logging: Implement log_append handler (#6066)

* logging: Implement `extra_log` handler

* Rename to `log_append`

* Rename `skip_log` to `log_skip`

---------

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

modules/caddyhttp/logging/caddyfile.go

@@ -0,0 +1,53 @@
+// 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 logging
+
+import (
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+)
+
+func init() {
+	httpcaddyfile.RegisterHandlerDirective("log_append", parseCaddyfile)
+}
+
+// parseCaddyfile sets up the log_append handler from Caddyfile tokens. Syntax:
+//
+//	log_append [<matcher>] <key> <value>
+func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
+	handler := new(LogAppend)
+	err := handler.UnmarshalCaddyfile(h.Dispenser)
+	return handler, err
+}
+
+// UnmarshalCaddyfile implements caddyfile.Unmarshaler.
+func (h *LogAppend) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	d.Next() // consume directive name
+	if !d.NextArg() {
+		return d.ArgErr()
+	}
+	h.Key = d.Val()
+	if !d.NextArg() {
+		return d.ArgErr()
+	}
+	h.Value = d.Val()
+	return nil
+}
+
+// Interface guards
+var (
+	_ caddyfile.Unmarshaler = (*LogAppend)(nil)
+)

modules/caddyhttp/logging/logadd.go

@@ -0,0 +1,94 @@
+// 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 logging
+
+import (
+	"net/http"
+	"strings"
+
+	"go.uber.org/zap"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+)
+
+func init() {
+	caddy.RegisterModule(LogAppend{})
+}
+
+// LogAppend implements a middleware that takes a key and value, where
+// the key is the name of a log field and the value is a placeholder,
+// or variable key, or constant value to use for that field.
+type LogAppend struct {
+	// Key is the name of the log field.
+	Key string `json:"key,omitempty"`
+
+	// Value is the value to use for the log field.
+	// If it is a placeholder (with surrounding `{}`),
+	// it will be evaluated when the log is written.
+	// If the value is a key that exists in the `vars`
+	// map, the value of that key will be used. Otherwise
+	// the value will be used as-is as a constant string.
+	Value string `json:"value,omitempty"`
+}
+
+// CaddyModule returns the Caddy module information.
+func (LogAppend) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "http.handlers.log_append",
+		New: func() caddy.Module { return new(LogAppend) },
+	}
+}
+
+func (h LogAppend) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
+	// Run the next handler in the chain first.
+	// If an error occurs, we still want to add
+	// any extra log fields that we can, so we
+	// hold onto the error and return it later.
+	handlerErr := next.ServeHTTP(w, r)
+
+	// On the way back up the chain, add the extra log field
+	ctx := r.Context()
+
+	vars := ctx.Value(caddyhttp.VarsCtxKey).(map[string]any)
+	repl := ctx.Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	extra := ctx.Value(caddyhttp.ExtraLogFieldsCtxKey).(*caddyhttp.ExtraLogFields)
+
+	var varValue any
+	if strings.HasPrefix(h.Value, "{") &&
+		strings.HasSuffix(h.Value, "}") &&
+		strings.Count(h.Value, "{") == 1 {
+		// the value looks like a placeholder, so get its value
+		varValue, _ = repl.Get(strings.Trim(h.Value, "{}"))
+	} else if val, ok := vars[h.Value]; ok {
+		// the value is a key in the vars map
+		varValue = val
+	} else {
+		// the value is a constant string
+		varValue = h.Value
+	}
+
+	// Add the field to the extra log fields.
+	// We use zap.Any because it will reflect
+	// to the correct type for us.
+	extra.Add(zap.Any(h.Key, varValue))
+
+	return handlerErr
+}
+
+// Interface guards
+var (
+	_ caddyhttp.MiddlewareHandler = (*LogAppend)(nil)
+)