caddyhttp: Implement CEL matcher (see #3051) (#3155)

* caddyhttp: Implement CEL matcher (see #3051)

CEL (Common Expression Language) is a very fast, flexible way to express
complex logic, useful for matching requests when the conditions are not
easy to express with JSON.

This matcher may be considered experimental even after the 2.0 release.

* Improve CEL module docs

4 files changed, 228 insertions, 3 deletions

diff --git a/go.mod b/go.mod
index 3a898fe..c5fc336 100644
--- a/go.mod
+++ b/go.mod
@@ -10,6 +10,7 @@ require (
 	github.com/dustin/go-humanize v1.0.1-0.20200219035652-afde56e7acac
 	github.com/go-acme/lego/v3 v3.5.0
 	github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e
+	github.com/google/cel-go v0.3.2
 	github.com/ilibs/json5 v1.0.1
 	github.com/jsternberg/zap-logfmt v1.2.0
 	github.com/klauspost/compress v1.10.3
@@ -29,6 +30,7 @@ require (
 	go.uber.org/zap v1.14.1
 	golang.org/x/crypto v0.0.0-20200317142112-1b76d66859c6
 	golang.org/x/net v0.0.0-20200301022130-244492dfa37a
+	google.golang.org/genproto v0.0.0-20200305110556-506484158171
 	gopkg.in/natefinch/lumberjack.v2 v2.0.0
 	gopkg.in/square/go-jose.v2 v2.4.1 // indirect
 	gopkg.in/yaml.v2 v2.2.8
diff --git a/go.sum b/go.sum
index f116fb7..20c5b82 100644
--- a/go.sum
+++ b/go.sum
@@ -104,6 +104,8 @@ github.com/andybalholm/brotli v1.0.0 h1:7UCwP93aiSfvWpapti8g88vVVGp2qqtGyePsSuDa
 github.com/andybalholm/brotli v1.0.0/go.mod h1:loMXtMfwqflxFJPmdbJO0a3KNoPuLBgiu3qAvBg8x/Y=
 github.com/anmitsu/go-shlex v0.0.0-20161002113705-648efa622239/go.mod h1:2FmKhYUyUczH0OGQWaF5ceTx0UBShxjsH6f8oGKYe2c=
 github.com/antihax/optional v0.0.0-20180407024304-ca021399b1a6/go.mod h1:V8iCPQYkqmusNa815XgQio277wI47sdRh1dUOLdyC6Q=
+github.com/antlr/antlr4 v0.0.0-20190819145818-b43a4c3a8015 h1:StuiJFxQUsxSCzcby6NFZRdEhPkXD5vxN7TZ4MD6T84=
+github.com/antlr/antlr4 v0.0.0-20190819145818-b43a4c3a8015/go.mod h1:T7PbCXFs94rrTttyxjbyT5+/1V8T2TYDejxUfHJjw1Y=
 github.com/apache/thrift v0.12.0/go.mod h1:cp2SuWMxlEZw2r+iP2GNCdIi4C1qmUzdZFSVb+bacwQ=
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
 github.com/asaskevich/govalidator v0.0.0-20180315120708-ccb8e960c48f/go.mod h1:lB+ZfQJz7igIIfQNfa7Ml4HSf2uFQQRzpGGRXenZAgY=
@@ -298,6 +300,9 @@ github.com/golangci/revgrep v0.0.0-20180812185044-276a5c0a1039/go.mod h1:qOQCunE
 github.com/golangci/unconvert v0.0.0-20180507085042-28b1c447d1f4/go.mod h1:Izgrg8RkN3rCIMLGE9CyYmU9pY2Jer6DgANEnZ/L/cQ=
 github.com/google/btree v0.0.0-20180813153112-4030bb1f1f0c/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
 github.com/google/btree v1.0.0/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
+github.com/google/cel-go v0.3.2 h1:72Lj/nrfpWSJkuXdeEGB/7jfdwVFtV8kPJSL2Mt9rog=
+github.com/google/cel-go v0.3.2/go.mod h1:DoRSdzaJzNiP1lVuWhp/RjSnHLDQr/aNPlyqSBasBqA=
+github.com/google/cel-spec v0.3.0/go.mod h1:MjQm800JAGhOZXI7vatnVpmIaFTR6L8FHcKk+piiKpI=
 github.com/google/certificate-transparency-go v1.0.21/go.mod h1:QeJfpSbVSfYc7RgB3gJFj9cbuQMMchQxrWXz8Ruopmg=
 github.com/google/certificate-transparency-go v1.1.0/go.mod h1:i+Q7XY+ArBveOUT36jiHGfuSK1fHICIg6sUkRxPAbCs=
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
diff --git a/modules/caddyhttp/celmatcher.go b/modules/caddyhttp/celmatcher.go
new file mode 100644
index 0000000..fcedea4
