[PATCH 00/11] Add ghost messages and fix thread linking

[PATCH 00/11] Add ghost messages and fix thread linking

[Austin Clements]

This series modifies our database representation of messages that have
been referenced by other messages, but for which we don't have the
message itself.  Currently, we store this information as Xapian
metadata, but this has several downsides for performance and
complexity and results in hard-to-fix thread linking bugs.  This patch
series implements "ghost messages", which replace this Xapian metadata
with Xapian documents that look and act very much like regular message
documents, but simply have no content.  This simplifies and speeds up
our thread linking algorithm and fixes the currently broken thread
linking test.

Ghost messages also open up interesting future possibilities, such as
"pre-seeding" tags for messages that are not yet indexed.  This could
be used to make notmuch insert simpler and more robust, as part of tag
synchronization, and to improve nmbug's behavior when tags arrive
before messages.

[PATCH 01/11] lib: Move message ID compression to _notmuch_message_create_for_message_id

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

Previously, this was performed by notmuch_database_add_message.  This
happens to be the only caller currently (which is why this was safe),
but we're about to introduce more callers, and it makes more sense to
put responsibility for ID compression in the lower-level function
rather than requiring each caller to handle it.
---
 lib/database.cc       | 16 ++++------------
 lib/message.cc        |  4 ++++
 lib/notmuch-private.h |  3 +++
 3 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index a47a71d..4e68706 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -390,8 +390,8 @@ find_document_for_doc_id (notmuch_database_t *notmuch, unsigned doc_id)
  *
  *	notmuch-sha1-<sha1_sum_of_message_id>
  */
-static char *
-_message_id_compressed (void *ctx, const char *message_id)
+char *
+_notmuch_message_id_compressed (void *ctx, const char *message_id)
 {
     char *sha1, *compressed;
 
@@ -415,7 +415,7 @@ notmuch_database_find_message (notmuch_database_t *notmuch,
 	return NOTMUCH_STATUS_NULL_POINTER;
 
     if (strlen (message_id) > NOTMUCH_MESSAGE_ID_MAX)
-	message_id = _message_id_compressed (notmuch, message_id);
+	message_id = _notmuch_message_id_compressed (notmuch, message_id);
 
     try {
 	status = _notmuch_database_find_unique_doc_id (notmuch, "id",
@@ -1728,7 +1728,7 @@ static char *
 _get_metadata_thread_id_key (void *ctx, const char *message_id)
 {
     if (strlen (message_id) > NOTMUCH_MESSAGE_ID_MAX)
-	message_id = _message_id_compressed (ctx, message_id);
+	message_id = _notmuch_message_id_compressed (ctx, message_id);
 
     return talloc_asprintf (ctx, NOTMUCH_METADATA_THREAD_ID_PREFIX "%s",
 			    message_id);
@@ -2100,14 +2100,6 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
 	     * better than no message-id at all. */
 	    if (message_id == NULL)
 		message_id = talloc_strdup (message_file, header);
-
-	    /* If a message ID is too long, substitute its sha1 instead. */
-	    if (message_id && strlen (message_id) > NOTMUCH_MESSAGE_ID_MAX) {
-		char *compressed = _message_id_compressed (message_file,
-							   message_id);
-		talloc_free (message_id);
-		message_id = compressed;
-	    }
 	}
 
 	if (message_id == NULL ) {
diff --git a/lib/message.cc b/lib/message.cc
index 7e82548..bbfc250 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -226,6 +226,10 @@ _notmuch_message_create_for_message_id (notmuch_database_t *notmuch,
     else if (*status_ret)
 	return NULL;
 
+    /* If the message ID is too long, substitute its sha1 instead. */
+    if (strlen (message_id) > NOTMUCH_MESSAGE_ID_MAX)
+	message_id = _notmuch_message_id_compressed (message, message_id);
+
     term = talloc_asprintf (NULL, "%s%s",
 			    _find_prefix ("id"), message_id);
     if (term == NULL) {
diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index 17f3061..36cc12b 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -174,6 +174,9 @@ typedef struct _notmuch_doc_id_set notmuch_doc_id_set_t;
 const char *
 _find_prefix (const char *name);
 
+char *
+_notmuch_message_id_compressed (void *ctx, const char *message_id);
+
 notmuch_status_t
 _notmuch_database_ensure_writable (notmuch_database_t *notmuch);
 
-- 
2.1.0

[PATCH 02/11] lib: Refactor _notmuch_database_link_message

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

This moves the code to retrieve and clear the metadata thread ID out
of _notmuch_database_link_message into its own function.  This will
simplify future changes.
---
 lib/database.cc | 69 +++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 43 insertions(+), 26 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index 4e68706..1c6ffc5 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -1958,6 +1958,37 @@ _notmuch_database_link_message_to_children (notmuch_database_t *notmuch,
     return ret;
 }
 
