RFR: JDK-8159047: Disable redundant build steps when creating buildjdk

[Tim Bell]

Erik:

> When creating the buildjdk (when cross compiling), the idea is to only 
> rebuild the parts that actually need to be rebuilt for the build 
> platform and reuse as much as possible from the normal build. 
> Currently, the interim-langtools step is rebuilt when it shouldn't be.
>
> The reason interim-langtools gets rebuilt is that the gensrc steps for 
> those modules get rebuilt. They in turn get rebuilt because the 
> buildjdk redefines the support outputdir, so for the buildjdk 
> makefiles, it looks like the gensrc steps for those modules have not 
> happened. The buildtools outputdir is not redefined however, so when 
> the interim-langtools get rebuilt, it overwrites the old build of it.
>
> While this is bad from an efficiency standpoint, it also causes 
> intermittent failures due to a race. When building the test lib, the 
> compilation of the test lib may happen at the same time as the 
> interim-langtools being rebuilt for the buildjdk. This means that the 
> class files being run for the interim javac compiler can be 
> overwritten while they are running. The failure can look like this:
>
> BuildTestLib.gmk:49: recipe for target 
> '/scratch/opt/jprt/T/P1/220139.ssteuck/s/build/linux-arm64/support/test/lib/test-lib_classes/_the.BUILD_TEST_LIB_JAR_batch' 
> failed
> make[3]: Leaving directory 
> '/scratch/opt/jprt/T/P1/220139.ssteuck/s/make/test'
> make/Main.gmk:413: recipe for target 'build-test-lib' failed
> Exception in thread "main" java.lang.ClassFormatError: Truncated class 
> file
>     at com.sun.tools.javac.main.CommandLine.parse(CommandLine.java:58)
>     at 
> com.sun.tools.sjavac.options.OptionHelper.traverse(OptionHelper.java:121)
>     at com.sun.tools.sjavac.options.Options.parseArgs(Options.java:187)
>     at com.sun.tools.sjavac.client.ClientMain.run(ClientMain.java:60)
>     at com.sun.tools.sjavac.client.ClientMain.run(ClientMain.java:49)
>     at com.sun.tools.sjavac.Main.go(Main.java:56)
>     at com.sun.tools.sjavac.Main.main(Main.java:46)
> make[3]: *** 
> [/scratch/opt/jprt/T/P1/220139.ssteuck/s/build/linux-arm64/support/test/lib/test-lib_classes/_the.BUILD_TEST_LIB_JAR_batch] 
> Error 1
> make[2]: *** [build-test-lib] Error 1
>
>
> My proposed fix for this problem is to add more guards in 
> make/Main.gmk that disables the interim and gensrc targets completely 
> when the buildjdk is being built.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8159047
> Webrev: http://cr.openjdk.java.net/~erikj/8159047/webrev.01/

Looks good to me.

/Tim