lib: add 'body:' field, stop indexing headers twice.

The new `body:` field (in Xapian terms) or prefix (in slightly
sloppier notmuch) terms allows matching terms that occur only in the
body.

Unprefixed query terms should continue to match anywhere (header or
body) in the message.

This follows a suggestion of Olly Betts to use the facility (since
Xapian 1.0.4) to add the same field with multiple prefixes. The double
indexing of previous versions is thus replaced with a query time
expension of unprefixed query terms to the various prefixed
equivalent.

Reindexing will be needed for 'body:' searches to work correctly;
otherwise they will also match messages where the term occur in
headers (demonstrated by the new tests in T530-upgrade.sh)

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/database-private.h b/lib/database-private.h
index a499b259..293f2db4 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -108,6 +108,12 @@ enum _notmuch_features {
      *
      * Introduced: version 3. */
     NOTMUCH_FEATURE_LAST_MOD = 1 << 6,
+
+    /* If set, unprefixed terms are stored only for the message body,
+     * not for headers.
+     *
+     * Introduced: version 3. */
+    NOTMUCH_FEATURE_UNPREFIX_BODY_ONLY = 1 << 7,
 };
 
 /* In C++, a named enum is its own type, so define bitwise operators