+/* Fetch and clear the stored thread_id for message, or NULL if none. */
+static char *
+_consume_metadata_thread_id (void *ctx, notmuch_database_t *notmuch,
+			     notmuch_message_t *message)
+{
+    const char *message_id;
+    string stored_id;
+    char *metadata_key;
+
+    message_id = notmuch_message_get_message_id (message);
+    metadata_key = _get_metadata_thread_id_key (ctx, message_id);
+
+    /* Check if we have already seen related messages to this one.
+     * If we have then use the thread_id that we stored at that time.
+     */
+    stored_id = notmuch->xapian_db->get_metadata (metadata_key);
+    if (stored_id.empty ()) {
+	return NULL;
+    } else {
+        Xapian::WritableDatabase *db;
+
+	db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
+
+	/* Clear the metadata for this message ID. We don't need it
+	 * anymore. */
+        db->set_metadata (metadata_key, "");
+
+        return talloc_strdup (ctx, stored_id.c_str ());
+    }
+}
+
 /* Given a (mostly empty) 'message' and its corresponding
  * 'message_file' link it to existing threads in the database.
  *
@@ -1988,42 +2019,25 @@ _notmuch_database_link_message (notmuch_database_t *notmuch,
 				notmuch_message_t *message,
 				notmuch_message_file_t *message_file)
 {
+    void *local = talloc_new (NULL);
     notmuch_status_t status;
-    const char *message_id, *thread_id = NULL;
-    char *metadata_key;
-    string stored_id;
-
-    message_id = notmuch_message_get_message_id (message);
-    metadata_key = _get_metadata_thread_id_key (message, message_id);
-
-    /* Check if we have already seen related messages to this one.
-     * If we have then use the thread_id that we stored at that time.
-     */
-    stored_id = notmuch->xapian_db->get_metadata (metadata_key);
-    if (! stored_id.empty()) {
-        Xapian::WritableDatabase *db;
-
-	db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
-
-	/* Clear the metadata for this message ID. We don't need it
-	 * anymore. */
-        db->set_metadata (metadata_key, "");
-        thread_id = stored_id.c_str();
+    const char *thread_id;
 
-        _notmuch_message_add_term (message, "thread", thread_id);
-    }
-    talloc_free (metadata_key);
+    /* 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);
 
     status = _notmuch_database_link_message_to_parents (notmuch, message,
 							message_file,
 							&thread_id);
     if (status)
-	return status;
+	goto DONE;
 
     status = _notmuch_database_link_message_to_children (notmuch, message,
 							 &thread_id);
     if (status)
-	return status;
+	goto DONE;
 
     /* If not part of any existing thread, generate a new thread ID. */
     if (thread_id == NULL) {
@@ -2032,7 +2046,10 @@ _notmuch_database_link_message (notmuch_database_t *notmuch,
 	_notmuch_message_add_term (message, "thread", thread_id);
     }
 
-    return NOTMUCH_STATUS_SUCCESS;
+ DONE:
+    talloc_free (local);
+
+    return status;
 }
 
 notmuch_status_t
-- 
2.1.0

Re: [PATCH 02/11] lib: Refactor _notmuch_database_link_message

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:
> +    void *local = talloc_new (NULL);

What's the advantage of using a local talloc context here? Is this just
an optimization?

d

Re: [PATCH 02/11] lib: Refactor _notmuch_database_link_message

[Austin Clements]

Quoth David Bremner on Oct 05 at  9:45 am:
> Austin Clements <aclements@csail.mit.edu> writes:
> > +    void *local = talloc_new (NULL);
> 
> What's the advantage of using a local talloc context here? Is this just
> an optimization?

There are a few allocations that wind up going in to this local
context because of the call to _consume_metadata_thread_id, so it's
more convenient to free this one context on return from
_notmuch_database_link_message than to worry about tracking these
various allocations.

Re: [PATCH 02/11] lib: Refactor _notmuch_database_link_message

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:

> Quoth David Bremner on Oct 05 at  9:45 am:
>> Austin Clements <aclements@csail.mit.edu> writes:
>> > +    void *local = talloc_new (NULL);
>> 
>> What's the advantage of using a local talloc context here? Is this just
>> an optimization?
>
> There are a few allocations that wind up going in to this local
> context because of the call to _consume_metadata_thread_id, so it's
> more convenient to free this one context on return from
> _notmuch_database_link_message than to worry about tracking these
> various allocations.

OK, but wouldn't the lazy solution be to use message as a talloc
context?

d

Re: [PATCH 02/11] lib: Refactor _notmuch_database_link_message

[Austin Clements]

Quoth David Bremner on Oct 06 at  8:04 am:
> Austin Clements <aclements@csail.mit.edu> writes:
> 
> > Quoth David Bremner on Oct 05 at  9:45 am:
> >> Austin Clements <aclements@csail.mit.edu> writes:
> >> > +    void *local = talloc_new (NULL);
> >> 
> >> What's the advantage of using a local talloc context here? Is this just
> >> an optimization?
> >
> > There are a few allocations that wind up going in to this local
> > context because of the call to _consume_metadata_thread_id, so it's
> > more convenient to free this one context on return from
> > _notmuch_database_link_message than to worry about tracking these
> > various allocations.
> 
> OK, but wouldn't the lazy solution be to use message as a talloc
> context?

That would be the lazy solution, but it would also leak a bunch of
allocations that don't need to live past the end of
_notmuch_database_link_message.

