documentation/poky-ref-manual/technical-details.xml: more on YOCTO #1500

More work on this bug for sstate.  This commit represents the third
pass through the new chapter four (Technical Details) that is
dedicated to YP components and sstate at the moment.  The material
is unreviewed by Richard as of yet.

(From yocto-docs rev: 3c0e5bac288c05ea3fd93b1d1d5866895c5c2d1e)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 378 insertions, 3 deletions

diff --git a/documentation/poky-ref-manual/technical-details.xml b/documentation/poky-ref-manual/technical-details.xml
index b341795..1657431 100644
--- a/documentation/poky-ref-manual/technical-details.xml
+++ b/documentation/poky-ref-manual/technical-details.xml
@@ -151,12 +151,386 @@
 
     <para>
         By design, the Yocto Project builds everything from scratch unless it can determine that
-        a given task's inputs have not changed.  
-        While building from scratch ensures that everything is current, it does also
-        mean that a lot of time could be spent rebuiding things that don't necessarily need built.
+        parts don't need to be rebuilt.
+        Fundamentally, building from scratch is an attraction as it means all parts are 
+        built fresh and there is no possibility of stale data causing problems. 
+        When developers hit problems, they typically default back to building from scratch
+        so they know the state of things from the start.
+    </para>
+
+    <para>  
+        Building an image from scratch is both an advantage and a disadvantage to the process. 
+        As mentioned in the previous paragraph, building from scratch ensures that 
+        everything is current and starts from a known state.
+        However, building from scratch also takes much longer as it generally means 
+        rebuiding things that don't necessarily need rebuilt.
+    </para>
+
+    <para>
+        The Yocto Project implements shared state code that supports incremental builds.
+        The implementation of the shared state code answers the following questions that
+        were fundamental roadblocks within the Yocto Project incremental build support system:
+        <itemizedlist>
+            <listitem>What pieces of the system have changed and what pieces have not changed?</listitem>
+            <listitem>How are changed pieces of software removed and replaced?</listitem>
+            <listitem>How are pre-built components that don't need to be rebuilt from scratch
+                used when they are available?</listitem>
+        </itemizedlist>
     </para>
 
     <para>
+        For the first question, the build system detects changes in the "inputs" to a given task by 
+        creating a checksum (or signature) of the task's inputs. 
+        If the checksum changes, the system assumes the inputs have changed and the task needs to be 
+        rerun.
+        For the second question, the shared state (sstate) code tracks which tasks add which output
+        to the build process. 
+        This means the output from a given task can be removed, upgraded or otherwise manipulated.
+        The third question is partly addressed by the solution for the second question
+        assuming the build system can fetch the sstate objects from remote locations and 
+        install them if they are deemed to be valid.
+    </para>
+
+    <para>
+        The rest of this section goes into detail about the overall incremental build
+        architecture, the checksums (signatures), shared state, and some tips and tricks.
+    </para>
+
+    <section id='overall-architecture'>
+        <title>Overall Architecture</title>
+
+        <para>
+            When determining what parts of the system need to be built, the Yocto Project 
+            uses a per-task basis and does not use a per-recipe basis.
+            You might wonder why using a per-task basis is preferred over a per-recipe basis.
+            To help explain, consider having the IPK packaging backend enabled and then switching to DEB. 
+            In this case, <filename>do_install</filename> and <filename>do_package</filename>
+            output are still valid.
+            However, with a per-recipe approach, the build would not include the 
+            <filename>.deb</filename> files.        
+            Consequently, you would have to invalidate the whole build and rerun it. 
+            Rerunning everything is not the best situation.
+            Also in this case, the core must be "taught" much about specific tasks. 
+            This methodology does not scale well and does not allow users to easily add new tasks 
+            in layers or as external recipes without touching the packaged-staging core.
+        </para>
+    </section>
+
+    <section id='checksums'>
+        <title>Checksums (Signatures)</title>
+
+        <para>
+            The Yocto Project uses a checksum, which is a unique signature of a task's 
+            inputs, to determine if a task needs to be run again. 
+            Because it is a change in a task's inputs that trigger a rerun, the process
+            needs to detect all the inputs to a given task. 
