RFR (L) JDK-8154239: -Xbootclasspath/a breaks exploded build

[Lois Foltan]

Hello,

Please review the following fix:

Webrev:
     http://cr.openjdk.java.net/~lfoltan/bug_jdk8154239/webrev/

Bug: -Xbootclasspath/a breaks exploded build
     https://bugs.openjdk.java.net/browse/JDK-8154239

Summary:
The JVM was incorrectly handling how to set up and search the 
ClassPathEntry's for directories representing the module definitions in 
an exploded module build situation for the boot class loader. The 
incorrect set up and search was particularly exposed when 
-Xbootclasspath/a was specified in conjunction with a exploded module 
build.  In an exploded module build, when Modules::define_modules was 
called to define a module to the boot loader, a ClassPathEntry was 
simply appended to the boot loader's search list.  At load class time, a 
class would then be searched by simply traversing that list without any 
regards to the module that class was defined in.  This is incorrect 
behavior.  The class should only be searched for in the location of the 
module's definition that it was defined within.  The fix now records 
each module and it's exploded build location in order to adhere to this 
rule.

The other piece to this problem was that if -Xbootclasspath/a was 
specified, it was incorrectly injected prior to the majority of 
ClassPathEntry's for the exploded modules, yielding a broken and 
incorrect search order for the boot loader.  Introducing the concept of 
a "base" or "core" piece for the boot loader which is either the java 
runtime image or the exploded modules build and changing the 
ClassLoader::_first_append_entry to truly be only appended entries added 
via -Xbootclasspath/a or jvmti appended entries, introduces a clean way 
for how this information is stored in the ClassLoader data structure 
that maps directly to how ClassLoader::load_class() searches that 
information.

  215   // The boot class path consists of 3 ordered pieces:
  216   //  1. the module/path pairs specified to -Xpatch
  217   //    -Xpatch:<module>=<file>(<pathsep><file>)*
  218   //  2. the base piece
  219   //    [jimage | build with exploded modules]
  220   //  3. boot loader append path
  221   //    [-Xbootclasspath/a]; [jvmti appended entries]
  222   //
  223   // The boot loader must obey this order when attempting
  224   // to load a class.


Testing:
Full hotspot nightly testing with a java runtime image build
JDK java/io, java/lang, java/util with both a java runtime image and an 
exploded module build
JCK lang & vm with both a java runtime image and an exploded module build
Hotspot jtreg tests with both a java runtime image and an exploded 
module build