[PATCH 03/11] lib: Handle empty date value

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

In the interest of robustness, avoid undefined behavior of
sortable_unserialise if the date value is missing.  This shouldn't
happen now, but ghost messages will have blank date values.
---
 lib/message.cc | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/message.cc b/lib/message.cc
index bbfc250..38bc929 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -896,6 +896,9 @@ notmuch_message_get_date (notmuch_message_t *message)
 	return 0;
     }
 
+    if (value.empty ())
+	/* sortable_unserialise is undefined on empty string */
+	return 0;
     return Xapian::sortable_unserialise (value);
 }
 
-- 
2.1.0

[PATCH 04/11] lib: Add a ghost messages database feature

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

This will be implemented over the next several patches.  The feature
is not yet "enabled" (this does not add it to
NOTMUCH_FEATURES_CURRENT).
---
 lib/database-private.h | 7 +++++++
 lib/database.cc        | 2 ++
 2 files changed, 9 insertions(+)

diff --git a/lib/database-private.h b/lib/database-private.h
index ca0751c..e2e4bc8 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -85,6 +85,13 @@ enum _notmuch_features {
      *
      * Introduced: version 2. */
     NOTMUCH_FEATURE_BOOL_FOLDER = 1 << 3,
+
+    /* If set, missing messages are stored in ghost mail documents.
+     * If unset, thread IDs of ghost messages are stored as database
+     * metadata instead of in ghost documents.
+     *
+     * Introduced: version 3. */
+    NOTMUCH_FEATURE_GHOSTS = 1 << 4,
 };
 
 /* In C++, a named enum is its own type, so define bitwise operators
diff --git a/lib/database.cc b/lib/database.cc
index 1c6ffc5..8fd7fad 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -286,6 +286,8 @@ static const struct {
       "from/subject/message-ID in database", "w" },
     { NOTMUCH_FEATURE_BOOL_FOLDER,
       "exact folder:/path: search", "rw" },
+    { NOTMUCH_FEATURE_GHOSTS,
+      "mail documents for missing messages", "w"},
 };
 
 const char *
-- 
2.1.0

[PATCH 05/11] lib: Update database schema doc for ghost messages

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

This describes the structure of ghost mail documents.  Ghost messages
are not yet implemented.
---
 lib/database.cc | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index 8fd7fad..c641bcd 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -50,8 +50,8 @@ typedef struct {
 
 /* Here's the current schema for our database (for NOTMUCH_DATABASE_VERSION):
  *
- * We currently have two different types of documents (mail and
- * directory) and also some metadata.
+ * We currently have three different types of documents (mail, ghost,
+ * and directory) and also some metadata.
  *
  * Mail document
  * -------------
@@ -109,6 +109,15 @@ typedef struct {
  *
  * The data portion of a mail document is empty.
  *
+ * Ghost mail document [if NOTMUCH_FEATURE_GHOSTS]
+ * -----------------------------------------------
+ * A ghost mail document is like a mail document, but where we don't
+ * have the message content.  These are used to track thread reference
+ * information for messages we haven't received.
+ *
+ * A ghost mail document has type: ghost; id and thread fields that
+ * are identical to the mail document fields; and a MESSAGE_ID value.
+ *
  * Directory document
  * ------------------
  * A directory document is used by a client of the notmuch library to
@@ -172,6 +181,13 @@ typedef struct {
  *			generated is 1 and the value will be
  *			incremented for each thread ID.
  *
+ * Obsolete metadata
+ * -----------------
+ *
+ * If ! NOTMUCH_FEATURE_GHOSTS, there are no ghost mail documents.
+ * Instead, the database has the following additional database
+ * metadata:
+ *
  *	thread_id_*	A pre-allocated thread ID for a particular
  *			message. This is actually an arbitrarily large
  *			family of metadata name. Any particular name is
-- 
2.1.0

[PATCH 06/11] lib: Internal support for querying and creating ghost messages

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

This updates the message abstraction to support ghost messages: it
adds a message flag that distinguishes regular messages from ghost
messages, and an internal function for initializing a newly created
(blank) message as a ghost message.
---
 lib/message.cc        | 50 ++++++++++++++++++++++++++++++++++++++++++++++++--
 lib/notmuch-private.h |  4 ++++
 lib/notmuch.h         |  9 ++++++++-
 3 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/lib/message.cc b/lib/message.cc
index 38bc929..ad832cf 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -39,6 +39,9 @@ struct visible _notmuch_message {
     notmuch_message_file_t *message_file;
     notmuch_message_list_t *replies;
     unsigned long flags;
+    /* For flags that are initialized on-demand, lazy_flags indicates
+     * if each flag has been initialized. */
+    unsigned long lazy_flags;
 
     Xapian::Document doc;
     Xapian::termcount termpos;
@@ -99,6 +102,7 @@ _notmuch_message_create_for_document (const void *talloc_owner,
 
     message->frozen = 0;
     message->flags = 0;
+    message->lazy_flags = 0;
 