+            For shell tasks, this turns out to be fairly easy because
+            the build process generates a "run" shell script for each task and 
+            it is possible to create a checksum that gives you a good idea of when 
+            the task's data changes.
+        </para>
+
+        <para>
+            To complicate the problem, there are things that should not be included in 
+            the checksum. 
+            First, there is the actual specific build path of a given task - 
+            the <filename>WORKDIR</filename>. 
+            It does not matter if the working directory changes because it should not 
+            affect the output for target packages.
+            Also, the build process has the objective of making native/cross packages relocatable. 
+            The checksum therefore needs to exclude <filename>WORKDIR</filename>.
+            The simplistic approach for excluding the worknig directory is to set 
+            <filename>WORKDIR</filename> to some fixed value and create the checksum
+            for the "run" script. 
+        </para>
+
+        <para>
+            Another problem results from the "run" scripts containing functions that 
+            might or might not get called.  
+            The Yocto Project contains code that figures out dependencies between shell 
+            functions.
+            This code is used to prune the "run" scripts down to the minimum set, 
+            thereby alleviating this problem and making the "run" scripts much more 
+            readable as a bonus.
+        </para>
+
+        <para>
+            So far we have solutions for shell scripts.
+            What about python tasks?
+            Handling these tasks are more difficult but the the same approach 
+            applies.
+            The process needs to figure out what variables a python function accesses 
+            and what functions it calls.
+            Again, the Yocto Project contains code that first figures out the variable and function 
+            dependencies, and then creates a checksum for the data used as the input to 
+            the task.
+        </para>
+
+        <para>
+            Like the <filename>WORKDIR</filename> case, situations exist where dependencies 
+            should be ignored.
+            For these cases, you can instruct the build process to ignore a dependency
+            by using a line like the following:
+            <literallayout class='monospaced'>
+     PACKAGE_ARCHS[vardepsexclude] = "MACHINE"
+            </literallayout>
+            This example ensures that the <filename>PACKAGE_ARCHS</filename> variable does not 
+            depend on the value of <filename>MACHINE</filename>, even if it does reference it.
+        </para>
+           
+        <para>
+            Equally, there are cases where we need to add in dependencies
+            BitBake is not able to find.
+            You can accomplish this by using a line like the following:
+            <literallayout class='monospaced'>
+      PACKAGE_ARCHS[vardeps] = "MACHINE"
+            </literallayout>
+            This example explicitly adds the <filename>MACHINE</filename> variable as a 
+            dependency for <filename>PACKAGE_ARCHS</filename>.
+        </para>
+
+        <para> 
+            Consider a case with inline python, for example, where BitBake is not
+            able to figure out dependencies. 
+            When running in debug mode (i.e. using <filename>-DDD</filename>), BitBake 
+            produces output when it discovers something for which it cannot figure out
+            dependencies. 
+            The Yocto Project team has currently not managed to cover those dependencies 
+            in detail and is aware of the need to fix this situation.
+        </para>
+
+        <para>
+            Thus far, this section has limited discussion to the direct inputs into a 
+            task.
+            Information based on direct inputs is referred to as the "basehash" in the code.
+            However, there is still the question of a task's indirect inputs, the things that 
+            were already built and present in the build directory. 
+            The checksum (or signature) for a particular task needs to add the hashes of all the
+            tasks the particular task depends upon. 
+            Choosing which dependencies to add is a policy decision.
+            However, the effect is to generate a master checksum that combines the 
+            basehash and the hashes of the task's dependencies.
+        </para>
+
+        <para>
+            While figuring out the dependencies and creating these checksums is good,
+            what does the Yocto Project build system do with the checksum information? 
+            The build system uses a signature handler that is responsible for 
+            processing the checksum information.
+            By default, there is a dummy "noop" signature handler enabled in BitBake.
+            This means that behaviour is unchanged from previous versions. 
+            OECore uses the "basic" signature handler through this setting in the
+            <filename>bitbake.conf</filename> file:
+            <literallayout class='monospaced'>
+     BB_SIGNATURE_HANDLER ?= "basic"
+            </literallayout>
+            Also within the BitBake configuration file, we can give BitBake
+            some extra information to help it handle this information.
