ref-manual: tasks: do_cleanall: recommend using '-f' instead

do_cleanall can produce failures when used in legitimate cases, such as
with recipe variants (foo and foo-native) or a shared DL_DIR. This is why
it is forbidden when writing tests that will run on the autobuilders
(https://docs.yoctoproject.org/test-manual/intro.html?highlight=cleanall#considerations-when-writing-tests).

Reword the documentation to clearly discourage, provide a safe alternative
(bitbake -f -c fetch), and the rationale with an example.

Reported-by: Sam Liddicott
Link: https://bootlin.com/blog/yocto-sharing-the-sstate-cache-and-download-directories/#comment-2650335
(From yocto-docs rev: bfeb00f9213b9d2650ccf03ea666387415227be4)

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 23 insertions, 3 deletions

diff --git a/documentation/ref-manual/tasks.rst b/documentation/ref-manual/tasks.rst
index 0db960b22f..16b48ca0bc 100644
--- a/documentation/ref-manual/tasks.rst
+++ b/documentation/ref-manual/tasks.rst
@@ -470,9 +470,29 @@ You can run this task using BitBake as follows::
 
    $ bitbake -c cleanall recipe
 
-Typically, you would not normally use the :ref:`ref-tasks-cleanall` task. Do so only
-if you want to start fresh with the :ref:`ref-tasks-fetch`
-task.
+You should never use the :ref:`ref-tasks-cleanall` task in a normal
+scenario. If you want to start fresh with the :ref:`ref-tasks-fetch` task,
+use instead::
+
+  $ bitbake -f -c fetch recipe
+
+.. note::
+
+   The reason to prefer ``bitbake -f -c fetch`` is that the
+   :ref:`ref-tasks-cleanall` task would break in some cases, such as::
+
+      $ bitbake -c fetch    recipe
+      $ bitbake -c cleanall recipe-native
+      $ bitbake -c unpack   recipe
+
+   because after step 1 there is a stamp file for the
+   :ref:`ref-tasks-fetch` task of ``recipe``, and it won't be removed at
+   step 2 because step 2 uses a different work directory. So the unpack task
+   at step 3 will try to extract the downloaded archive and fail as it has
+   been deleted in step 2.
+
+   Note that this also applies to BitBake from concurrent processes when a
+   shared download directory (:term:`DL_DIR`) is setup.
 
 .. _ref-tasks-cleansstate: