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SCons / doc / design / issues.xml

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<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;
]>

<chapter id="chap-issues"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0/scons.xsd scons.xsd">
<title>Other Issues</title>

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 <para>

   No build tools is perfect.
   Here are some &SCons; issues that
   do not yet have solutions.

 </para>

 <section>
 <title>Interaction with SC-config</title>

   <para>

   The SC-config tool will be used in the &SCons; installation
   process to generate an appropriate default construction environment
   so that building most software works "out of the box" on the
   installed platform.  The SC-config tool will find reasonable default
   compilers (C, C++, Fortran), linkers/loaders, library archive tools,
   etc. for specification in the default &SCons; construction
   environment.

   </para>

 </section>

 <section>
 <title>Interaction with test infrastructures</title>

   <para>

   &SCons; can be configured to use SC-test (or some other test tool)
   to provide controlled, automated testing of software.  The &Link;
   method could link a <filename>test</filename> subdirectory to a build
   subdirectory:

   </para>

    <programlisting>
	Link('test', 'build')
	SConscript('test/SConscript')</programlisting>

   <para>

   Any test cases checked in with the source code will be linked
   into the <filename>test</filename> subdirectory and executed.  If
   &SConscript; files and test cases are written with this in mind, then
   invoking:

   </para>

    <programlisting>
	% sccons test</programlisting>

   <para>

   Would run all the automated test cases that depend on any changed
   software.

   </para>


   <!--

	 YYY	integrate with SC-test to provide sanity check on new tools
		"discovery testing" of new tools
		results recorded in a central database
		central database can be somewhere else on web

   -->

 </section>

 <section>
 <title>Java dependencies</title>

   <para>

   Java dependencies are difficult for an external dependency-based
   construction tool to accomodate.  Determining Java class dependencies
   is more complicated than the simple pattern-matching of C or C++
   <literal>#include</literal> files.  From the point of view of an
   external build tool, the Java compiler behaves "unpredictably"
   because it may create or update multiple output class files and
   directories as a result of its internal class dependencies.

   </para>

   <para>

   An obvious &SCons; implementation would be to have the &Scanner;
   object parse output from <command>Java -depend -verbose</command> to
   calculate dependencies, but this has the distinct disadvantage of
   requiring two separate compiler invocations, thereby slowing down
   builds.

   </para>

 </section>

 <section>
 <title>Limitations of digital signature calculation</title>

   <para>

   In practice, calculating digital signatures of a file's contents is a
   more robust mechanism than time stamps for determining what needs
   building.  However:

   </para>

   <orderedlist numeration="arabic">

    <listitem>
    <para>

     Developers used to the time stamp model of &Make; can initially
     find digital signatures counter-intuitive.  The assumption that:

     <programlisting>
	% touch file.c</programlisting>

     will cause a rebuild of <filename>file</filename> is strong...

    </para>
    </listitem>

    <listitem>
    <para>

     Abstracting dependency calculation into a single digital signature
     loses a little information:  It is no longer possible to tell
     (without laborious additional calculation) which input file dependency
     caused a rebuild of a given target file.  A feature that could
     report, "I'm rebuilding file X because it's out-of-date with respect
     to file Y," would be good, but an digital-signature implementation of
     such a feature is non-obvious.

    </para>
    </listitem>

   </orderedlist>

 </section>

 <section>
 <title>Remote execution</title>

   <para>

   The ability to use multiple build systems through remote execution
   of tools would be good.  This should be implementable through the
   &Job; class.  Construction environments would need modification
   to specify build systems.

   </para>

 </section>

 <section>
 <title>Conditional builds</title>

   <para>

   The ability to check run-time conditions as suggested on the
   sc-discuss mailing list ("build X only if: the machine is idle /
   the file system has Y megabytes free space") would also be good,
   but is not part of the current design.

   </para>

 </section>

</chapter>