     /* Each of these will be lazily created as needed. */
     message->message_id = NULL;
@@ -192,7 +196,7 @@ _notmuch_message_create (const void *talloc_owner,
  *
  *     There is already a document with message ID 'message_id' in the
  *     database. The returned message can be used to query/modify the
- *     document.
+ *     document. The message may be a ghost message.
  *
  *   NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND:
  *
@@ -305,6 +309,7 @@ _notmuch_message_ensure_metadata (notmuch_message_t *message)
     const char *thread_prefix = _find_prefix ("thread"),
 	*tag_prefix = _find_prefix ("tag"),
 	*id_prefix = _find_prefix ("id"),
+	*type_prefix = _find_prefix ("type"),
 	*filename_prefix = _find_prefix ("file-direntry"),
 	*replyto_prefix = _find_prefix ("replyto");
 
@@ -337,10 +342,23 @@ _notmuch_message_ensure_metadata (notmuch_message_t *message)
 	message->message_id =
 	    _notmuch_message_get_term (message, i, end, id_prefix);
 
+    /* Get document type */
+    assert (strcmp (id_prefix, type_prefix) < 0);
+    if (! (message->lazy_flags & (1 << NOTMUCH_MESSAGE_FLAG_GHOST))) {
+	i.skip_to (type_prefix);
+	if (*i == "Tmail")
+	    message->flags &= ~(1 << NOTMUCH_MESSAGE_FLAG_GHOST);
+	else if (*i == "Tghost")
+	    message->flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
+	else
+	    INTERNAL_ERROR ("Message without type term");
+	message->lazy_flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
+    }
+
     /* Get filename list.  Here we get only the terms.  We lazily
      * expand them to full file names when needed in
      * _notmuch_message_ensure_filename_list. */
-    assert (strcmp (id_prefix, filename_prefix) < 0);
+    assert (strcmp (type_prefix, filename_prefix) < 0);
     if (!message->filename_term_list && !message->filename_list)
 	message->filename_term_list =
 	    _notmuch_database_get_terms_with_prefix (message, i, end,
@@ -371,6 +389,11 @@ _notmuch_message_invalidate_metadata (notmuch_message_t *message,
 	message->tag_list = NULL;
     }
 
+    if (strcmp ("type", prefix_name) == 0) {
+	message->flags &= ~(1 << NOTMUCH_MESSAGE_FLAG_GHOST);
+	message->lazy_flags &= ~(1 << NOTMUCH_MESSAGE_FLAG_GHOST);
+    }
+
     if (strcmp ("file-direntry", prefix_name) == 0) {
 	talloc_free (message->filename_term_list);
 	talloc_free (message->filename_list);
@@ -869,6 +892,10 @@ notmuch_bool_t
 notmuch_message_get_flag (notmuch_message_t *message,
 			  notmuch_message_flag_t flag)
 {
+    if (flag == NOTMUCH_MESSAGE_FLAG_GHOST &&
+	! (message->lazy_flags & (1 << flag)))
+	_notmuch_message_ensure_metadata (message);
+
     return message->flags & (1 << flag);
 }
 
@@ -880,6 +907,7 @@ notmuch_message_set_flag (notmuch_message_t *message,
 	message->flags |= (1 << flag);
     else
 	message->flags &= ~(1 << flag);
+    message->lazy_flags |= (1 << flag);
 }
 
 time_t
@@ -989,6 +1017,24 @@ _notmuch_message_delete (notmuch_message_t *message)
     return NOTMUCH_STATUS_SUCCESS;
 }
 
+/* Transform a blank message into a ghost message.  The caller must
+ * _notmuch_message_sync the message. */
+notmuch_private_status_t
+_notmuch_message_initialize_ghost (notmuch_message_t *message,
+				   const char *thread_id)
+{
+    notmuch_private_status_t status;
+
+    status = _notmuch_message_add_term (message, "type", "ghost");
+    if (status)
+	return status;
+    status = _notmuch_message_add_term (message, "thread", thread_id);
+    if (status)
+	return status;
+
+    return NOTMUCH_PRIVATE_STATUS_SUCCESS;
+}
+
 /* Ensure that 'message' is not holding any file object open. Future
  * calls to various functions will still automatically open the
  * message file as needed.
diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index 36cc12b..2fbd38e 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -297,6 +297,10 @@ _notmuch_message_sync (notmuch_message_t *message);
 notmuch_status_t
 _notmuch_message_delete (notmuch_message_t *message);
 
+notmuch_private_status_t
+_notmuch_message_initialize_ghost (notmuch_message_t *message,
+				   const char *thread_id);
+
 void
 _notmuch_message_close (notmuch_message_t *message);
 
diff --git a/lib/notmuch.h b/lib/notmuch.h
index dae0416..92594b9 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -1221,7 +1221,14 @@ notmuch_message_get_filenames (notmuch_message_t *message);
  */
 typedef enum _notmuch_message_flag {
     NOTMUCH_MESSAGE_FLAG_MATCH,
-    NOTMUCH_MESSAGE_FLAG_EXCLUDED
+    NOTMUCH_MESSAGE_FLAG_EXCLUDED,
+
+    /* This message is a "ghost message", meaning it has no filenames
+     * or content, but we know it exists because it was referenced by
+     * some other message.  A ghost message has only a message ID and
+     * thread ID.
+     */
+    NOTMUCH_MESSAGE_FLAG_GHOST,
 } notmuch_message_flag_t;
 
 /**
-- 
2.1.0

Re: [PATCH 06/11] lib: Internal support for querying and creating ghost messages

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:

> +	    message->flags &= ~(1 << NOTMUCH_MESSAGE_FLAG_GHOST);

What do you think about using bit set / clear / read macros?  I don't
insist, but I wonder if it would make this part more readable.

