$ hg init repo

  $ cd repo

  $ touch a.html b.html c.py d.py

  $ cat > frontend.sparse << EOF

  > [include]

  > *.html

  > EOF

  $ hg -q commit -A -m initial

  $ echo 1 > a.html

  $ echo 1 > c.py

  $ hg commit -m 'commit 1'

Enable sparse profile

  $ cat .hg/requires

  dotencode

  fncache

  generaldelta

  revlogv1

  store

  testonly-simplestore (reposimplestore !)

  $ hg debugsparse --config extensions.sparse= --enable-profile frontend.sparse

  $ ls

  a.html

  b.html

Requirement for sparse added when sparse is enabled

  $ cat .hg/requires

  dotencode

  exp-sparse

  fncache

  generaldelta

  revlogv1

  store

  testonly-simplestore (reposimplestore !)

Client without sparse enabled reacts properly

  $ hg files

  abort: repository is using sparse feature but sparse is not enabled; enable the "sparse" extensions to access!

  [255]

Requirement for sparse is removed when sparse is disabled

  $ hg debugsparse --reset --config extensions.sparse=

  $ cat .hg/requires

  dotencode

  fncache

  generaldelta

  revlogv1

  store

  testonly-simplestore (reposimplestore !)

And client without sparse can access

  $ hg files

  a.html

  b.html

  c.py

  d.py

  frontend.sparse