--- /dev/null
+++ b/modules/caddyhttp/celmatcher.go
@@ -0,0 +1,221 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddyhttp
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"reflect"
+	"regexp"
+	"strings"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/checker/decls"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+	"github.com/google/cel-go/interpreter/functions"
+	exprpb "google.golang.org/genproto/googleapis/api/expr/v1alpha1"
+)
+
+func init() {
+	caddy.RegisterModule(MatchExpression{})
+}
+
+// MatchExpression matches requests by evaluating a
+// [CEL](https://github.com/google/cel-spec) expression.
+// This enables complex logic to be expressed using a comfortable,
+// familiar syntax.
+//
+// COMPATIBILITY NOTE: This module is still experimental and is not
+// subject to Caddy's compatibility guarantee.
+type MatchExpression struct {
+	// The CEL expression to evaluate. Any Caddy placeholders
+	// will be expanded and situated into proper CEL function
+	// calls before evaluating.
+	Expr string
+
+	expandedExpr string
+	prg          cel.Program
+}
+
+// CaddyModule returns the Caddy module information.
+func (MatchExpression) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "http.matchers.expression",
+		New: func() caddy.Module { return new(MatchExpression) },
+	}
+}
+
+// MarshalJSON marshals m's expression.
+func (m MatchExpression) MarshalJSON() ([]byte, error) {
+	return json.Marshal(m.Expr)
+}
+
+// UnmarshalJSON unmarshals m's expression.
+func (m *MatchExpression) UnmarshalJSON(data []byte) error {
+	return json.Unmarshal(data, &m.Expr)
+}
+
+// Provision sets ups m.
+func (m *MatchExpression) Provision(_ caddy.Context) error {
+	// replace placeholders with a function call - this is just some
+	// light (and possibly naïve) syntactic sugar
+	m.expandedExpr = placeholderRegexp.ReplaceAllString(m.Expr, placeholderExpansion)
+
+	// create the CEL environment
+	env, err := cel.NewEnv(
+		cel.Declarations(
+			decls.NewIdent("request", httpRequestObjectType, nil),
+			decls.NewFunction(placeholderFuncName,
+				decls.NewOverload(placeholderFuncName+"_httpRequest_string",
+					[]*exprpb.Type{httpRequestObjectType, decls.String},
+					decls.String)),
+		),
+		cel.CustomTypeAdapter(celHTTPRequestTypeAdapter{}),
+	)
+	if err != nil {
+		return fmt.Errorf("setting up CEL environment: %v", err)
+	}
+
+	// parse the expression
+	parsed, issues := env.Parse(m.expandedExpr)
+	if issues != nil && issues.Err() != nil {
+		return fmt.Errorf("parsing CEL program: %s", issues.Err())
+	}
+
+	// type-check it
+	checked, issues := env.Check(parsed)
+	if issues != nil && issues.Err() != nil {
+		return fmt.Errorf("type-checking CEL program: %s", issues.Err())
+	}
+
+	// compile the "program"
+	m.prg, err = env.Program(checked,
+		cel.Functions(
+			&functions.Overload{
+				Operator: placeholderFuncName,
+				Binary:   caddyPlaceholderFunc,
+			},
+		),
+	)
+
+	if err != nil {
+		return fmt.Errorf("compiling CEL program: %s", err)
+	}
+	return nil
+}
+
+// Match returns true if r matches m.
+func (m MatchExpression) Match(r *http.Request) bool {
+	out, _, _ := m.prg.Eval(map[string]interface{}{
+		"request": celHTTPRequest{r},
+	})
+	if outBool, ok := out.Value().(bool); ok {
+		return outBool
+	}
