For A Parallel World. Case Study n.2: misknowing your make rules

Here comes another case study about parallel make failures and fixes. This time I’m going to write about a much less common, and more difficult to understand, type of failure. I have spotted and fixed this failure in gtk# (yes I have it installed).

Let’s see the failure to begin with:

Creating policy.2.4.glib-sharp.dll
Creating policy.2.4.glib-sharp.dll
Creating policy.2.4.glib-sharp.dll
ALINK: error A1019: Metadata failure creating assembly -- System.IO.FileNotFoundException: Could not find file "/var/tmp/portage/dev-dotnet/gtk-sharp-2.10.2/work/gtk-sharp-2.10.2/glib/policy.2.4.glib-sharp.dll".
File name: "/var/tmp/portage/dev-dotnet/gtk-sharp-2.10.2/work/gtk-sharp-2.10.2/glib/policy.2.4.glib-sharp.dll"
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share) [0x00000] 
  at (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare)
  at System.IO.File.OpenRead (System.String path) [0x00000] 
  at Mono.Security.StrongName.Sign (System.String fileName) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName, PortableExecutableKinds portableExecutableKind, ImageFileMachine imageFileMachine) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName) [0x00000] 
  at Mono.AssemblyLinker.AssemblyLinker.DoIt () [0x00000] 
ALINK: error A1019: Metadata failure creating assembly -- System.IO.IOException: Sharing violation on path /var/tmp/portage/dev-dotnet/gtk-sharp-2.10.2/work/gtk-sharp-2.10.2/glib/policy.2.4.glib-sharp.dll
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean isAsync, Boolean anonymous) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access) [0x00000] 
  at (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess)
  at System.Reflection.Emit.ModuleBuilder.Save () [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName, PortableExecutableKinds portableExecutableKind, ImageFileMachine imageFileMachine) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName) [0x00000] 
  at Mono.AssemblyLinker.AssemblyLinker.DoIt () [0x00000] 
Creating policy.2.6.glib-sharp.dll
Creating policy.2.6.glib-sharp.dll
Creating policy.2.6.glib-sharp.dll
ALINK: error A1019: Metadata failure creating assembly -- System.IO.IOException: Sharing violation on path /var/tmp/portage/dev-dotnet/gtk-sharp-2.10.2/work/gtk-sharp-2.10.2/glib/policy.2.6.glib-sharp.dll
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean isAsync, Boolean anonymous) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access) [0x00000] 
  at (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess)
  at System.Reflection.Emit.ModuleBuilder.Save () [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName, PortableExecutableKinds portableExecutableKind, ImageFileMachine imageFileMachine) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName) [0x00000] 
  at Mono.AssemblyLinker.AssemblyLinker.DoIt () [0x00000] 
Creating policy.2.8.glib-sharp.dll
Creating policy.2.8.glib-sharp.dll
Creating policy.2.8.glib-sharp.dll
ALINK: error A1019: Metadata failure creating assembly -- System.IO.IOException: Sharing violation on path /var/tmp/portage/dev-dotnet/gtk-sharp-2.10.2/work/gtk-sharp-2.10.2/glib/policy.2.8.glib-sharp.dll
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] 
  at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share) [0x00000] 
  at (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare)
  at System.IO.File.OpenWrite (System.String path) [0x00000] 
  at Mono.Security.StrongName.Sign (System.String fileName) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName, PortableExecutableKinds portableExecutableKind, ImageFileMachine imageFileMachine) [0x00000] 
  at System.Reflection.Emit.AssemblyBuilder.Save (System.String assemblyFileName) [0x00000] 
  at Mono.AssemblyLinker.AssemblyLinker.DoIt () [0x00000] 
make[3]: *** [policy.2.4.glib-sharp.dll] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [all-recursive] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

Okay so there are some failures during calling “alink”, in particular it reports “sharing violations”. I suppose the name of the error message is derived from the original .NET as “sharing violation” is what Windows reports when two applications try to write to the same file at once, or one tries to write to a file that is locked down by someone else.

But I want to put some emphasis on something in particular:

Creating policy.2.4.glib-sharp.dll
Creating policy.2.4.glib-sharp.dll
Creating policy.2.4.glib-sharp.dll
[...]
Creating policy.2.6.glib-sharp.dll
Creating policy.2.6.glib-sharp.dll
Creating policy.2.6.glib-sharp.dll
[...]
Creating policy.2.8.glib-sharp.dll
Creating policy.2.8.glib-sharp.dll
Creating policy.2.8.glib-sharp.dll
[...]
make[3]: *** [policy.2.4.glib-sharp.dll] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [all-recursive] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

As you can see each policy is reportedly created thrice. If you, like me, know what to look for in a parallel make failure, you’ll also notice that there are three policies being created there. This is quite important and interesting, as it already suggests to an experienced eye what the problem is, but let’s go on step by step.

