guile: avoid body duplication

In message, don't re-fill when unneeded. Add tests.

Fixes #2802.

guile/mu-guile-message.cc

@@ -333,7 +333,7 @@ SCM_DEFINE(get_parts,
 		attlist = scm_cons(elm, attlist);
 	}
 
-	/* explicitly close the file backend, so we won't run of fds */
+	/* explicitly close the file backend, so we won't run out of fds */
 	msg->unload_mime_message();
 
 	return attlist;

guile/tests/test-mu-guile.scm

@@ -74,7 +74,10 @@ exec guile -e main -s $0 $@
     (str-equal-or-exit (mu:subject msg) "Fwd: rfc822")
     (str-equal-or-exit (mu:to msg) "martin")
     (str-equal-or-exit (mu:from msg) "foobar <foo@example.com>")
+    (str-equal-or-exit (mu:body msg) "Hello world, forwarding some RFC822 message\n")
     (str-equal-or-exit (mu:header msg "X-Mailer") "Ximian Evolution 1.4.5")
+    ;; issue #2802
+    (str-equal-or-exit (mu:body msg) "Hello world, forwarding some RFC822 message\n")
 
     (if (not (equal? (mu:priority msg) mu:prio:normal))
 	(error-exit "Expected ~A, got ~A"  (mu:priority msg) mu:prio:normal)))

lib/message/mu-message.cc

@@ -72,6 +72,10 @@ struct Message::Private {
 
 	Option<std::string> language; /* body ISO language code */
 
+	// after unload_mime_message, we don't need to re-parse the message.
+	// issue #2802.
+	bool document_filled{};
+
 private:
 	Document::Options doc_opts(Message::Options mopts) {
 		return any_of(opts & Message::Options::SupportNgrams) ?
@@ -675,6 +679,10 @@ doc_add_reply_to(Document& doc, const MimeMessage& mime_msg)
 static void
 fill_document(Message::Private& priv)
 {
+	if (priv.document_filled) {
+		return; // nothing to do.
+	}
+
 	/* hunt & gather info from message tree */
 	Document& doc{priv.doc};
 	MimeMessage& mime_msg{priv.mime_msg.value()};
@@ -691,7 +699,7 @@ fill_document(Message::Private& priv)
 	doc_add_list_post(doc, mime_msg); /* only in sexp */
 	doc_add_reply_to(doc, mime_msg);  /* only in sexp */
 
-	field_for_each([&](auto&& field) {
+	field_for_each([&](const auto& field) {
 		/* insist on explicitly handling each */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic error "-Wswitch"
@@ -775,6 +783,8 @@ fill_document(Message::Private& priv)
 #pragma GCC diagnostic pop
 
 	});
+
+	priv.document_filled = true;
 }
 
 Option<std::string>