> +	else if (*i == "Tghost")
> +	    message->flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
> +	else

It makes me faintly unhappy to have the prefix hardcoded here.
Not sure if there is a sensible solution.

Re: [PATCH 06/11] lib: Internal support for querying and creating ghost messages

[Austin Clements]

Quoth David Bremner on Oct 05 at 10:30 am:
> Austin Clements <aclements@csail.mit.edu> writes:
> 
> > +	    message->flags &= ~(1 << NOTMUCH_MESSAGE_FLAG_GHOST);
> 
> What do you think about using bit set / clear / read macros?  I don't
> insist, but I wonder if it would make this part more readable.

I'm used to reading this stuff, so either way is fine with me.  Do we
have bit set / clear / read macros?

> > +	else if (*i == "Tghost")
> > +	    message->flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
> > +	else
> 
> It makes me faintly unhappy to have the prefix hardcoded here.
> Not sure if there is a sensible solution.

I agree, but I also don't want to construct the test string every time
or deconstruct the term string every time.  I could move the "T"
prefix string to a #define and use that both here and in
BOOLEAN_PREFIX_INTERNAL, but that solution may be worse than the
problem.  What do you think?

Re: [PATCH 06/11] lib: Internal support for querying and creating ghost messages

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:

>
> I'm used to reading this stuff, so either way is fine with me.  Do we
> have bit set / clear / read macros?
>

I guess not. the things we have in query.cc are related but different.

>> > +	else if (*i == "Tghost")
>> > +	    message->flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
>> > +	else
>> 
>> It makes me faintly unhappy to have the prefix hardcoded here.
>> Not sure if there is a sensible solution.
>
> I agree, but I also don't want to construct the test string every time
> or deconstruct the term string every time.  I could move the "T"
> prefix string to a #define and use that both here and in
> BOOLEAN_PREFIX_INTERNAL, but that solution may be worse than the
> problem.  What do you think?

Maybe just a comment to point to BOOLEAN_PREFIX_INTERNAL.

Or maybe define a macro right beside BOOLEAN_PREFIX_INTERNAL like

#define ADD_TYPE_PREFIX(s) "T" s

At least then the duplication is all in one place.

Re: [PATCH 06/11] lib: Internal support for querying and creating ghost messages

[Austin Clements]

On Mon, 06 Oct 2014, David Bremner <david@tethera.net> wrote:
> Austin Clements <aclements@csail.mit.edu> writes:
>
>>
>> I'm used to reading this stuff, so either way is fine with me.  Do we
>> have bit set / clear / read macros?
>>
>
> I guess not. the things we have in query.cc are related but different.

I added some macros for doing this to notmuch-private.h and converted
the other bit twiddling for message flags to use these.

>>> > +	else if (*i == "Tghost")
>>> > +	    message->flags |= (1 << NOTMUCH_MESSAGE_FLAG_GHOST);
>>> > +	else
>>> 
>>> It makes me faintly unhappy to have the prefix hardcoded here.
>>> Not sure if there is a sensible solution.
>>
>> I agree, but I also don't want to construct the test string every time
>> or deconstruct the term string every time.  I could move the "T"
>> prefix string to a #define and use that both here and in
>> BOOLEAN_PREFIX_INTERNAL, but that solution may be worse than the
>> problem.  What do you think?
>
> Maybe just a comment to point to BOOLEAN_PREFIX_INTERNAL.
>
> Or maybe define a macro right beside BOOLEAN_PREFIX_INTERNAL like
>
> #define ADD_TYPE_PREFIX(s) "T" s
>
> At least then the duplication is all in one place.

A #define by BOOLEAN_PREFIX_INTERNAL won't help because
BOOLEAN_PREFIX_INTERNAL lives in database.cc and this code is in
message.cc.  I would have to put the #define in one of the private
headers, but I could use it in BOOLEAN_PREFIX_INTERNAL so there wouldn't
be any duplication of the "T" string.

I added a comment pointing to BOOLEAN_PREFIX_INTERNAL.  Maybe that's
enough?

I'll post v2 later today, when I can run the test suite (currently
running on battery).

