This exposes the committed database revision to library users along
with a UUID that can be used to detect when revision numbers are no
longer comparable (e.g., because the database has been replaced).
---
 lib/database-private.h |  1 +
 lib/database.cc        | 11 +++++++++++
 lib/notmuch.h          | 18 ++++++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/lib/database-private.h b/lib/database-private.h
index 465065d..0977229 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -157,6 +157,7 @@ struct _notmuch_database {
      * under a higher revision number, which can be generated with
      * notmuch_database_new_revision. */
     unsigned long revision;
+    const char *uuid;
 
     Xapian::QueryParser *query_parser;
     Xapian::TermGenerator *term_gen;
diff --git a/lib/database.cc b/lib/database.cc
index 45d32ab..9bec170 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -905,6 +905,8 @@ notmuch_database_open (const char *path,
 	    notmuch->revision = 0;
 	else
 	    notmuch->revision = Xapian::sortable_unserialise (last_mod);
+	notmuch->uuid = talloc_strdup (
+	    notmuch, notmuch->xapian_db->get_uuid ().c_str ());
 
 	notmuch->query_parser = new Xapian::QueryParser;
 	notmuch->term_gen = new Xapian::TermGenerator;
@@ -1562,6 +1564,15 @@ DONE:
     return NOTMUCH_STATUS_SUCCESS;
 }
 
+unsigned long
+notmuch_database_get_revisison (notmuch_database_t *notmuch,
+				const char **uuid)
+{
+    if (*uuid)
+	*uuid = notmuch->uuid;
+    return notmuch->revision;
+}
+
 /* We allow the user to use arbitrarily long paths for directories. But
  * we have a term-length limit. So if we exceed that, we'll use the
  * SHA-1 of the path for the database term.
diff --git a/lib/notmuch.h b/lib/notmuch.h
index 92594b9..898f7b9 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -433,6 +433,24 @@ notmuch_status_t
 notmuch_database_end_atomic (notmuch_database_t *notmuch);
 
 /**
+ * Return the committed database revision and UUID.
+ *
+ * The database revision number increases monotonically with each
+ * commit to the database.  Hence, all messages and message changes
+ * committed to the database (that is, visible to readers) have a last
+ * modification revision <= the committed database revision.  Any
+ * messages committed in the future will be assigned a modification
+ * revision > the committed database revision.
+ *
+ * The UUID is a NUL-terminated opaque string that uniquely identifies
+ * this database.  Two revision numbers are only comparable if they
+ * have the same database UUID.
+ */
+unsigned long
+notmuch_database_get_revisison (notmuch_database_t *notmuch,
+				const char **uuid);
+
+/**
  * Retrieve a directory object from the database for 'path'.
  *
  * Here, 'path' should be a path relative to the path of 'database'
-- 
2.1.0