[PATCH v3 5/9] lib: Implement ghost-based thread linking

[Austin Clements <aclements@csail.mit.edu>]

From: Austin Clements <amdragon@mit.edu>

This updates the thread linking code to use ghost messages instead of
user metadata to link messages into threads.

In contrast with the old approach, this is actually correct.
Previously, thread merging updated only the thread IDs of message
documents, not thread IDs stored in user metadata.  As originally
diagnosed by Mark Walters [1] and as demonstrated by the broken
T260-thread-order test, this can cause notmuch to fail to link
messages even though they're in the same thread.  In principle the old
approach could have been fixed by updating the user metadata thread
IDs as well, but these are not indexed and hence this would have
required a full scan of all stored thread IDs.  Ghost messages solve
this problem naturally by reusing the exact same thread ID and message
ID representation and indexing as regular messages.

Furthermore, thanks to this greater symmetry, ghost messages are also
algorithmically simpler.  We continue to support the old user metadata
format, so this patch can't delete any code, but when we do remove
support for the old format, several functions can simply be deleted.

[1] id:8738h7kv2q.fsf@qmul.ac.uk
---
 lib/database.cc | 99 +++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 83 insertions(+), 16 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index c641bcd..92a92d9 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -1752,6 +1752,12 @@ _get_metadata_thread_id_key (void *ctx, const char *message_id)
 			    message_id);
 }
 
+static notmuch_status_t
+_resolve_message_id_to_thread_id_old (notmuch_database_t *notmuch,
+				      void *ctx,
+				      const char *message_id,
+				      const char **thread_id_ret);
+
 /* Find the thread ID to which the message with 'message_id' belongs.
  *
  * Note: 'thread_id_ret' must not be NULL!
@@ -1760,9 +1766,9 @@ _get_metadata_thread_id_key (void *ctx, const char *message_id)
  *
  * Note: If there is no message in the database with the given
  * 'message_id' then a new thread_id will be allocated for this
- * message and stored in the database metadata, (where this same
+ * message ID and stored in the database metadata so that the
  * thread ID can be looked up if the message is added to the database
- * later).
+ * later.
  */
 static notmuch_status_t
 _resolve_message_id_to_thread_id (notmuch_database_t *notmuch,
@@ -1770,6 +1776,49 @@ _resolve_message_id_to_thread_id (notmuch_database_t *notmuch,
 				  const char *message_id,
 				  const char **thread_id_ret)
 {
+    notmuch_private_status_t status;
+    notmuch_message_t *message;
+
+    if (! (notmuch->features & NOTMUCH_FEATURE_GHOSTS))
+	return _resolve_message_id_to_thread_id_old (notmuch, ctx, message_id,
+						     thread_id_ret);
+
+    /* Look for this message (regular or ghost) */
+    message = _notmuch_message_create_for_message_id (
+	notmuch, message_id, &status);
+    if (status == NOTMUCH_PRIVATE_STATUS_SUCCESS) {
+	/* Message exists */
+	*thread_id_ret = talloc_steal (
+	    ctx, notmuch_message_get_thread_id (message));
+    } else if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
+	/* Message did not exist.  Give it a fresh thread ID and
+	 * populate this message as a ghost message. */
+	*thread_id_ret = talloc_strdup (
+	    ctx, _notmuch_database_generate_thread_id (notmuch));
+	if (! *thread_id_ret) {
+	    status = NOTMUCH_PRIVATE_STATUS_OUT_OF_MEMORY;
+	} else {
+	    status = _notmuch_message_initialize_ghost (message, *thread_id_ret);
+	    if (status == 0)
+		/* Commit the new ghost message */
+		_notmuch_message_sync (message);
+	}
+    } else {
+	/* Create failed. Fall through. */
+    }
+
+    notmuch_message_destroy (message);
+
+    return COERCE_STATUS (status, "Error creating ghost message");
+}
+
+/* Pre-ghost messages _resolve_message_id_to_thread_id */
+static notmuch_status_t
+_resolve_message_id_to_thread_id_old (notmuch_database_t *notmuch,
+				      void *ctx,
+				      const char *message_id,
+				      const char **thread_id_ret)
+{
     notmuch_status_t status;
     notmuch_message_t *message;
     string thread_id_string;
@@ -2007,13 +2056,16 @@ _consume_metadata_thread_id (void *ctx, notmuch_database_t *notmuch,
     }
 }
 
