update-index doc: note the caveat with "could not open..."

Note the caveat where 2.17 is stricter about index validation
potentially causing "could not open directory" warnings when git is
upgraded. See the preceding "dir.c: stop ignoring opendir() error in
open_cached_dir()" change.

This caused some mayhem when I upgraded git to a version with this
series at Booking.com, and other users have doubtless enabled the UC
extension and are in for a surprise when they upgrade. Let's give them
a headsup in the docs.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index f0e08d6a4c..b8d3c0d72e 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -480,6 +480,16 @@ a directory with a file when it comes to the internal structures of
 the untracked cache, but no case has been reported where this resulted in
 wrong "git status" output.
 
+There are also cases where existing indexes written by git versions
+before 2.17 will reference directories that don't exist anymore,
+potentially causing many "could not open directory" warnings to be
+printed on "git status". These are new warnings for existing issues
+that were previously silently discarded.
+
+As with the bug described above the solution is to one-off do a "git
+status" run with `core.untrackedCache=false` to flush out the leftover
+bad data.
+
 File System Monitor
 -------------------