We faced with "could not create parent directories" exception during the concurrent file compilation on the fast multi-cpu machine. If seems that the problem exists in the com.sun.tools.javac.util.DefaultFileManager. Let's consider the code below:
...
private void ensureParentDirectoriesExist() throws IOException {
            if (!hasParents) {
                File parent = f.getParentFile();
                if (parent != null && !parent.exists()) {
                //point #1
                    if (!parent.mkdirs()) {
		//point #2
                        throw new IOException("could not create parent directories");
                    }
                }
                hasParents = true;
            }	    
	}
...
Let's imagine the we are compiling two classes a.b.c.d.e.Cl and a.b.c.d.C2 in parallel and directory a/b/c/d/e has not been created yet. Processes 1 and 2 steps on point #1 simultaneously. The one of the processes creates the directory structure, for example 'a/b/c/d' and returns successfully. At the same moment other process invokes parent.mkdirs() and gets false becase some subdiretories has been already created by the process 1. As a result process 2 steps on point #2 and throws an exception.
I have reporoduced this problem using dmake. Please see /home/ag153348/tmp/compiler_bug/Makefile.

Command output for concurrent mode:
----------------------------------
ag153348@oink$ rm -rf classes; dmake all
dmake: defaulting to parallel mode.
See the man page dmake(1) for more information on setting up the .dmakerc file.
oink --> 1 job
/java/re/jdk/6/latest/binaries/solaris-sparc/bin/javac -d classes C1.java
oink --> 2 jobs
/java/re/jdk/6/latest/binaries/solaris-sparc/bin/javac -d classes C2.java
oink --> Job output
/java/re/jdk/6/latest/binaries/solaris-sparc/bin/javac -d classes C2.java
C2.java:2: error while writing a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.C2: could not create parent directories
class C2 {
^
1 error
*** Error code 1
dmake: Fatal error: Command failed for target `C2.ok'
Waiting for 1 job to finish
-------------------------------
Command output for single job mode:
----------------------------------
ag153348@oink$ rm -rf classes; dmake -j 1 all
dmake: defaulting to parallel mode.
See the man page dmake(1) for more information on setting up the .dmakerc file.
oink --> 1 job
/java/re/jdk/6/latest/binaries/solaris-sparc/bin/javac -d classes C1.java
oink --> 1 job
/java/re/jdk/6/latest/binaries/solaris-sparc/bin/javac -d classes C2.java
ag153348@oink$
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