[PATCH 07/11] lib: Implement ghost-based thread linking

[Austin Clements]

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 | 86 +++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 75 insertions(+), 11 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index c641bcd..fdcc526 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;
@@ -2007,7 +2056,7 @@ _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
@@ -2035,16 +2084,22 @@ _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 +2134,7 @@ 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 +2227,20 @@ 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

[PATCH 08/11] lib: Implement upgrade to ghost messages feature

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

Somehow this is the first upgrade pass that actually does *any* error
checking, so this also adds the bit of necessary infrastructure to
handle that.
---
 lib/database.cc | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 62 insertions(+), 2 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index fdcc526..ff6a7f6 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -1231,6 +1231,7 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
     notmuch_bool_t timer_is_active = FALSE;
     enum _notmuch_features target_features, new_features;
     notmuch_status_t status;
+    notmuch_private_status_t private_status;
     unsigned int count = 0, total = 0;
 
     status = _notmuch_database_ensure_writable (notmuch);
@@ -1275,6 +1276,11 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
 	for (t = db->allterms_begin ("XTIMESTAMP"); t != t_end; t++)
 	    ++total;
     }
+    if (new_features & NOTMUCH_FEATURE_GHOSTS) {
+	t_end = db->metadata_keys_end ("thread_id_");
+	for (t = db->metadata_keys_begin ("thread_id_"); t != t_end; ++t)
+	    ++total;
+    }
 
     /* Perform the upgrade in a transaction. */
     db->begin_transaction (true);
@@ -1378,10 +1384,64 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
 	}
     }
 
+    /* Perform metadata upgrades. */
+
+    /* Prior to NOTMUCH_FEATURE_GHOSTS, thread IDs for missing
+     * messages were stored as database metadata. Change these to
+     * ghost messages.
+     */
+    if (new_features & NOTMUCH_FEATURE_GHOSTS) {
+	notmuch_message_t *message;
+	std::string message_id, thread_id;
+
+	t_end = db->metadata_keys_end (NOTMUCH_METADATA_THREAD_ID_PREFIX);
+	for (t = db->metadata_keys_begin (NOTMUCH_METADATA_THREAD_ID_PREFIX);
+	     t != t_end; ++t) {
+	    if (do_progress_notify) {
+		progress_notify (closure, (double) count / total);
+		do_progress_notify = 0;
+	    }
+
+	    message_id = (*t).substr (
+		strlen (NOTMUCH_METADATA_THREAD_ID_PREFIX));
+	    thread_id = db->get_metadata (*t);
+
+	    /* Create ghost message */
+	    message = _notmuch_message_create_for_message_id (
+		notmuch, message_id.c_str (), &private_status);
+	    if (private_status == NOTMUCH_PRIVATE_STATUS_SUCCESS) {
+		/* Document already exists; ignore the stored thread ID */
+	    } else if (private_status ==
+		       NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
+		private_status = _notmuch_message_initialize_ghost (
+		    message, thread_id.c_str ());
+		if (! private_status)
+		    _notmuch_message_sync (message);
+	    }
+
+	    if (private_status) {
+		fprintf (stderr,
+			 "Upgrade failed while creating ghost messages.\n");
+		status = COERCE_STATUS (private_status, "Unexpected status from _notmuch_message_initialize_ghost");
+		goto DONE;
+	    }
+
+	    /* Clear saved metadata thread ID */
+	    db->set_metadata (*t, "");
+
+	    ++count;
+	}
+    }
+
+    status = NOTMUCH_STATUS_SUCCESS;
     db->set_metadata ("features", _print_features (local, notmuch->features));
     db->set_metadata ("version", STRINGIFY (NOTMUCH_DATABASE_VERSION));
 
-    db->commit_transaction ();
+ DONE:
+    if (status == NOTMUCH_STATUS_SUCCESS)
+	db->commit_transaction ();
+    else
+	db->cancel_transaction ();
 
     if (timer_is_active) {
 	/* Now stop the timer. */
@@ -1397,7 +1457,7 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
     }
 
     talloc_free (local);
-    return NOTMUCH_STATUS_SUCCESS;
+    return status;
 }
 
 notmuch_status_t
-- 
2.1.0

Re: [PATCH 08/11] lib: Implement upgrade to ghost messages feature

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:
> +    if (new_features & NOTMUCH_FEATURE_GHOSTS) {
> +	t_end = db->metadata_keys_end ("thread_id_");
> +	for (t = db->metadata_keys_begin ("thread_id_"); t != t_end; ++t)
> +	    ++total;
> +    }

It would be nice to have the comment below, or something like it, for
the loop above.

> +    /* Perform metadata upgrades. */
> +
> +    /* Prior to NOTMUCH_FEATURE_GHOSTS, thread IDs for missing
> +     * messages were stored as database metadata. Change these to
> +     * ghost messages.
> +     */

Re: [PATCH 08/11] lib: Implement upgrade to ghost messages feature

[Austin Clements]

Quoth David Bremner on Oct 05 at 10:56 am:
> Austin Clements <aclements@csail.mit.edu> writes:
> > +    if (new_features & NOTMUCH_FEATURE_GHOSTS) {
> > +	t_end = db->metadata_keys_end ("thread_id_");
> > +	for (t = db->metadata_keys_begin ("thread_id_"); t != t_end; ++t)
> > +	    ++total;
> > +    }
> 
> It would be nice to have the comment below, or something like it, for
> the loop above.

  /* The ghost message upgrade converts all thread_id_*
   * metadata values into ghost message documents. */
sound good?

