Store "from" and "subject" headers in the database.

This is a rebase and cleanup of Istvan Marko's patch from
id:m3pqnj2j7a.fsf@zsu.kismala.com

Search retrieves these headers for every message in the search
results.  Previously, this required opening and parsing every message
file.  Storing them directly in the database significantly reduces IO
and computation, speeding up search by between 50% and 10X.

Taking full advantage of this requires a database rebuild, but it will
fall back to the old behavior for messages that do not have headers
stored in the database.

lib/database.cc

@@ -1725,7 +1725,7 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
 		goto DONE;
 
 	    date = notmuch_message_file_get_header (message_file, "date");
-	    _notmuch_message_set_date (message, date);
+	    _notmuch_message_set_header_values (message, date, from, subject);
 
 	    _notmuch_message_index_file (message, filename);
 	} else {

lib/message.cc

@@ -412,6 +412,21 @@ _notmuch_message_ensure_message_file (notmuch_message_t *message)
 const char *
 notmuch_message_get_header (notmuch_message_t *message, const char *header)
 {
+    std::string value;
+
+    /* Fetch header from the appropriate xapian value field if
+     * available */
+    if (strcasecmp (header, "from") == 0)
+	value = message->doc.get_value (NOTMUCH_VALUE_FROM);
+    else if (strcasecmp (header, "subject") == 0)
+	value = message->doc.get_value (NOTMUCH_VALUE_SUBJECT);
+    else if (strcasecmp (header, "message-id") == 0)
+	value = message->doc.get_value (NOTMUCH_VALUE_MESSAGE_ID);
+
+    if (!value.empty())
+	return talloc_strdup (message, value.c_str ());
+
+    /* Otherwise fall back to parsing the file */
     _notmuch_message_ensure_message_file (message);
     if (message->message_file == NULL)
 	return NULL;
@@ -795,8 +810,10 @@ notmuch_message_set_author (notmuch_message_t *message,
 }
 
 void
-_notmuch_message_set_date (notmuch_message_t *message,
+_notmuch_message_set_header_values (notmuch_message_t *message,
-			   const char *date)
+				    const char *date,
+				    const char *from,
+				    const char *subject)
 {
     time_t time_value;
 
@@ -809,6 +826,8 @@ _notmuch_message_set_date (notmuch_message_t *message,
 
     message->doc.add_value (NOTMUCH_VALUE_TIMESTAMP,
 			    Xapian::sortable_serialise (time_value));
+    message->doc.add_value (NOTMUCH_VALUE_FROM, from);
+    message->doc.add_value (NOTMUCH_VALUE_SUBJECT, subject);
 }
 
 /* Synchronize changes made to message->doc out into the database. */

lib/notmuch-private.h

@@ -93,7 +93,9 @@ NOTMUCH_BEGIN_DECLS
 
 typedef enum {
     NOTMUCH_VALUE_TIMESTAMP = 0,
-    NOTMUCH_VALUE_MESSAGE_ID
+    NOTMUCH_VALUE_MESSAGE_ID,
+    NOTMUCH_VALUE_FROM,
+    NOTMUCH_VALUE_SUBJECT
 } notmuch_value_t;
 
 /* Xapian (with flint backend) complains if we provide a term longer
@@ -269,9 +271,10 @@ void
 _notmuch_message_ensure_thread_id (notmuch_message_t *message);
 
 void
-_notmuch_message_set_date (notmuch_message_t *message,
+_notmuch_message_set_header_values (notmuch_message_t *message,
-			   const char *date);
+				    const char *date,
-
+				    const char *from,
+				    const char *subject);
 void
 _notmuch_message_sync (notmuch_message_t *message);