[evolution] I#2029 - prefer-plain: Runtime warnings for "Only ever show plain text"

[Milan Crha <mcrha src gnome org>]

commit 48dbfb4b1e19df8f0f2bb3cd66c3daaa97ca0fc6
Author: Milan Crha <mcrha redhat com>
Date:   Fri Sep 16 09:10:53 2022 +0200

    I#2029 - prefer-plain: Runtime warnings for "Only ever show plain text"
    
    Closes https://gitlab.gnome.org/GNOME/evolution/-/issues/2029

 .../prefer-plain/e-mail-parser-prefer-plain.c      | 81 +++++++++++++++++++++-
 1 file changed, 79 insertions(+), 2 deletions(-)
---
diff --git a/src/modules/prefer-plain/e-mail-parser-prefer-plain.c 
b/src/modules/prefer-plain/e-mail-parser-prefer-plain.c
index 86480ccf02..af6acb0644 100644
--- a/src/modules/prefer-plain/e-mail-parser-prefer-plain.c
+++ b/src/modules/prefer-plain/e-mail-parser-prefer-plain.c
@@ -28,6 +28,70 @@
 
 #include "e-mail-parser-prefer-plain.h"
 
+/* ------------------------------------------------------------------------ */
+
+#define E_TYPE_NULL_REQUEST e_null_request_get_type ()
+G_DECLARE_FINAL_TYPE (ENullRequest, e_null_request, E, NULL_REQUEST, GObject)
+
+struct _ENullRequest {
+       GObject parent;
+};
+
+static void e_null_request_content_request_init (EContentRequestInterface *iface);
+
+G_DEFINE_TYPE_WITH_CODE (ENullRequest, e_null_request, G_TYPE_OBJECT,
+       G_IMPLEMENT_INTERFACE (E_TYPE_CONTENT_REQUEST, e_null_request_content_request_init))
+
+static gboolean
+e_null_request_can_process_uri (EContentRequest *request,
+                               const gchar *uri)
+{
+       return TRUE;
+}
+
+static gboolean
+e_null_request_process_sync (EContentRequest *request,
+                            const gchar *uri,
+                            GObject *requester,
+                            GInputStream **out_stream,
+                            gint64 *out_stream_length,
+                            gchar **out_mime_type,
+                            GCancellable *cancellable,
+                            GError **error)
+{
+       if (g_cancellable_set_error_if_cancelled (cancellable, error))
+               return FALSE;
+
+       g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED, "not supported");
+
+       return FALSE;
+}
+
+static void
+e_null_request_content_request_init (EContentRequestInterface *iface)
+{
+       iface->can_process_uri = e_null_request_can_process_uri;
+       iface->process_sync = e_null_request_process_sync;
+}
+
+static void
+e_null_request_class_init (ENullRequestClass *klass)
+{
+}
+
+static void
+e_null_request_init (ENullRequest *request)
+{
+}
+
+static EContentRequest *
+e_null_request_new (void)
+{
+       return g_object_new (E_TYPE_NULL_REQUEST, NULL);
+}
+
+/* ------------------------------------------------------------------------ */
+
 typedef struct _EMailParserPreferPlain EMailParserPreferPlain;
 typedef struct _EMailParserPreferPlainClass EMailParserPreferPlainClass;
 
@@ -274,13 +338,26 @@ static gboolean
 mail_parser_prefer_plain_convert_text (gpointer user_data)
 {
        AsyncContext *async_context = user_data;
+       EContentRequest *content_request;
+       EWebView *web_view;
        gchar *script;
 
        g_return_val_if_fail (async_context != NULL, FALSE);
 
-       async_context->web_view = WEBKIT_WEB_VIEW (g_object_ref_sink (e_web_view_new ()));
+       web_view = E_WEB_VIEW (g_object_ref_sink (e_web_view_new ()));
+       async_context->web_view = WEBKIT_WEB_VIEW (web_view);
+
+       /* Register schemes used by the EMailDisplay, but do not process them.
+          It avoids a runtime warning from the EWebView when it cannot find
+          the scheme handler. */
+       content_request = e_null_request_new ();
+       e_web_view_register_content_request_for_scheme (web_view, "evo-http", content_request);
+       e_web_view_register_content_request_for_scheme (web_view, "evo-https", content_request);
+       e_web_view_register_content_request_for_scheme (web_view, "mail", content_request);
+       e_web_view_register_content_request_for_scheme (web_view, "cid", content_request);
+       g_object_unref (content_request);
 
-       e_web_view_load_uri (E_WEB_VIEW (async_context->web_view), "evo://disable-remote-content");
+       e_web_view_load_uri (web_view, "evo://disable-remote-content");
 
        script = e_web_view_jsc_printf_script (
                "var elem;\n"