+            The following statements effectively result in a list of global
+            list of variable dependency excludes - variables never included in 
+            any checksum:
+            <literallayout class='monospaced'>
+     BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH"
+     BB_HASHBASE_WHITELIST += "DL_DIR SSTATE_DIR THISDIR FILESEXTRAPATHS"
+     BB_HASHBASE_WHITELIST += "FILE_DIRNAME HOME LOGNAME SHELL TERM USER"
+     BB_HASHBASE_WHITELIST += "FILESPATH USERNAME STAGING_DIR_HOST STAGING_DIR_TARGET"
+     BB_HASHTASK_WHITELIST += "(.*-cross$|.*-native$|.*-cross-initial$| \
+         .*-cross-intermediate$|^virtual:native:.*|^virtual:nativesdk:.*)"
+            </literallayout>
+            This example is actually where <filename>WORKDIR</filename>
+            is excluded since <filename>WORKDIR</filename> is constructed as a
+            path within <filename>TMPDIR</filename>, which is on the whitelist.
+        </para>
+
+        <para>
+            The <filename>BB_HASHTASK_WHITELIST</filename> covers dependent tasks and 
+            excludes certain kinds of tasks from the dependency chains. 
+            The effect of the previous example is to isolate the native, target,
+            and cross components.
+            So, for example, toolchain changes do not force a rebuild of the whole system.
+        </para>
+
+        <para>
+            The end result of the "basic" handler is to make some dependency and
+            hash information available to the build. 
+            This includes:
+            <literallayout class='monospaced'>
+     BB_BASEHASH_task-&lt;taskname&gt; - the base hashes for each task in the recipe
+     BB_BASEHASH_&lt;filename:taskname&gt; - the base hashes for each dependent task
+     BBHASHDEPS_&lt;filename:taskname&gt; - The task dependencies for each task
+     BB_TASKHASH - the hash of the currently running task
+            </literallayout>
+            There is also a "basichash" <filename>BB_SIGNATURE_HANDLER</filename>,
+            which is the same as the basic version but adds the task hash to the stamp files. 
+            This results in any metadata change that changes the task hash,
+            automatically causing the task to be run again. 
+            This removes the need to bump <filename>PR</filename>
+            values and changes to metadata automatically ripple across the build.
+            Currently, this behavior is not the default behavior.
+            However, it is likely that the Yocto Project team will go forward with this 
+            behavior in the future since all the functionality exists. 
+            The reason for the delay is the potential impact to the distribution feed 
+            creation as they need increasing <filename>PR</filename> fields
+            and the Yocto Project currently lacks a mechanism to automate incrementing 
+            this field.
+        </para>
+    </section>
+
+    <section id='shared-state'>
+        <title>Shared State</title>
+
+        <para>
+            Checksums and dependencies as discussed in the previous section solves half the 
+            problem.
+            The other part of the problem is being able to use checksum information during the build
+            and being able to reuse or rebuild specific components.
+        </para>
+
+        <para>
+            The shared state class (<filename>sstate.bbclass</filename>) 
+            is a relatively generic implementation of how to
+            "capture" a snapshot of a given task. 
+            The idea is that the build process does not care about the source of a
+            task's output.
+            Output could be freshly built or it could be downloaded and unpacked from
+            somewhere - the build process doesn't need to worry about its source.
+        </para>
+
+        <para>
+            There are two types of output, one is just about creating a directory
+            in <filename>WORKDIR</filename>.
+            A good example is the output of either <filename>do_install</filename> or 
+            <filename>do_package</filename>. 
+            The other type of output occurs when a set of data is merged into a shared directory 
+            tree such as the sysroot.
+        </para>
+
+        <para>
+            The Yocto Project team has tried to keep the details of the implementation hidden in 
+            <filename>sstate.bbclass</filename>. 
+            From a user's perspective, adding shared state wrapping to a task
+            is as simple as this <filename>do_deploy</filename> example taken from 
+            <filename>do_deploy.bbclass</filename>:
+            <literallayout class='monospaced'>
+     DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
+     SSTATETASKS += "do_deploy"
+     do_deploy[sstate-name] = "deploy"
+     do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
+     do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"
+
+     python do_deploy_setscene () {
+         sstate_setscene(d)
+     }
+     addtask do_deploy_setscene
+            </literallayout>
+            In the example, we add some extra flags to the task, a name field ("deploy"), an
+            input directory where the task sends data, and the output
+            directory where the data from the task should eventually be copied. 
