A few minor comments below.
At a higher level, I'm curious what the tag synchronization protocol
you're building on top of this is. I can't think of one that doesn't
have race conditions, but maybe I'm not thinking about it right.
Quoth Thomas Jost on Dec 13 at 6:11 pm:
> This is a time_t value, similar to the message date (TIMESTAMP). It is first set
> when the message is added to the database, and is then updated every time a tag
> is added or removed. It can thus be used for doing incremental dumps of the
> database or for synchronizing it between several computers.
>
> This value can be read freely (with notmuch_message_get_mtime()) but for now it
> can't be set to an arbitrary value: it can only be set to "now" when updated.
> There's no specific reason for this except that I don't really see a real use
> case for setting it to an arbitrary value.
> ---
> lib/database.cc | 7 ++++++-
> lib/message.cc | 32 ++++++++++++++++++++++++++++++++
> lib/notmuch-private.h | 6 +++++-
> lib/notmuch.h | 4 ++++
> 4 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/lib/database.cc b/lib/database.cc
> index 2025189..6dc6f73 100644
> --- a/lib/database.cc
> +++ b/lib/database.cc
> @@ -81,7 +81,7 @@ typedef struct {
> * STRING is the name of a file within that
> * directory for this mail message.
> *
> - * A mail document also has four values:
> + * A mail document also has five values:
> *
> * TIMESTAMP: The time_t value corresponding to the message's
> * Date header.
> @@ -92,6 +92,9 @@ typedef struct {
> *
> * SUBJECT: The value of the "Subject" header
> *
> + * MTIME: The time_t value corresponding to the last time
> + * a tag was added or removed on the message.
> + *
> * In addition, terms from the content of the message are added with
> * "from", "to", "attachment", and "subject" prefixes for use by the
> * user in searching. Similarly, terms from the path of the mail
> @@ -1735,6 +1738,8 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
> date = notmuch_message_file_get_header (message_file, "date");
> _notmuch_message_set_header_values (message, date, from, subject);
>
> + _notmuch_message_update_mtime (message);
Indentation.
> +
> _notmuch_message_index_file (message, filename);
> } else {
> ret = NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID;
> diff --git a/lib/message.cc b/lib/message.cc
> index 0075425..0c98589 100644
> --- a/lib/message.cc
> +++ b/lib/message.cc
> @@ -830,6 +830,34 @@ _notmuch_message_set_header_values (notmuch_message_t *message,
> message->doc.add_value (NOTMUCH_VALUE_SUBJECT, subject);
> }
>
> +/* Get the message mtime, i.e. when it was added or the last time a tag was
> + * added/removed. */
> +time_t
> +notmuch_message_get_mtime (notmuch_message_t *message)
> +{
> + std::string value;
> +
> + try {
> + value = message->doc.get_value (NOTMUCH_VALUE_MTIME);
> + } catch (Xapian::Error &error) {
> + INTERNAL_ERROR ("Failed to read mtime value from document.");
> + return 0;
> + }
For compatibility, this should handle the case when
NOTMUCH_VALUE_MTIME is missing, probably by just returning 0. As it
is, value will be an empty string and sortable_unserialise is
undefined on strings that weren't produced by sortable_serialise.
> +
> + return Xapian::sortable_unserialise (value);
> +}
> +
> +/* Set the message mtime to "now". */
> +void
> +_notmuch_message_update_mtime (notmuch_message_t *message)
> +{
> + time_t time_value;
> +
> + time_value = time (NULL);
> + message->doc.add_value (NOTMUCH_VALUE_MTIME,
> + Xapian::sortable_serialise (time_value));
Indentation.
> +}
> +
> /* Synchronize changes made to message->doc out into the database. */
> void
> _notmuch_message_sync (notmuch_message_t *message)
> @@ -994,6 +1022,8 @@ notmuch_message_add_tag (notmuch_message_t *message, const char *tag)
> private_status);
> }
>
> + _notmuch_message_update_mtime (message);
> +
> if (! message->frozen)
> _notmuch_message_sync (message);
>
> @@ -1022,6 +1052,8 @@ notmuch_message_remove_tag (notmuch_message_t *message, const char *tag)
> private_status);
> }
>
> + _notmuch_message_update_mtime (message);
> +
> if (! message->frozen)
> _notmuch_message_sync (message);
>
> diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
> index 60a932f..9859872 100644
> --- a/lib/notmuch-private.h
> +++ b/lib/notmuch-private.h
> @@ -95,7 +95,8 @@ typedef enum {
> NOTMUCH_VALUE_TIMESTAMP = 0,
> NOTMUCH_VALUE_MESSAGE_ID,
> NOTMUCH_VALUE_FROM,
> - NOTMUCH_VALUE_SUBJECT
> + NOTMUCH_VALUE_SUBJECT,
> + NOTMUCH_VALUE_MTIME
> } notmuch_value_t;
>
> /* Xapian (with flint backend) complains if we provide a term longer
> @@ -276,6 +277,9 @@ _notmuch_message_set_header_values (notmuch_message_t *message,
> const char *from,
> const char *subject);
> void
> +_notmuch_message_update_mtime (notmuch_message_t *message);
> +
> +void
> _notmuch_message_sync (notmuch_message_t *message);
>
> notmuch_status_t
> diff --git a/lib/notmuch.h b/lib/notmuch.h
> index 9f23a10..643ebce 100644
> --- a/lib/notmuch.h
> +++ b/lib/notmuch.h
> @@ -910,6 +910,10 @@ notmuch_message_set_flag (notmuch_message_t *message,
> time_t
> notmuch_message_get_date (notmuch_message_t *message);
>
> +/* Get the mtime of 'message' as a time_t value. */
> +time_t
> +notmuch_message_get_mtime (notmuch_message_t *message);
> +
> /* Get the value of the specified header from 'message'.
> *
> * The value will be read from the actual message file, not from the