> > +    /* Perform metadata upgrades. */
> > +
> > +    /* Prior to NOTMUCH_FEATURE_GHOSTS, thread IDs for missing
> > +     * messages were stored as database metadata. Change these to
> > +     * ghost messages.
> > +     */

Re: [PATCH 08/11] lib: Implement upgrade to ghost messages feature

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:

> Quoth David Bremner on Oct 05 at 10:56 am:
>> Austin Clements <aclements@csail.mit.edu> writes:
>> > +    if (new_features & NOTMUCH_FEATURE_GHOSTS) {
>> > +	t_end = db->metadata_keys_end ("thread_id_");
>> > +	for (t = db->metadata_keys_begin ("thread_id_"); t != t_end; ++t)
>> > +	    ++total;
>> > +    }
>> 
>> It would be nice to have the comment below, or something like it, for
>> the loop above.
>
>   /* The ghost message upgrade converts all thread_id_*
>    * metadata values into ghost message documents. */
> sound good?

Fine.

d

[PATCH 09/11] lib: Enable ghost messages feature

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

This fixes the broken thread order test.
---
 lib/database-private.h    | 2 +-
 test/T260-thread-order.sh | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/lib/database-private.h b/lib/database-private.h
index e2e4bc8..15e03cc 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -166,7 +166,7 @@ struct _notmuch_database {
  * databases will have it). */
 #define NOTMUCH_FEATURES_CURRENT \
     (NOTMUCH_FEATURE_FILE_TERMS | NOTMUCH_FEATURE_DIRECTORY_DOCS | \
-     NOTMUCH_FEATURE_BOOL_FOLDER)
+     NOTMUCH_FEATURE_BOOL_FOLDER | NOTMUCH_FEATURE_GHOSTS)
 
 /* Return the list of terms from the given iterator matching a prefix.
  * The prefix will be stripped from the strings in the returned list.
diff --git a/test/T260-thread-order.sh b/test/T260-thread-order.sh
index b435d79..99f5833 100755
--- a/test/T260-thread-order.sh
+++ b/test/T260-thread-order.sh
@@ -30,7 +30,6 @@ expected=$(for ((i = 0; i < $nthreads; i++)); do
 test_expect_equal "$output" "$expected"
 
 test_begin_subtest "Messages with all parents get linked in all delivery orders"
-test_subtest_known_broken
 # Here we do the same thing as the previous test, but each message
 # references all of its parents.  Since every message references the
 # root of the thread, each thread should always be fully joined.  This
-- 
2.1.0

[PATCH 10/11] test: Test upgrade to ghost messages feature

[Austin Clements]

---
 test/T530-upgrade.sh | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/test/T530-upgrade.sh b/test/T530-upgrade.sh
index c4c4ac8..6b42a69 100755
--- a/test/T530-upgrade.sh
+++ b/test/T530-upgrade.sh
@@ -116,4 +116,25 @@ MAIL_DIR/bar/new/21:2,
 MAIL_DIR/bar/new/22:2,
 MAIL_DIR/cur/51:2,"
 
+# Ghost messages are difficult to test since they're nearly invisible.
+# However, if the upgrade works correctly, the ghost message should
+# retain the right thread ID even if all of the original messages in
+# the thread are deleted.  That's what we test.  This won't detect if
+# the upgrade just plain didn't happen, but it should detect if
+# something went wrong.
+test_begin_subtest "ghost message retains thread ID"
+# Upgrade database
+notmuch new
+# Get thread ID of real message
+thread=$(notmuch search --output=threads id:4EFC743A.3060609@april.org)
+# Delete all real messages in that thread
+rm $(notmuch search --output=files $thread)
+notmuch new
+# "Deliver" ghost message
+add_message '[subject]=Ghost' '[id]=4EFC3931.6030007@april.org'
+# If the ghost upgrade worked, the new message should be attached to
+# the existing thread ID.
+nthread=$(notmuch search --output=threads id:4EFC3931.6030007@april.org)
+test_expect_equal "$thread" "$nthread"
+
 test_done
-- 
2.1.0

[PATCH 11/11] lib: Remove unnecessary thread linking steps when using ghost messages

[Austin Clements]

From: Austin Clements <amdragon@mit.edu>

Previously, it was necessary to link new messages to children to work
around some (though not all) problems with the old metadata-based
approach to stored thread IDs.  With ghost messages, this is no longer
necessary, so don't bother with child linking when ghost messages are
in use.
---
 lib/database.cc | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index ff6a7f6..4655f59 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -2167,10 +2167,23 @@ _notmuch_database_link_message (notmuch_database_t *notmuch,
     if (status)
 	goto DONE;
 
-    status = _notmuch_database_link_message_to_children (notmuch, message,
-							 &thread_id);
-    if (status)
-	goto DONE;
+    if (! (notmuch->features & NOTMUCH_FEATURE_GHOSTS)) {
+	/* In general, it shouldn't be necessary to link children,
+	 * since the earlier indexing of those children will have
+	 * stored a thread ID for the missing parent.  However, prior
+	 * to ghost messages, these stored thread IDs were NOT
+	 * rewritten during thread merging (and there was no
+	 * performant way to do so), so if indexed children were
+	 * pulled into a different thread ID by a merge, it was
+	 * necessary to pull them *back* into the stored thread ID of
+	 * the parent.  With ghost messages, we just rewrite the
+	 * stored thread IDs during merging, so this workaround isn't
+	 * necessary. */
+	status = _notmuch_database_link_message_to_children (notmuch, message,
+							     &thread_id);
+	if (status)
+	    goto DONE;
+    }
 
