+ *
+ * Database metadata
+ * -----------------
+ * Xapian allows us to store arbitrary name-value pairs as
+ * "metadata". We currently use the following metadata names with the
+ * given meanings:
+ *
+ * version The database schema version, (which is distinct
+ * from both the notmuch package version (see
+ * notmuch --version) and the libnotmuch library
+ * version. The version is stored as an base-10
+ * ASCII integer. The initial database version
+ * was 1, (though a schema existed before that
+ * were no "version" database value existed at
+ * all). Successive versions are allocated as
+ * changes are made to the database (such as by
+ * indexing new fields).
+ *
+ * last_thread_id The last thread ID generated. This is stored
+ * as a 16-byte hexadecimal ASCII representation
+ * of a 64-bit unsigned integer. The first ID
+ * generated is 1 and the value will be
+ * incremented for each thread ID.
+ *
+ * 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
+ * formed by concatenating "thread_id_" with a message
+ * ID (or the SHA1 sum of a message ID if it is very
+ * long---see description of 'id' in the mail
+ * document). The value stored is a thread ID.
+ *
+ * These thread ID metadata values are stored
+ * whenever a message references a parent message
+ * that does not yet exist in the database. A
+ * thread ID will be allocated and stored, and if
+ * the message is later added, the stored thread
+ * ID will be used (and the metadata value will
+ * be cleared).
+ *
+ * Even before a message is added, it's
+ * pre-allocated thread ID is useful so that all
+ * descendant messages that reference this common
+ * parent can be recognized as belonging to the
+ * same thread.