lib: reword comment about XFOLDER: prefix

I believe the current one is misleading, because in my experiments
Xapian did not add : when prefix and term were both upper case. Indeed,
it's hard to see how it could, because prefixes are added at a layer
above Xapian in our code. See _notmuch_message_add_term for an example.

Also try to explain why this is a good idea.  As far as I can ascertain,
this is more of an issue for a system trying to work with an unknown set
of prefixes. Since notmuch has a fixed set of prefixes, and we can
hopefully be trusted not to add XGOLD and XGOLDEN as prefixes, it is
harder for problems to arise.

diff --git a/lib/database.cc b/lib/database.cc
index 57a98c94204f5db2e2714b2568d2424ccb66264c..5577aaf9f48ce5974d692721735122c7ff6ba5d3 100644 (file)
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -260,10 +260,10 @@ static prefix_t BOOLEAN_PREFIX_EXTERNAL[] = {
     { "id",                    "Q" },
     { "path",                  "P" },
     /*
-     * Without the ":", since this is a multi-letter prefix, Xapian
-     * will add a colon itself if the first letter of the path is
-     * upper-case ASCII. Including the ":" forces there to always be a
-     * colon, which keeps our own logic simpler.
+     * Unconditionally add ':' to reduce potential ambiguity with
+     * overlapping prefixes and/or terms that start with capital
+     * letters. See Xapian document termprefixes.html for related
+     * discussion.
      */
     { "folder",                        "XFOLDER:" },
 };