Implement brotli encoder; improve validation of other encoders

modules/caddyhttp/encode/gzip/gzip.go

@@ -3,6 +3,7 @@ package caddygzip
 import (
 	"compress/flate"
 	"compress/gzip" // TODO: consider using https://github.com/klauspost/compress/gzip
+	"fmt"
 
 	"github.com/caddyserver/caddy2"
 	"github.com/caddyserver/caddy2/modules/caddyhttp/encode"
@@ -20,17 +21,37 @@ type Gzip struct {
 	Level int `json:"level,omitempty"`
 }
 
-// NewEncoder returns a new gzip writer.
-func (g Gzip) NewEncoder() encode.Encoder {
-	if g.Level <= flate.NoCompression {
+// Provision provisions g's configuration.
+func (g *Gzip) Provision(ctx caddy2.Context) error {
+	if g.Level == 0 {
 		g.Level = defaultGzipLevel
 	}
-	if g.Level > flate.BestCompression {
-		g.Level = flate.BestCompression
+	return nil
+}
+
+// Validate validates g's configuration.
+func (g Gzip) Validate() error {
+	if g.Level < flate.NoCompression {
+		return fmt.Errorf("quality too low; must be >= %d", flate.NoCompression)
 	}
+	if g.Level > flate.BestCompression {
+		return fmt.Errorf("quality too high; must be <= %d", flate.BestCompression)
+	}
+	return nil
+}
+
+// NewEncoder returns a new gzip writer.
+func (g Gzip) NewEncoder() encode.Encoder {
 	writer, _ := gzip.NewWriterLevel(nil, g.Level)
 	return writer
 }
 
 // Informed from http://blog.klauspost.com/gzip-performance-for-go-webservers/
 var defaultGzipLevel = 5
+
+// Interface guards
+var (
+	_ encode.Encoding    = (*Gzip)(nil)
+	_ caddy2.Provisioner = (*Gzip)(nil)
+	_ caddy2.Validator   = (*Gzip)(nil)
+)