     /* If not part of any existing thread, generate a new thread ID. */
     if (thread_id == NULL) {
-- 
2.1.0

Re: [PATCH 00/11] Add ghost messages and fix thread linking

[Tomi Ollila]

On Fri, Oct 03 2014, Austin Clements <aclements@csail.mit.edu> wrote:

> This series modifies our database representation of messages that have
> been referenced by other messages, but for which we don't have the
> message itself.  Currently, we store this information as Xapian
> metadata, but this has several downsides for performance and
> complexity and results in hard-to-fix thread linking bugs.  This patch
> series implements "ghost messages", which replace this Xapian metadata
> with Xapian documents that look and act very much like regular message
> documents, but simply have no content.  This simplifies and speeds up
> our thread linking algorithm and fixes the currently broken thread
> linking test.
>
> Ghost messages also open up interesting future possibilities, such as
> "pre-seeding" tags for messages that are not yet indexed.  This could
> be used to make notmuch insert simpler and more robust, as part of tag
> synchronization, and to improve nmbug's behavior when tags arrive
> before messages.

The code looks OK to me -- there are IMO some strange things but those
don't change the status quo -- I did look a little past the diffs into
the code to understand something...

I am now having these patches applied to my 'home' notmuch and haven't yet
seen anything strange there. In this setup I have 27 emails missing that
nmbug expects there to be -- let's see if I can get ghost messages there.

... and tests pass, ran while writing the above part...

I'll put these in use next week on one of my 'work' notmuch. There I have
seen a problem that while I am on one thread, 
`notmuch-poll-and-refresh-buffer' picks new mail but suddenly this thread
now has changed it's thread id :O -- making the search buffer go blank.
I'll see whether it still happens with these (may be totally unrelated)
and perhaps investigate further...


Tomi

Re: [PATCH 00/11] Add ghost messages and fix thread linking

[Austin Clements]

On Sat, 04 Oct 2014, Tomi Ollila <tomi.ollila@iki.fi> wrote:
> On Fri, Oct 03 2014, Austin Clements <aclements@csail.mit.edu> wrote:
>
>> This series modifies our database representation of messages that have
>> been referenced by other messages, but for which we don't have the
>> message itself.  Currently, we store this information as Xapian
>> metadata, but this has several downsides for performance and
>> complexity and results in hard-to-fix thread linking bugs.  This patch
>> series implements "ghost messages", which replace this Xapian metadata
>> with Xapian documents that look and act very much like regular message
>> documents, but simply have no content.  This simplifies and speeds up
>> our thread linking algorithm and fixes the currently broken thread
>> linking test.
>>
>> Ghost messages also open up interesting future possibilities, such as
>> "pre-seeding" tags for messages that are not yet indexed.  This could
>> be used to make notmuch insert simpler and more robust, as part of tag
>> synchronization, and to improve nmbug's behavior when tags arrive
>> before messages.
>
> The code looks OK to me -- there are IMO some strange things but those
> don't change the status quo -- I did look a little past the diffs into
> the code to understand something...
>
> I am now having these patches applied to my 'home' notmuch and haven't yet
> seen anything strange there. In this setup I have 27 emails missing that
> nmbug expects there to be -- let's see if I can get ghost messages there.

In its current form, it won't help with these missing messages from
nmbug.  In the future, nmbug could use ghost messages to record the tags
for those messages anyway and then it becomes an interface question of
whether nmbug should report such missing messages.

> ... and tests pass, ran while writing the above part...
>
> I'll put these in use next week on one of my 'work' notmuch. There I have
> seen a problem that while I am on one thread, 
> `notmuch-poll-and-refresh-buffer' picks new mail but suddenly this thread
> now has changed it's thread id :O -- making the search buffer go blank.
> I'll see whether it still happens with these (may be totally unrelated)
> and perhaps investigate further...

I have this same problem with some people I communicate with.  I haven't
looked in to it, but I assume their messages reference only the
immediate parent, so if it's a reply to another unindexed message then
the thread ID may change depending on indexing order and other arbitrary
factors.  I may be wrong, but if this is the case, I don't think ghost
messages will change this behavior.  Solving this may require more
intelligently picking the "winning" thread ID when merging threads;
e.g., pick the thread ID with the most messages, rather than picking
arbitrarily.

Re: [PATCH 00/11] Add ghost messages and fix thread linking

[David Bremner]

Austin Clements <aclements@csail.mit.edu> writes:

> This series modifies our database representation of messages that have
> been referenced by other messages, but for which we don't have the
> message itself.  Currently, we store this information as Xapian
> metadata, but this has several downsides for performance and
> complexity and results in hard-to-fix thread linking bugs.  This patch
> series implements "ghost messages", which replace this Xapian metadata
> with Xapian documents that look and act very much like regular message
> documents, but simply have no content.  This simplifies and speeds up
> our thread linking algorithm and fixes the currently broken thread
> linking test.

Other than some style / documentation quibbles already noted, 
the series looks good to me.

d