-/* Given a (mostly empty) 'message' and its corresponding
+/* Given a blank or ghost 'message' and its corresponding
  * 'message_file' link it to existing threads in the database.
  *
- * The first check is in the metadata of the database to see if we
- * have pre-allocated a thread_id in advance for this message, (which
- * would have happened if a message was previously added that
- * referenced this one).
+ * First, if is_ghost, this retrieves the thread ID already stored in
+ * the message (which will be the case if a message was previously
+ * added that referenced this one).  If the message is blank
+ * (!is_ghost), it doesn't have a thread ID yet (we'll generate one
+ * later in this function).  If the database does not support ghost
+ * messages, this checks for a thread ID stored in database metadata
+ * for this message ID.
  *
  * Second, we look at 'message_file' and its link-relevant headers
  * (References and In-Reply-To) for message IDs.
@@ -2021,7 +2073,7 @@ _consume_metadata_thread_id (void *ctx, notmuch_database_t *notmuch,
  * Finally, we look in the database for existing message that
  * reference 'message'.
  *
- * In all cases, we assign to the current message the first thread_id
+ * In all cases, we assign to the current message the first thread ID
  * found (through either parent or child). We will also merge any
  * existing, distinct threads where this message belongs to both,
  * (which is not uncommon when messages are processed out of order).
@@ -2035,16 +2087,22 @@ _consume_metadata_thread_id (void *ctx, notmuch_database_t *notmuch,
 static notmuch_status_t
 _notmuch_database_link_message (notmuch_database_t *notmuch,
 				notmuch_message_t *message,
-				notmuch_message_file_t *message_file)
+				notmuch_message_file_t *message_file,
+				notmuch_bool_t is_ghost)
 {
     void *local = talloc_new (NULL);
     notmuch_status_t status;
-    const char *thread_id;
+    const char *thread_id = NULL;
 
     /* Check if the message already had a thread ID */
-    thread_id = _consume_metadata_thread_id (local, notmuch, message);
-    if (thread_id)
-	_notmuch_message_add_term (message, "thread", thread_id);
+    if (notmuch->features & NOTMUCH_FEATURE_GHOSTS) {
+	if (is_ghost)
+	    thread_id = notmuch_message_get_thread_id (message);
+    } else {
+	thread_id = _consume_metadata_thread_id (local, notmuch, message);
+	if (thread_id)
+	    _notmuch_message_add_term (message, "thread", thread_id);
+    }
 
     status = _notmuch_database_link_message_to_parents (notmuch, message,
 							message_file,
@@ -2079,6 +2137,7 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
     notmuch_message_t *message = NULL;
     notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS, ret2;
     notmuch_private_status_t private_status;
+    notmuch_bool_t is_ghost = false;
 
     const char *date, *header;
     const char *from, *to, *subject;
@@ -2171,12 +2230,20 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
 
 	_notmuch_message_add_filename (message, filename);
 
-	/* Is this a newly created message object? */
-	if (private_status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
+	/* Is this a newly created message object or a ghost
+	 * message?  We have to be slightly careful: if this is a
+	 * blank message, it's not safe to call
+	 * notmuch_message_get_flag yet. */
+	if (private_status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND ||
+	    (is_ghost = notmuch_message_get_flag (
+		message, NOTMUCH_MESSAGE_FLAG_GHOST))) {
 	    _notmuch_message_add_term (message, "type", "mail");
+	    if (is_ghost)
+		/* Convert ghost message to a regular message */
+		_notmuch_message_remove_term (message, "type", "ghost");
 
 	    ret = _notmuch_database_link_message (notmuch, message,
-						  message_file);
+						  message_file, is_ghost);
 	    if (ret)
 		goto DONE;
 
-- 
2.1.0