dev-manual: add bblock documentation

bblock is a helper tool to lock/unlock tasks and recipes to specific
signatures. Add a documentation page for it.

(From yocto-docs rev: a082aa39840587d3af6c3f4a2c2747564ca37414)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/bblock.rst

@@ -0,0 +1,129 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+Locking and Unlocking Recipes Using ``bblock``
+**********************************************
+
+By design, the OpenEmbedded build system builds everything from scratch
+unless BitBake determines that specific tasks do not require rebuilding.
+At startup, it computes a signature for all tasks, based on the task's input.
+Then, it compares these signatures with the ones from the sstate cache (if they
+exist). Any changes cause the task to rerun.
+
+During development, changes might trigger BitBake to rebuild certain
+recipes, even when we know they do not require rebuilding at that stage.
+For example, modifying a recipe can lead to rebuilding its native
+counterpart, which might prove unnecessary. Editing the ``python3`` recipe,
+for instance, can prompt BitBake to rebuild ``python3-native`` along with any
+recipes that depend on it.
+
+To prevent this, use ``bblock`` to lock specific tasks or recipes to
+specific signatures, forcing BitBake to use the sstate cache for them.
+
+.. attention::
+
+   Use ``bblock`` only during the development phase.
+
+   Forcing BitBake to use the sstate cache, regardless of input changes, means
+   the recipe metadata no longer directly reflect the output. Use this feature
+   with caution. If you do not understand why signatures change, see the section
+   on :yocto_wiki:`understanding what changed </TipsAndTricks/Understanding_what_changed_(diffsigs_etc)>`.
+
+
+Locking tasks and recipes
+-------------------------
+
+To lock a recipe, use::
+
+   $ bblock recipe
+
+You can also use a space-separated list of recipes to lock multiple recipes::
+
+   $ bblock recipe1 recipe2
+
+Locking a recipe means locking all tasks of the recipe. If you need to
+lock only particular tasks, use the `-t` option with a comma-separated
+list of tasks::
+
+  $ bblock -t task1,task2 recipe
+
+
+Unlocking tasks and recipes
+---------------------------
+
+To unlock a recipe, use the ``-r`` option::
+
+   $ bblock -r recipe
+
+You can also use a space-separated list of recipes to unlock multiple recipes::
+
+   $ bblock -r recipe1 recipe2
+
+Unlocking a recipe means unlocking all tasks of the recipe. If you need to
+unlock only particular tasks use the ``-t`` option with a comma-separated
+list of tasks::
+
+  $ bblock -r -t task1,task2 recipe
+
+To unlock all recipes, do not specify any recipe::
+
+  $ bblock -r
+
+
+Configuration file
+------------------
+
+``bblock`` will dump the signatures in the ``build/conf/bblock.conf`` file,
+included by default in :oe_git:`meta/conf/bitbake.conf </openembedded-core/tree/meta/conf/bitbake.conf>`.
+
+To dump the file, use the ``-d`` option::
+
+  $ bblock -d
+
+
+Locking mechanism
+-----------------
+
+``bblock`` computes the signature(s) of the task(s) and sets the 3 following
+variables: :term:`SIGGEN_LOCKEDSIGS`, :term:`SIGGEN_LOCKEDSIGS_TYPES`
+and :term:`SIGGEN_LOCKEDSIGS_TASKSIG_CHECK`.
+
+In particular, ``bblock`` sets::
+
+  SIGGEN_LOCKEDSIGS_TASKSIG_CHECK = "info"
+  SIGGEN_LOCKEDSIGS_TYPES += "${PACKAGE_ARCHS}"
+
+  SIGGEN_LOCKEDSIGS_<package_arch> += "<recipe>:<task>:<signature>"
+
+This produces architecture specific locks and reminds user that some tasks
+have locked signatures.
+
+Example
+-------
+
+When working on the ``python3`` recipe, we can lock ``python3-native`` with
+the following::
+
+  $ bblock python3-native
+  $ bblock -d
+  # Generated by bblock
+  SIGGEN_LOCKEDSIGS_TASKSIG_CHECK = "info"
+  SIGGEN_LOCKEDSIGS_TYPES += "${PACKAGE_ARCHS}"
+
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_patch:865859c27e603ba42025b7bb766c3cd4c0f477e4962cfd39128c0619d695fce7"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_populate_sysroot:f8fa5d3194cef638416000252b959e86d0a19f6b7898e1f56b643c588cdd8605"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_prepare_recipe_sysroot:fe295ac505d9d1143313424b201c6f3f2a0a90da40a13a905b86b874705f226a"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_fetch:1b6e4728fee631bc7a8a7006855c5b8182a8224579e32e3d0a2db77c26459f25"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_unpack:2ad74d6f865ef75c35c0e6bbe3f9a90923a6b2c62c18a3ddef514ea31fbc588f"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_deploy_source_date_epoch:15f89b8483c1ad7507480f337619bb98c26e231227785eb3543db163593e7b42"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_configure:7960c13d23270fdb12b3a7c426ce1da0d2f5c7cf5e5d3f5bdce5fa330eb7d482"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_compile:012e1d4a63f1a78fc2143bd90d704dbcf5865c5257d6272aa7540ec1cd3063d9"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_install:d3401cc2afa4c996beb154beaad3e45fa0272b9c56fb86e9db14ec3544c68f9d"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_build:fa88bb7afb9046c0417c24a3fa98a058653805a8b00eda2c2d7fea68fc42f882"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_collect_spdx_deps:cc9c53ba7c495567e9a38ec4801830c425c0d1f895aa2fc66930a2edd510d9b4"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_create_spdx:766a1d09368438b7b5a1a8e2a8f823b2b731db44b57e67d8b3196de91966f9c5"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_create_package_spdx:46f80faeab25575e9977ba3bf14c819489c3d489432ae5145255635108c21020"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_recipe_qa:cb960cdb074e7944e894958db58f3dc2a0436ecf87c247feb3e095e214fec0e4"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_populate_lic:15657441621ee83f15c2e650e7edbb036870b56f55e72e046c6142da3c5783fd"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_create_manifest:24f0abbec221d27bbb2909b6e846288b12cab419f1faf9f5006ed80423d37e28"
+  SIGGEN_LOCKEDSIGS_x86_64 += "python3-native:do_addto_recipe_sysroot:bcb6a1905f113128de3f88d702b706befd6a786267c045ee82532759a7c214d7"
+

