This feature is intended to mark the database as supporting a
key-value store in the document data area. The actual key-value store
is implemented in a subsequent commit.
---
 lib/database-private.h |  8 +++++++-
 lib/database.cc        | 12 ++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/database-private.h b/lib/database-private.h
index a499b259..3a15fd16 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, messages store a serialized string-map in their data
+     * area
+     *
+     * Introduced: version 3. */
+    NOTMUCH_FEATURE_MESSAGE_DATA = 1 << 7,
 };
 
 /* In C++, a named enum is its own type, so define bitwise operators
@@ -233,7 +239,7 @@ struct _notmuch_database {
 #define NOTMUCH_FEATURES_CURRENT \
     (NOTMUCH_FEATURE_FILE_TERMS | NOTMUCH_FEATURE_DIRECTORY_DOCS | \
      NOTMUCH_FEATURE_BOOL_FOLDER | NOTMUCH_FEATURE_GHOSTS | \
-     NOTMUCH_FEATURE_LAST_MOD)
+     NOTMUCH_FEATURE_LAST_MOD | NOTMUCH_FEATURE_MESSAGE_DATA )
 
 /* Return the list of terms from the given iterator matching a prefix.
  * The prefix will be stripped from the strings in the returned list.
diff --git a/lib/database.cc b/lib/database.cc
index 9cf8062c..47e903d5 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -383,6 +383,8 @@ static const struct {
       "indexed MIME types", "w"},
     { NOTMUCH_FEATURE_LAST_MOD,
       "modification tracking", "w"},
+    { NOTMUCH_FEATURE_MESSAGE_DATA,
+      "per message data", "rw"},
 };
 
 const char *
@@ -1342,6 +1344,16 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
     if (! notmuch_database_needs_upgrade (notmuch))
 	return NOTMUCH_STATUS_SUCCESS;
 
+    /*
+     * the upgrade for NOTMUCH_FEATURE_MESSAGE_DATA is actually done
+     * by NOTMUCH_FEATURE_FILE_TERMS
+     */
+    if ((new_features & NOTMUCH_FEATURE_MESSAGE_DATA) &&
+	!(target_features & NOTMUCH_FEATURE_FILE_TERMS)) {
+	_notmuch_database_log (notmuch, "inconsistent feature set");
+	return NOTMUCH_STATUS_UNSUPPORTED_OPERATION;
+    }
+
     if (progress_notify) {
 	/* Set up our handler for SIGALRM */
 	memset (&action, 0, sizeof (struct sigaction));
-- 
2.17.1

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