+            We also add a <filename>_setscene</filename> variant of the task and add the task
+            name to the <filename>SSTATETASKS</filename> list.
+        </para>
+
+        <para>
+            If you have a directory whose contents you need to preserve, 
+            you can do this with a line like the following:
+            <literallayout class='monospaced'>
+     do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
+            </literallayout>
+            This method, as well as the following example, also works for mutliple directories.
+            <literallayout class='monospaced'>
+     do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
+     do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
+     do_package[sstate-lockfile] = "${PACKAGELOCK}"
+            </literallayout>
+            These methods also include the ability to take a lockfile when manipulating
+            shared state directory structures since some cases are sensitive to file
+            additions or removals.
+        </para>
+
+        <para>
+            Behind the scenes, the shared state code works by looking in 
+            <filename>SSTATE_DIR</filename> and  
+            <filename>SSTATE_MIRRORS</filename> for shared state files. 
+            Here is an example:
+            <literallayout class='monospaced'>
+     SSTATE_MIRRORS ?= "\
+     file://.* http://someserver.tld/share/sstate/ \n \
+     file://.* file:///some/local/dir/sstate/"
+            </literallayout>
+        </para>
+
+        <para>
+            The shared state package validity can be detected just by looking at the
+            filename since the filename contains the task checksum (or signature) as
+            described earlier in this section. 
+            If a valid shared state package is found, the build process downloads it 
+            and uses it to accelerate the task.
+        </para>
+
+        <para>
+            The build processes uses the <filename>*_setscene</filename> tasks
+            for the task acceleration phase.
+            BitBake goes through this phase before the main execution code and tries
+            to accelerate any tasks for which it can find shared state packages. 
+            If a shared state package for a task is available, the shared state
+            package is used.
+            This means the task and any tasks on which it is dependent are not 
+            executed.
+        </para>
+
+        <para>
+            As a real world example, the aim is when building an IPK-based image,
+            only the <filename>do_package_write_ipk</filename> tasks would have their 
+            shared state packages fetched and extracted. 
+            Since the sysroot is not used, it would never get extracted. 
+            This is another reason to prefer the task-based approach over a 
+            recipe-based approach, which would have to install the output from every task.
+        </para>
+    </section>
+
+    <section id='tips-and-tricks'>
+        <title>Tips and Tricks</title>
+
+        <para>
+            The code in the Yocto Project that supports incremental builds is not 
+            simple code. 
+            Consequently, when things go wrong, debugging needs to be straightforward. 
+            Because of this, the Yocto Project team included strong debugging
+            tools.
+        </para>
+
+        <para>
+            First, whenever a shared state package is written out, so is a
+            corresponding <filename>.siginfo</filename> file. 
+            This practice results in a pickled python database of all
+            the metadata that went into creating the hash for a given shared state
+            package.
+        </para>
+
+        <para>
+            Second, if BitBake is run with the <filename>--dump-signatures</filename>
+            (or <filename>-S</filename>) option, BitBake dumps out 
+            <filename>.siginfo</filename> files in
+            the stamp directory for every task it would have executed instead of
+            building the target package specified.
+        </para>
+
+        <para>
+            Finally, there is a <filename>bitbake-diffsigs</filename> command that
+            can process these <filename>.siginfo</filename> files. 
+            If one file is specified, it will dump out the dependency
+            information in the file. 
+            If two files are specified, it will compare the
+            two files and dump out the differences between the two.
+            This allows the question of "What changed between X and Y?" to be
+            answered easily.
+        </para>
+    </section>
+</section>
+
+<!--
+
+    <para>
         The Yocto Project build process uses a shared state caching scheme to avoid having to 
         rebuild software when it is not necessary.  
         Because the build time for a Yocto image can be significant, it is helpful to try and 
@@ -222,6 +596,7 @@
         <ulink url='http://git.yoctoproject.org/cgit.cgi/poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54'>commit</ulink>.
     </note>
 </section>
+-->
 
 </chapter>
 <!--