 // Host represents a remote host which can be proxied to.
@@ -223,12 +225,19 @@ func fillDialInfo(upstream *Upstream, repl *caddy.Replacer) (DialInfo, error) {
 	}, nil
 }
 
-// DialInfoCtxKey is used to store a DialInfo
-// in a context.Context.
-const DialInfoCtxKey = caddy.CtxKey("dial_info")
+// GetDialInfo gets the upstream dialing info out of the context,
+// and returns true if there was a valid value; false otherwise.
+func GetDialInfo(ctx context.Context) (DialInfo, bool) {
+	dialInfo, ok := caddyhttp.GetVar(ctx, dialInfoVarKey).(DialInfo)
+	return dialInfo, ok
+}
 
 // hosts is the global repository for hosts that are
 // currently in use by active configuration(s). This
 // allows the state of remote hosts to be preserved
 // through config reloads.
 var hosts = caddy.NewUsagePool()
+
+// dialInfoVarKey is the key used for the variable that holds
+// the dial info for the upstream connection.
+const dialInfoVarKey = "reverse_proxy.dial_info"
diff --git a/modules/caddyhttp/reverseproxy/httptransport.go b/modules/caddyhttp/reverseproxy/httptransport.go
index 75f2786..044bb83 100644
--- a/modules/caddyhttp/reverseproxy/httptransport.go
+++ b/modules/caddyhttp/reverseproxy/httptransport.go
@@ -92,8 +92,7 @@ func (h *HTTPTransport) newTransport() (*http.Transport, error) {
 	rt := &http.Transport{
 		DialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
 			// the proper dialing information should be embedded into the request's context
-			if dialInfoVal := ctx.Value(DialInfoCtxKey); dialInfoVal != nil {
-				dialInfo := dialInfoVal.(DialInfo)
+			if dialInfo, ok := GetDialInfo(ctx); ok {
 				network = dialInfo.Network
 				address = dialInfo.Address
 			}
diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index d0c4833..acb5213 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -320,8 +320,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyht
 		// attach to the request information about how to dial the upstream;
 		// this is necessary because the information cannot be sufficiently
 		// or satisfactorily represented in a URL
-		ctx := context.WithValue(r.Context(), DialInfoCtxKey, dialInfo)
-		r = r.WithContext(ctx)
+		caddyhttp.SetVar(r.Context(), dialInfoVarKey, dialInfo)
 
 		// set placeholders with information about this upstream
 		repl.Set("http.reverse_proxy.upstream.address", dialInfo.String())