Second checkpoint

pkg/httputil/httputil.go

@@ -0,0 +1,92 @@
+// Package httputil provides small, generic helpers for JSON HTTP handlers:
+// response writing, request decoding, and validation-error formatting.
+package httputil
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"github.com/go-playground/validator/v10"
+)
+
+// maxBodyBytes caps the size of a decoded JSON request body.
+const maxBodyBytes = 1 << 20 // 1 MiB
+
+// WriteJSON writes data as a JSON response with the given status code.
+func WriteJSON(w http.ResponseWriter, status int, data any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if data != nil {
+		_ = json.NewEncoder(w).Encode(data)
+	}
+}
+
+// ErrorResponse is the JSON error envelope.
+type ErrorResponse struct {
+	Error string `json:"error"`
+}
+
+// ValidationErrorResponse is the field-level validation error envelope.
+type ValidationErrorResponse struct {
+	Errors map[string]string `json:"errors"`
+}
+
+// WriteError writes a JSON error envelope: {"error": "..."}.
+func WriteError(w http.ResponseWriter, status int, msg string) {
+	WriteJSON(w, status, ErrorResponse{Error: msg})
+}
+
+// DecodeJSON reads and validates a JSON body into a value of type T. It caps the
+// body size and rejects unknown fields.
+func DecodeJSON[T any](w http.ResponseWriter, r *http.Request) (T, error) {
+	var v T
+	r.Body = http.MaxBytesReader(w, r.Body, maxBodyBytes)
+
+	dec := json.NewDecoder(r.Body)
+	dec.DisallowUnknownFields()
+	if err := dec.Decode(&v); err != nil {
+		return v, err
+	}
+	return v, nil
+}
+
+// WriteValidationErrors renders validator.ValidationErrors as a field->message
+// map under {"errors": {...}} with a 422 status.
+func WriteValidationErrors(w http.ResponseWriter, err error) {
+	var ve validator.ValidationErrors
+	if !errors.As(err, &ve) {
+		WriteError(w, http.StatusBadRequest, "invalid request")
+		return
+	}
+
+	problems := make(map[string]string, len(ve))
+	for _, fe := range ve {
+		problems[fe.Field()] = messageForTag(fe)
+	}
+	WriteJSON(w, http.StatusUnprocessableEntity, ValidationErrorResponse{Errors: problems})
+}
+
+func messageForTag(fe validator.FieldError) string {
+	switch fe.Tag() {
+	case "required":
+		return "is required"
+	case "email":
+		return "must be a valid email address"
+	case "uuid", "uuid4":
+		return "must be a valid UUID"
+	case "min":
+		return fmt.Sprintf("must be at least %s characters", fe.Param())
+	case "max":
+		return fmt.Sprintf("must be at most %s characters", fe.Param())
+	case "gte":
+		return fmt.Sprintf("must be %s or greater", fe.Param())
+	case "lte":
+		return fmt.Sprintf("must be %s or less", fe.Param())
+	case "oneof":
+		return fmt.Sprintf("must be one of: %s", fe.Param())
+	default:
+		return "is invalid"
+	}
+}