scripts/setlocalversion: add more comments to -dirty flag detection

This script stumbled on the read-only source tree over again:

 - a2bb90a08cb3 ("kbuild: fix delay in setlocalversion on readonly
   source")

 - cdf2bc632ebc ("scripts/setlocalversion on write-protected source
   tree")

 - 8ef14c2c41d9 ("Revert "scripts/setlocalversion: git: Make -dirty
   check more robust"")

 - ff64dd485730 ("scripts/setlocalversion: Improve -dirty check with
   git-status --no-optional-locks")

Add comments to clarify that this script should never ever try to write
to the source tree.

'git describe --dirty' might look as a simple solution for appending
the -dirty string, but we cannot use it because it creates the
.git/index.lock file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>

diff --git a/scripts/setlocalversion b/scripts/setlocalversion
index f3084d6..6865df6 100755 (executable)
--- a/scripts/setlocalversion
+++ b/scripts/setlocalversion
@@ -80,6 +80,10 @@ scm_version()
                fi
 
                # Check for uncommitted changes.
+               # This script must avoid any write attempt to the source tree,
+               # which might be read-only.
+               # You cannot use 'git describe --dirty' because it tries to
+               # create .git/index.lock .
                # First, with git-status, but --no-optional-locks is only
                # supported in git >= 2.14, so fall back to git-diff-index if
                # it fails. Note that git-diff-index does not refresh the