documentation/dev-manual/index.rst

@@ -48,5 +48,6 @@ Yocto Project Development Tasks Manual
    error-reporting-tool
    wayland
    qemu
+   bblock
 
 .. include:: /boilerplate.rst

documentation/ref-manual/structure.rst

@@ -335,6 +335,15 @@ Once the build process gets the sample file, it uses ``sed`` to substitute final
    version of the ``bblayers.conf.sample`` file in the ``meta-poky/conf/templates/default``
    directory.
 
+.. _structure-build-conf-bblock.conf:
+
+``build/conf/bblock.conf``
+--------------------------
+
+This configuration file is generated by :doc:`bblock </dev-manual/bblock>` and
+contains the signatures locked by ``bblock``. By default, it does not exist
+and will be created upon the first invocation of ``bblock``.
+
 .. _structure-build-downloads:
 
 ``build/downloads/``

documentation/ref-manual/variables.rst

@@ -7901,6 +7901,9 @@ system and gives an overview of their function and contents.
      Then you can look at files in ``build/tmp/stamps/<arch>/bc`` and look for
      files like: ``<PV>.do_compile.sigdata.09772aa4532512baf96d433484f27234d4b7c11dd9cda0d6f56fa1b7ce6f25f0``.
 
+     Alternatively, you can also use :doc:`bblock </dev-manual/bblock>` to
+     generate this line for you.
+
    :term:`SIGGEN_LOCKEDSIGS_TASKSIG_CHECK`
      Specifies the debug level of task signature check. 3 levels are supported: