From 9ad19e4454ff98342a6c0ec5a7e44c8c516e6fcd Mon Sep 17 00:00:00 2001
From: David Bremner <david@tethera.net>
Date: Fri, 16 Apr 2021 09:00:39 -0300
Subject: [PATCH] lib: directly traverse postlists in _n_message_delete

This is intended to fix the slow behaviour of "notmuch new" (and possibly
"notmuch reindex") when large numbers of files are deleted.

The underlying issue [1] seems to be the Xapian glass backend spending
a large amount of time in db.has_positions when running queries with
large-ish amounts of unflushed changes.

This commit removes two uses of Xapian queries [2], and replaces them with
an approximation of what Xapian would do after optimizing the
queries. This avoids the calls to has_positions (which are in any case
un-needed because we are only using boolean terms here).

[1] Thanks to "andres" on IRC for narrowing down the performance
bottleneck.

[2] Thanks to Olly Betts of Xapian fame for talking me a through a fix
that does not require people to update Xapian.
---
 lib/message.cc | 68 +++++++++++++++++++++++++++++---------------------
 1 file changed, 39 insertions(+), 29 deletions(-)

diff --git a/lib/message.cc b/lib/message.cc
index 0c2eeab5..42d56acb 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -1356,11 +1356,10 @@ notmuch_status_t
 _notmuch_message_delete (notmuch_message_t *message)
 {
     notmuch_status_t status;
-    const char *mid, *tid, *query_string;
+    const char *mid, *tid;
     notmuch_message_t *ghost;
     notmuch_private_status_t private_status;
     notmuch_database_t *notmuch;
-    notmuch_query_t *query;
     unsigned int count = 0;
     bool is_ghost;
 
@@ -1382,16 +1381,33 @@ _notmuch_message_delete (notmuch_message_t *message)
     if (is_ghost)
 	return NOTMUCH_STATUS_SUCCESS;
 
-    query_string = talloc_asprintf (message, "thread:%s", tid);
-    query = notmuch_query_create (notmuch, query_string);
-    if (query == NULL)
-	return NOTMUCH_STATUS_OUT_OF_MEMORY;
-    status = notmuch_query_count_messages (query, &count);
-    if (status) {
-	notmuch_query_destroy (query);
-	return status;
-    }
+    /* look for a non-ghost message in the same thread */
+    try {
+	Xapian::PostingIterator thread_doc, thread_doc_end;
+	Xapian::PostingIterator mail_doc, mail_doc_end;
 
+	_notmuch_database_find_doc_ids (message->notmuch, "thread", tid, &thread_doc,
+					&thread_doc_end);
+	_notmuch_database_find_doc_ids (message->notmuch, "type", "mail", &mail_doc, &mail_doc_end);
+
+	while (count == 0 &&
+	       thread_doc != thread_doc_end &&
+	       mail_doc != mail_doc_end) {
+	    thread_doc.skip_to (*mail_doc);
+	    if (thread_doc != thread_doc_end) {
+		if (*thread_doc == *mail_doc) {
+		    count++;
+		} else {
+		    mail_doc.skip_to (*thread_doc);
+		    if (mail_doc != mail_doc_end && *thread_doc == *mail_doc)
+			count++;
+		}
+	    }
+	}
+    } catch (Xapian::Error &error) {
+	LOG_XAPIAN_EXCEPTION (message, error);
+	return NOTMUCH_STATUS_XAPIAN_EXCEPTION;
+    }
     if (count > 0) {
 	/* reintroduce a ghost in its place because there are still
 	 * other active messages in this thread: */
@@ -1410,27 +1426,21 @@ _notmuch_message_delete (notmuch_message_t *message)
 	notmuch_message_destroy (ghost);
 	status = COERCE_STATUS (private_status, "Error converting to ghost message");
     } else {
-	/* the thread is empty; drop all ghost messages from it */
-	notmuch_messages_t *messages;
-	status = _notmuch_query_search_documents (query,
-						  "ghost",
-						  &messages);
-	if (status == NOTMUCH_STATUS_SUCCESS) {
-	    notmuch_status_t last_error = NOTMUCH_STATUS_SUCCESS;
-	    while (notmuch_messages_valid (messages)) {
-		message = notmuch_messages_get (messages);
-		status = _notmuch_message_delete (message);
-		if (status) /* we'll report the last failure we see;
-					 * if there is more than one failure, we
-					 * forget about previous ones */
-		    last_error = status;
-		notmuch_message_destroy (message);
-		notmuch_messages_move_to_next (messages);
+	/* the thread now contains only ghosts: delete them */
+	try {
+	    Xapian::PostingIterator doc, doc_end;
+
+	    _notmuch_database_find_doc_ids (message->notmuch, "thread", tid, &doc, &doc_end);
+
+	    for (; doc != doc_end; doc++) {
+		message->notmuch->writable_xapian_db->delete_document (*doc);
 	    }
-	    status = last_error;
+	} catch (Xapian::Error &error) {
+	    LOG_XAPIAN_EXCEPTION (message, error);
+	    return NOTMUCH_STATUS_XAPIAN_EXCEPTION;
 	}
+
     }
-    notmuch_query_destroy (query);
     return status;
 }