all: sync with master

internal/aghhttp/aghhttp.go

@@ -2,10 +2,12 @@
 package aghhttp
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/version"
 	"github.com/AdguardTeam/golibs/log"
 )
 
@@ -34,3 +36,40 @@ func Error(r *http.Request, w http.ResponseWriter, code int, format string, args
 	log.Error("%s %s: %s", r.Method, r.URL, text)
 	http.Error(w, text, code)
 }
+
+// UserAgent returns the ID of the service as a User-Agent string.  It can also
+// be used as the value of the Server HTTP header.
+func UserAgent() (ua string) {
+	return fmt.Sprintf("AdGuardHome/%s", version.Version())
+}
+
+// textPlainDeprMsg is the message returned to API users when they try to use
+// an API that used to accept "text/plain" but doesn't anymore.
+const textPlainDeprMsg = `using this api with the text/plain content-type is deprecated; ` +
+	`use application/json`
+
+// WriteTextPlainDeprecated responds to the request with a message about
+// deprecation and removal of a plain-text API if the request is made with the
+// "text/plain" content-type.
+func WriteTextPlainDeprecated(w http.ResponseWriter, r *http.Request) (isPlainText bool) {
+	if r.Header.Get(HdrNameContentType) != HdrValTextPlain {
+		return false
+	}
+
+	Error(r, w, http.StatusUnsupportedMediaType, textPlainDeprMsg)
+
+	return true
+}
+
+// WriteJSONResponse sets the content-type header in w.Header() to
+// "application/json", encodes resp to w, calls Error on any returned error, and
+// returns it as well.
+func WriteJSONResponse(w http.ResponseWriter, r *http.Request, resp any) (err error) {
+	w.Header().Set(HdrNameContentType, HdrValApplicationJSON)
+	err = json.NewEncoder(w).Encode(resp)
+	if err != nil {
+		Error(r, w, http.StatusInternalServerError, "encoding resp: %s", err)
+	}
+
+	return err
+}

internal/aghhttp/header.go

@@ -0,0 +1,22 @@
+package aghhttp
+
+// HTTP Headers
+
+// HTTP header name constants.
+//
+// TODO(a.garipov): Remove unused.
+const (
+	HdrNameAcceptEncoding           = "Accept-Encoding"
+	HdrNameAccessControlAllowOrigin = "Access-Control-Allow-Origin"
+	HdrNameContentType              = "Content-Type"
+	HdrNameContentEncoding          = "Content-Encoding"
+	HdrNameServer                   = "Server"
+	HdrNameTrailer                  = "Trailer"
+	HdrNameUserAgent                = "User-Agent"
+)
+
+// HTTP header value constants.
+const (
+	HdrValApplicationJSON = "application/json"
+	HdrValTextPlain       = "text/plain"
+)