caddyconfig: register adapters as Caddy modules (#3132)

* admin: Refactor /load endpoint out of caddy package

This eliminates the caddy package's dependency on the caddyconfig
package, which helps prevent import cycles.

* v2: adapter: register config adapters as Caddy modules

* v2: adapter: simplify adapter registration as adapters and modules

* v2: adapter: let RegisterAdapter be in charge of registering adapters as modules

* v2: adapter: remove underscrores placeholders

* v2: adapter: explicitly ignore the error of writing response of writing warnings back to client

* Implicitly wrap config adapters as modules

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

3 files changed, 173 insertions, 84 deletions

diff --git a/admin.go b/admin.go
index 7a5d0b6..212637f 100644
--- a/admin.go
+++ b/admin.go
@@ -21,7 +21,6 @@ import (
 	"expvar"
 	"fmt"
 	"io"
-	"mime"
 	"net/http"
 	"net/http/pprof"
 	"net/url"
@@ -33,7 +32,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/caddyserver/caddy/v2/caddyconfig"
 	"go.uber.org/zap"
 )
 
@@ -115,7 +113,6 @@ func (admin AdminConfig) newAdminHandler(listenAddr string) adminHandler {
 	}
 
 	// register standard config control endpoints
-	addRoute("/load", AdminHandlerFunc(handleLoad))
 	addRoute("/"+rawConfigKey+"/", AdminHandlerFunc(handleConfig))
 	addRoute("/id/", AdminHandlerFunc(handleConfigID))
 	addRoute("/stop", AdminHandlerFunc(handleStop))
@@ -407,86 +404,6 @@ func (h adminHandler) originAllowed(origin string) bool {
 	return false
 }
 
-func handleLoad(w http.ResponseWriter, r *http.Request) error {
-	if r.Method != http.MethodPost {
-		return APIError{
-			Code: http.StatusMethodNotAllowed,
-			Err:  fmt.Errorf("method not allowed"),
-		}
-	}
-
-	buf := bufPool.Get().(*bytes.Buffer)
-	buf.Reset()
-	defer bufPool.Put(buf)
-
-	_, err := io.Copy(buf, r.Body)
-	if err != nil {
-		return APIError{
-			Code: http.StatusBadRequest,
-			Err:  fmt.Errorf("reading request body: %v", err),
-		}
-	}
-	body := buf.Bytes()
-
-	// if the config is formatted other than Caddy's native
-	// JSON, we need to adapt it before loading it
-	if ctHeader := r.Header.Get("Content-Type"); ctHeader != "" {
-		ct, _, err := mime.ParseMediaType(ctHeader)
-		if err != nil {
-			return APIError{
-				Code: http.StatusBadRequest,
-				Err:  fmt.Errorf("invalid Content-Type: %v", err),
-			}
-		}
-		if !strings.HasSuffix(ct, "/json") {
-			slashIdx := strings.Index(ct, "/")
-			if slashIdx < 0 {
-				return APIError{
-					Code: http.StatusBadRequest,
-					Err:  fmt.Errorf("malformed Content-Type"),
-				}
-			}
-			adapterName := ct[slashIdx+1:]
-			cfgAdapter := caddyconfig.GetAdapter(adapterName)
-			if cfgAdapter == nil {
-				return APIError{
-					Code: http.StatusBadRequest,
-					Err:  fmt.Errorf("unrecognized config adapter '%s'", adapterName),
-				}
-			}
-			result, warnings, err := cfgAdapter.Adapt(body, nil)
-			if err != nil {
-				return APIError{
-					Code: http.StatusBadRequest,
-					Err:  fmt.Errorf("adapting config using %s adapter: %v", adapterName, err),
-				}
-			}
-			if len(warnings) > 0 {
-				respBody, err := json.Marshal(warnings)
-				if err != nil {
-					Log().Named("admin.api.load").Error(err.Error())
-				}
-				w.Write(respBody)
-			}
-			body = result
-		}
-	}
-
-	forceReload := r.Header.Get("Cache-Control") == "must-revalidate"
-
-	err = Load(body, forceReload)
-	if err != nil {
-		return APIError{
-			Code: http.StatusBadRequest,
-			Err:  fmt.Errorf("loading config: %v", err),
-		}
-	}
-
-	Log().Named("admin.api").Info("load complete")
-
-	return nil
-}
-
 func handleConfig(w http.ResponseWriter, r *http.Request) error {
 	switch r.Method {
 	case http.MethodGet:
diff --git a/caddyconfig/configadapters.go b/caddyconfig/configadapters.go
index 071221f..96d7e10 100644
--- a/caddyconfig/configadapters.go
+++ b/caddyconfig/configadapters.go
@@ -17,6 +17,8 @@ package caddyconfig
 import (
 	"encoding/json"
 	"fmt"
+
+	"github.com/caddyserver/caddy/v2"
 )
 
 // Adapter is a type which can adapt a configuration to Caddy JSON.
@@ -105,7 +107,7 @@ func RegisterAdapter(name string, adapter Adapter) error {
 		return fmt.Errorf("%s: already registered", name)
 	}
 	configAdapters[name] = adapter
-	return nil
+	return caddy.RegisterModule(adapterModule{name, adapter})
 }
 
