message: fall-back to received for dateless

For the (rare) messages without a Date: header, attempt to guess a date
from the top (most-recent) Received: header. Not perfect, of course, but
better than nothing.

Requires re-indexing to apply to already indexed messages.

Should help for #1083.

lib/message/mu-message.cc

@@ -741,7 +741,10 @@ fill_document(Message::Private& priv)
 				doc.add(field.id, priv.ctime);
 			break;
 		case Field::Id::Date:
-			if (const auto& date{mime_msg.date()}; date) {
+			/* for the rare message without a Date: header, fall back
+			 * to the most recent Received: header */
+			if (const auto date{mime_msg.date().or_else([&]{
+					return mime_msg.received(); })}; date) {
 				doc.add(field.id, date->first);
 				doc.add(Field::Id::UtcOffset, date->second);
 			}

lib/message/mu-mime-object.cc

@@ -22,7 +22,9 @@
 #include "gmime/gmime-message.h"
 #include "utils/mu-utils.hh"
 #include "utils/mu-utils-file.hh"
+#include <algorithm>
 #include <mutex>
+#include <ranges>
 #include <regex>
 #include <fcntl.h>
 #include <sys/stat.h>
@@ -334,19 +336,47 @@ MimeMessage::make_from_text(const std::string& text)
 		return make_from_stream(std::move(stream));
 }
 
+static MimeMessage::Date date_from_date_time(GDateTime* dt)
+{
+	constexpr auto usecs_per_sec = 1'000'000;
+	return MimeMessage::Date {g_date_time_to_unix(dt),
+				  g_date_time_get_utc_offset(dt) / usecs_per_sec };
+}
+
+
 Option<MimeMessage::Date>
 MimeMessage::date() const noexcept
 {
 	if (/*const*/GDateTime *dt{g_mime_message_get_date(self())}; !dt)
 		return Nothing;
 	else {
-		constexpr auto usecs_per_sec = 1'000'000;
-		return Date{ g_date_time_to_unix(dt),
-			     g_date_time_get_utc_offset(dt) / usecs_per_sec };
-
+		return date_from_date_time(dt);
 	}
 }
 
+Option<MimeMessage::Date>
+MimeMessage::received() const noexcept
+{
+	const auto recv{header("Received")};
+	if (!recv)
+		return Nothing;
+
+	/* per RFC 5322, "Received:" is *received-token ";" date-time, so the
+	 * date-time follows the _last_ semicolon. */
+	const auto pos{recv->rfind(';')};
+	if (pos == std::string::npos)
+		return Nothing;
+
+	GDateTime *dt{g_mime_utils_header_decode_date(recv->c_str() + pos + 1)};
+	if (!dt)
+		return Nothing;
+
+	const auto date{date_from_date_time(dt)};
+	g_date_time_unref(dt);
+
+	return date;
+}
+
 constexpr Option<GMimeAddressType>
 address_type(Contact::Type ctype)
 {
@@ -372,20 +402,20 @@ address_type(Contact::Type ctype)
 static Mu::Contacts
 all_contacts(const MimeMessage& msg)
 {
-	Contacts contacts;
-
-	for (auto&& cctype: {
+	constexpr auto ctypes = std::to_array({
 			Contact::Type::Sender,
 			Contact::Type::From,
 			Contact::Type::ReplyTo,
 			Contact::Type::To,
 			Contact::Type::Cc,
 			Contact::Type::Bcc
-		}) {
-		auto addrs{msg.contacts(cctype)};
-		std::move(addrs.begin(), addrs.end(),
+		});
+
+	Contacts contacts;
+	std::ranges::move(ctypes | std::views::transform([&](auto ctype) {
+				return msg.contacts(ctype); })
+			         | std::views::join,
 			  std::back_inserter(contacts));
-	}
 
 	return contacts;
 }

lib/message/mu-mime-object.hh

@@ -1051,16 +1051,23 @@ public:
 	/**< Date type */
 
 	/**
-	 * Gets a date, utf-offset pair if it exists, or nullopt otherwise.
+	 * Gets a { date, utf-offset } pair if it exists, or nullopt otherwise.
 	 *
 	 * The date is the number of seconds since epoch (i.e., unix time),
 	 * while utf-offset is the number of seconds offset from UTC
 	 * (i.e., negative numbers west of GMT, positive numbers for east)
 	 *
-	 * @return a time_t value (expressed as a 64-bit number) or nullopt
+	 * @return a Date pair or nullopt
 	 */
 	Option<Date> date() const noexcept;
 
+	/**
+	 * Gets a { date, utc-offset } pair from the first (more recent)
+	 * Received: header, or nullopt if there is none.
+	 *
+	 * @return a Date pair or nullopt
+	 */
+	Option<Date> received() const noexcept;
 
 	/**
 	 * Get the references for this message (including in-reply-to), in the