+	return false
+
+}
+
+// UnmarshalCaddyfile implements caddyfile.Unmarshaler.
+func (m *MatchExpression) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	for d.Next() {
+		m.Expr = strings.Join(d.RemainingArgs(), " ")
+	}
+	return nil
+}
+
+// httpRequestCELType is the type representation of a native HTTP request.
+var httpRequestCELType = types.NewTypeValue("http.Request", traits.ReceiverType)
+
+// cellHTTPRequest wraps an http.Request with
+// methods to satisfy the ref.Val interface.
+type celHTTPRequest struct {
+	*http.Request
+}
+
+func (cr celHTTPRequest) ConvertToNative(typeDesc reflect.Type) (interface{}, error) {
+	return cr.Request, nil
+}
+func (celHTTPRequest) ConvertToType(typeVal ref.Type) ref.Val {
+	panic("not implemented")
+}
+func (cr celHTTPRequest) Equal(other ref.Val) ref.Val {
+	if o, ok := other.Value().(celHTTPRequest); ok {
+		return types.Bool(o.Request == cr.Request)
+	}
+	return types.ValOrErr(other, "%v is not comparable type", other)
+}
+func (celHTTPRequest) Type() ref.Type        { return httpRequestCELType }
+func (cr celHTTPRequest) Value() interface{} { return cr }
+
+// celHTTPRequestTypeAdapter can adapt a
+// celHTTPRequest to a CEL value.
+type celHTTPRequestTypeAdapter struct{}
+
+func (celHTTPRequestTypeAdapter) NativeToValue(value interface{}) ref.Val {
+	if celReq, ok := value.(celHTTPRequest); ok {
+		return celReq
+	}
+	return types.DefaultTypeAdapter.NativeToValue(value)
+}
+
+// Variables used for replacing Caddy placeholders in CEL
+// expressions with a proper CEL function call; this is
+// just for syntactic sugar.
+var (
+	placeholderRegexp    = regexp.MustCompile(`{([\w.-]+)}`)
+	placeholderExpansion = `caddyPlaceholder(request, "${1}")`
+)
+
+var httpRequestObjectType = decls.NewObjectType("http.Request")
+
+// The name of the CEL function which accesses Replacer values.
+const placeholderFuncName = "caddyPlaceholder"
+
+// caddyPlaceholderFunc implements the custom CEL function that
+// accesses the Replacer on a request and gets values from it.
+func caddyPlaceholderFunc(lhs, rhs ref.Val) ref.Val {
+	celReq, ok := lhs.(celHTTPRequest)
+	if !ok {
+		return types.NewErr(
+			"invalid request of type '%v' to "+placeholderFuncName+"(request, placeholderVarName)",
+			lhs.Type())
+	}
+	phStr, ok := rhs.(types.String)
+	if !ok {
+		return types.NewErr(
+			"invalid placeholder variable name of type '%v' to "+placeholderFuncName+"(request, placeholderVarName)",
+			rhs.Type())
+	}
+
+	repl := celReq.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	val, _ := repl.Get(string(phStr))
+
+	return types.String(val)
+}
+
+// Interface guards
+var (
+	_ caddy.Provisioner     = (*MatchExpression)(nil)
+	_ RequestMatcher        = (*MatchExpression)(nil)
+	_ caddyfile.Unmarshaler = (*MatchExpression)(nil)
+	_ json.Marshaler        = (*MatchExpression)(nil)
+	_ json.Unmarshaler      = (*MatchExpression)(nil)
+)
diff --git a/modules/caddyhttp/matchers.go b/modules/caddyhttp/matchers.go
index 6bfb31e..043831f 100644
--- a/modules/caddyhttp/matchers.go
+++ b/modules/caddyhttp/matchers.go
@@ -107,9 +107,6 @@ type (
 
 		Matchers MatcherSet `json:"-"`
 	}
-
-	// MatchTable matches requests by values in the table.
-	MatchTable string // TODO: finish implementing
 )
 
 func init() {