 // GetAdapter returns the adapter with the given name,
@@ -114,4 +116,21 @@ func GetAdapter(name string) Adapter {
 	return configAdapters[name]
 }
 
+// adapterModule is a wrapper type that can turn any config
+// adapter into a Caddy module, which has the benefit of being
+// counted with other modules, even though they do not
+// technically extend the Caddy configuration structure.
+// See caddyserver/caddy#3132.
+type adapterModule struct {
+	name string
+	Adapter
+}
+
+func (am adapterModule) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  caddy.ModuleID("caddy.adapters." + am.name),
+		New: func() caddy.Module { return am },
+	}
+}
+
 var configAdapters = make(map[string]Adapter)
diff --git a/caddyconfig/load.go b/caddyconfig/load.go
new file mode 100644
index 0000000..4855b46
--- /dev/null
+++ b/caddyconfig/load.go
@@ -0,0 +1,153 @@
+// 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 caddyconfig
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"mime"
+	"net/http"
+	"strings"
+	"sync"
+
+	"github.com/caddyserver/caddy/v2"
+)
+
+func init() {
+	caddy.RegisterModule(adminLoad{})
+}
+
+// adminLoad is a module that provides the /load endpoint
+// for the Caddy admin API. The only reason it's not baked
+// into the caddy package directly is because of the import
+// of the caddyconfig package for its GetAdapter function.
+// If the caddy package depends on the caddyconfig package,
+// then the caddyconfig package will not be able to import
+// the caddy package, and it can more easily cause backward
+// edges in the dependency tree (i.e. import cycle).
+// Fortunately, the admin API has first-class support for
+// adding endpoints from modules.
+type adminLoad struct{}
+
+// CaddyModule returns the Caddy module information.
+func (adminLoad) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "admin.api.load",
+		New: func() caddy.Module { return new(adminLoad) },
+	}
+}
+
+// Routes returns a route for the /load endpoint.
+func (al adminLoad) Routes() []caddy.AdminRoute {
+	return []caddy.AdminRoute{
+		{
+			Pattern: "/load",
+			Handler: caddy.AdminHandlerFunc(al.handleLoad),
+		},
+	}
+}
+
+// handleLoad replaces the entire current configuration with
+// a new one provided in the response body. It supports config
+// adapters through the use of the Content-Type header. A
+// config that is identical to the currently-running config
+// will be a no-op unless Cache-Control: must-revalidate is set.
+func (adminLoad) handleLoad(w http.ResponseWriter, r *http.Request) error {
+	if r.Method != http.MethodPost {
+		return caddy.APIError{
+			Code: http.StatusMethodNotAllowed,
+			Err:  fmt.Errorf("method not allowed"),
+		}
+	}
+
+	buf := bufPool.Get().(*bytes.Buffer)
+	buf.Reset()
+	defer bufPool.Put(buf)
+
+	_, err := io.Copy(buf, r.Body)
+	if err != nil {
+		return caddy.APIError{
+			Code: http.StatusBadRequest,
+			Err:  fmt.Errorf("reading request body: %v", err),
+		}
+	}
+	body := buf.Bytes()
+
+	// if the config is formatted other than Caddy's native
+	// JSON, we need to adapt it before loading it
+	if ctHeader := r.Header.Get("Content-Type"); ctHeader != "" {
+		ct, _, err := mime.ParseMediaType(ctHeader)
+		if err != nil {
+			return caddy.APIError{
+				Code: http.StatusBadRequest,
+				Err:  fmt.Errorf("invalid Content-Type: %v", err),
+			}
+		}
+		if !strings.HasSuffix(ct, "/json") {
+			slashIdx := strings.Index(ct, "/")
+			if slashIdx < 0 {
+				return caddy.APIError{
+					Code: http.StatusBadRequest,
+					Err:  fmt.Errorf("malformed Content-Type"),
+				}
+			}
+			adapterName := ct[slashIdx+1:]
+			cfgAdapter := GetAdapter(adapterName)
+			if cfgAdapter == nil {
+				return caddy.APIError{
+					Code: http.StatusBadRequest,
+					Err:  fmt.Errorf("unrecognized config adapter '%s'", adapterName),
+				}
+			}
+			result, warnings, err := cfgAdapter.Adapt(body, nil)
+			if err != nil {
+				return caddy.APIError{
+					Code: http.StatusBadRequest,
+					Err:  fmt.Errorf("adapting config using %s adapter: %v", adapterName, err),
+				}
+			}
+			if len(warnings) > 0 {
+				respBody, err := json.Marshal(warnings)
+				if err != nil {
+					caddy.Log().Named("admin.api.load").Error(err.Error())
+				}
+				_, _ = w.Write(respBody)
+			}
+			body = result
+		}
+	}
+
+	forceReload := r.Header.Get("Cache-Control") == "must-revalidate"
+
+	err = caddy.Load(body, forceReload)
+	if err != nil {
+		return caddy.APIError{
+			Code: http.StatusBadRequest,
+			Err:  fmt.Errorf("loading config: %v", err),
+		}
+	}
+
+	caddy.Log().Named("admin.api").Info("load complete")
+
+	return nil
+}
+
+var bufPool = sync.Pool{
+	New: func() interface{} {
+		return new(bytes.Buffer)
+	},
+}