lib: Add a ghost messages database feature

This will be implemented over the next several patches.  The feature
is not yet "enabled" (this does not add it to
NOTMUCH_FEATURES_CURRENT).

2 files changed, 9 insertions, 0 deletions

diff --git a/lib/database-private.h b/lib/database-private.h
index ca0751cf..e2e4bc87 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 1c6ffc57..8fd7fad6 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 *