Once again, we know the software is built with automake so you don’t expect parallel make failures, not from the default rules at least. But C#/Mono is not one of the languages that automake supports out of the box. Which means that almost surely there are custom rules involved.

As they are using custom rules, rather than automake the problem involves knowledge of GNU make (or any other make, but let’s assume GNU for now, it’s the most common in Free Software after all, for good or bad).

Let’s look for the “Creating” line in the Makefile.am file:

$(POLICY_ASSEMBLIES): $(top_builddir)/policy.config gtk-sharp.snk
        @for i in $(POLICY_VERSIONS); do        
          echo "Creating policy.$$i.$(ASSEMBLY)";       
          sed -e "s/@ASSEMBLY_NAME@/$(ASSEMBLY_NAME)/" -e "s/@POLICY@/$$i/" $(top_builddir)/policy.config > policy.$$i.config;  
          $(AL) -link:policy.$$i.config -out:policy.$$i.$(ASSEMBLY) -keyfile:gtk-sharp.snk;     
        done

If you had to deal with a similar failure before (as I did), you knew already what you were going to find in that rule. I’m referring to the for loop. It’s a common mistake for people not knowing make well enough to create a rule like this. They expect that declaring multiple targets in the rule means, for make “build all of these with a single command”, while it actually means “for any of these files, use this command to generate it”.

The result is that, as you’re going to need three different files, make will launch three times that code in parallel. Which not only will waste a huge amount of time but will also fail, as the three of them might try to access the same resource at once (like is happening here).

The solution for this kind of problem is not really obvious, as it often requires to rewrite the rules entirely. My usual way of thinking of the problem here is that whoever wrote the rule didn’t know make well enough and made a mistake, and it’s easier to just rewrite the rule.

Let’s decompose the rule then, ignoring the for loop, and the echo line, what we have is these two commands:

sed -e "s/@ASSEMBLY_NAME@/$(ASSEMBLY_NAME)/" -e "s/@POLICY@/$$i/" $(top_builddir)/policy.config > policy.$$i.config
$(AL) -link:policy.$$i.config -out:policy.$$i.$(ASSEMBLY) -keyfile:gtk-sharp.snk

Both of these two commands create a different file, one is intermediate, and is the policy configuration, the other is the final one. This again shows there’s a lack of understanding of how make is supposed to work, again a very common one, so I’m not blaming the developer here, make is a strange language. So there are two dependent steps involved here: the final requested result is the policy file, but to generate that you need the policy configuration.

Let’s start with the policy configuration then, the actual generation command is a simple sed call that takes the generic configuration and sets the assembly name and policy version in it. The problem here is obviously to replace the use of $$i (the variable used in the for loop) with the actual policy name. Just so we’re clear, the policy version is the 2.4, 2.6 and 2.8 string we have seen before. Luckily this is a pretty common task for a software like make and there is a construct that gets in our help: static pattern rules.

The name of the generated file is always in the format policy.$VERSION.config, and we need to know the $VERSION part for using it in sed. Nothing more suited for this than static pattern rules. Let’s replace the variable section of the filename with the magic symbol %, make will take care of expanding that as needed, and will also provide us a special variable in the rule, $* that will take the value of its expansion. The rule then becomes this:

policy.%.config: $(top_builddir)/policy.config
    sed -e "s/@ASSEMBLY_NAME@/$(ASSEMBLY_NAME)/" -e "s/@POLICY@/$*/" $(top_builddir)/policy.config > $@

And here we’ve created our policy configuration files, in a parallel build friendly way as none of them is dependent on the other, the three sed commands can easily be executed in parallel.

Now it’s time to create the actual policy assembly, again, we’re going to make use of the static pattern rules, and making the best use of the fact that you can also declare dependencies based on static patterns.

Instead of a simple two-entries rule, this is going to be a three-entries rule, the first entry defines the list of targets that this rule may apply to, that is the same as it was before ($(POLICY_ASSEMBLIES)), the second and third are the usual ones, defining target and dependencies.

While the original rule depended directly on the generic policy config, this one will only depend on the actual final config, as the rule we just wrote for the configuration files will take care of it. So the final rule to generate the wanted assembly will be:

$(POLICY_ASSEMBLIES) : policy.%.$(ASSEMBLY): policy.%.config gtk-sharp.snk
    $(AL) -link:policy.$*.config -out:$@ -keyfile:gtk-sharp.snk

At this point, the same has just to be applied to all the involved Makefile.am files in the package, like I did on the patch I submitted, and the package becomes totally parallel build friendly.

There is another nice addition to this: you’re trading one complex, difficult to read and broken rule with two one-liner rules, which makes the code much more readable and understandable if you’re looking for a mistake.