Handle decode error

internal/server/form.go

@@ -1,7 +1,13 @@
 // test
 package server
 
-import "git.runcible.io/learning/ratchet/internal/validator"
+import (
+	"errors"
+	"net/http"
+
+	"git.runcible.io/learning/ratchet/internal/validator"
+	"github.com/go-playground/form/v4"
+)
 
 // Define a snippetCreateForm struct to represent the form data and validation
 // errors for the form fields. Note that all the struct fields are deliberately
@@ -25,3 +31,23 @@ type snippetCreateForm struct {
 	Expires             int    `form:"expires"`
 	validator.Validator `form:"-"`
 }
+
+func decodePostForm(r *http.Request, fd *form.Decoder, dst any) error {
+	err := r.ParseForm()
+	if err != nil {
+		return err
+	}
+
+	err = fd.Decode(dst, r.PostForm)
+	if err != nil {
+		var invalidDecoderError *form.InvalidDecoderError
+
+		if errors.As(err, &invalidDecoderError) {
+			// if called in the handler, recovery middleware
+			// will log and send 500 response back
+			panic(err)
+		}
+		return err
+	}
+	return nil
+}

internal/server/handlers.go

@@ -174,11 +174,15 @@ func handleSnippetCreatePost(logger *slog.Logger, tc *TemplateCache, formDecoder
 				return
 			}
 
+			// initializes the struct with zero values. So &form is non-nil
 			var form snippetCreateForm
 
 			// AUTOMATIC FORM PROCESSING
-
-			err = formDecoder.Decode(&form, r.PostForm)
+			// When we call app.formDecoder.Decode() it requires a non-nil pointer as the target
+			// decode destination. If we try to pass in something that isn’t a non-nil pointer, then
+			//  Decode() will return a form.InvalidDecoderError error. That error should be handled
+			// differently form a 400 error since it's a server side problem (ie 5xx error)
+			err = decodePostForm(r, formDecoder, &form)
 
 			if err != nil {
 				clientError(w, http.StatusBadRequest)