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+
+# Java for Hurd (and vice versa)
+
+Contact information:
+
+  * Full name: Jérémie Koenig
+  * Email: jk@jk.fr.eu.org
+  * IRC: jkoenig on Freenode and OFTC
+
+## Introductions
+
+I am a first year M.Sc. student
+in Computer Science at University of Strasbourg (France).
+My interests include capability-based security,
+programming languages and formal methods
+(in particular, object-capability languages and proof-carrying code).
+
+### Proposal summary
+
+This project would consist in improving Java support on Hurd.
+The first part would consist in
+fixing bugs and porting Java-related packages.
+The second part would consist in
+creating low-level Java bindings for the Hurd interfaces,
+as well as libraries to make translator development easier.
+
+### Previous involvement
+
+I started contributing to Hurd last summer,
+during which I participated to Google Summer of Code
+as a student for the Debian project.
+I worked on porting Debian-Installer to Hurd.
+This project was mostly a success,
+although we still have to use a special mirror for installation
+with a few modified packages
+and tweaked priorities
+to work around some uninstallable packages
+with Priority: standard.
+
+Shortly afterwards,
+I rewrote the procfs translator
+to fix some issues with memory leaks,
+make it more reliable,
+and improve compatibility with Linux-based tools
+such as `procps` or `htop`.
+
+Although I have not had as much time
+as I would have liked to dedicate to the Hurd
+since that time,
+I have continued to maintain the mirror in question,
+and I have started to work
+on implementing POSIX threads signal semantics in glibc.
+
+### Project-related skills and interests
+
+I have used Java mostly for university assignments.
+This includes non-trivial projects
+using threads and distributed programming frameworks
+such as Java RMI or CORBA.
+I have also used it to experiment with
+Google App Engine
+(web applications)
+and Google Web Toolkit
+(a compiler from Java to Javascript which helps with AJAX code),
+and I have some limited experience with JNI
+(the Java Native Interface, to link Java with C code).
+
+My knowledge of the Hurd and Debian GNU/Hurd is reasonable,
+as the Debian-Installer and procfs projects
+gave me the opportunity to fiddle with many parts of the system.
+
+Initially,
+I started working on this project because I wanted to use
+[Joe-E](http://code.google.com/p/joe-e/)
+(a subset of Java)
+to investigate the potential
+[[applications of object-capability languages|objcap]]
+in a Hurd context.
+I also believe that improving Java support on Hurd
+would be an important milestone.
+
+### Organisational matters
+
+I am subscribed to bug-hurd@g.o and
+I do have a permanent internet connexion.
+
+I would be able to attend the regular IRC meetings,
+and otherwise communicate with my mentor
+through any means they would prefer
+(though I expect email and IRC would be the most practical).
+Since I'm already familiar with the Hurd,
+I don't expect I would require too much time from them.
+
+My exams end on May 20 so I would be able to start coding
+right at the beginning of the GSoC period.
+Next year's term would probably begin around September 15,
+so that would not be an issue either.
+I expect I would work around 40 hours per week,
+and my waking hours would be flexible.
+
+I don't have any other plans for the summer
+and would not make any if my project were to be accepted.
+
+Full disclosure:
+I also submitted a proposal to the Jikes RVM project
+(which is a research-oriented Java Virtual Machine,
+itself written in Java)
+for implementing a new garbage collector into the MMTk subsystem.
+
+## Improve Java support
+
+### Justification
+
+Java is a popular language and platform used by many desktop and web
+applications (mostly on the server side). As a consequence, competitive Java
+support is important for any general-purpose operating system.
+Better Java support would also be a prerequisite
+for the second part of my proposal.
+
+### Current situation
+
+Java is currently supported on Hurd with the GNU Java suite:
+
+   * [GCJ](http://gcc.gnu.org/java/),
+     the GNU Compiler for Java, is part of GCC and can compile Java
+     source code to Java bytecode, and both source code and bytecode to
+     native code;
+   * libgcj is the implementation of the Java runtime which GCJ uses.
+     It is based on [GNU Classpath](http://www.gnu.org/software/classpath/).
+     It includes a bytecode interpreter which enables
+     Java applications compiled to native code to dynamically load and execute
+     Java bytecode from class files.
+   * The gij command is a wrapper around the above-mentioned virtual machine
+     functionality of libgcj and can be used as a replacement for the java
+     command.
+
+However, GCJ does not work flawlessly on Hurd.r
+For instance, some parts of libgcj relies on
+the POSIX threads signal semantics, which are not yet implemented.
+In particular, this makes ant hang waiting for child processes,
+which makes some packages fail to build on Hurd
+(“ant” is the “make” of the Java world).
+
+### Tasks
+
+   * **Finish implementing POSIX thread semantics** in glibc (high priority).
+     According to POSIX, signal dispositions should be global to a process,
+     while signal blocking masks should be thread-specific. Signals sent to the
+     process as a whole are to be delivered to any thread which does not block
+     them. By contrast, Hurd has per-thread signal dispositions and signals
+     sent to a process are delivered to the main thread only. I have been
+     working on refactoring the glibc signal code and implementing the POSIX
+     semantics as a per-thread option. However, due to lack of time I have not
+     yet been able to test and debug my code properly. Finishing this work
+     would be my first task.
+   * **Fix further problems with GCJ on Hurd** (high priority). While I’m not
+     aware of any other problems with GCJ at the moment, I suspect some might
+     turn up as I progress with the other tasks. Fixing these problems would
+     also be a high-priority task.
+   * **Port OpenJDK 6** (medium priority). While GCJ is fine, it is not yet
+     100% complete. It is also slower than OpenJDK on architectures where a
+     just-in-time compiler is available. Porting OpenJDK would therefore
+     improve Java support on Hurd in scope and quality. Besides, it would also
+     be a good way to test GCJ, which is used for bootstrapping by the Debian
+     OpenJDK packages. Also note that OpenJDK 6 is now the default Java
+     Runtime Environment on all released Linux-based Debian architectures;
+     bringing Hurd in line with this would probably be a good thing.
+   * **Port Eclipse and other Java applications** (low priority). Eclipse is a
+     popular, state-of-the-art IDE and tool suite used for Java and other
+     languages. It is a dependency of the Joe-E verifier (see part 3 of this
+     proposal). Porting Eclipse would be a good opportunity to test GCJ and
+     OpenJDK.
+
+### Deliverables
+
+  * The glibc pthreads patch and any other fixes on the Hurd side
+    would be submitted upstream
+  * Patches against Debian source packages
+    required to make them build on Hurd would be submitted
+    to the [Debian bug tracking system](http://bugs.debian.org/).
+
+
+## Create Java bindings for the Hurd interfaces
+
+### Justification
+
+Java is used for many applications and often taught to
+introduce object-oriented programming. The fact that Java is a
+garbage-collected language makes it easier to use, especially for the less
+experienced programmers. Besides, its object-oriented nature is a
+natural fit for the capability-based design of Hurd.
+The JVM is also used as a target for many other languages,
+all of which would benefit from the access provided by these bindings.
+
+Advantages over other garbage-collected, object-oriented languages include
+performance, type safety and the possibility to compile a Java translator to
+native code and
+[link it statically](http://gcc.gnu.org/wiki/Statically_linking_libgcj)
+using GCJ, should anyone want to use a
+translator written in Java for booting.
+Note that Java is
+[being](http://www.linuxjournal.com/article/8757)
+[used](http://oss.readytalk.com/avian/)
+in this manner for embedded development.
+Since GCJ can take bytecode as its input,
+this expect this possibility would apply to any JVM-based language.
+
+Java bindings would lower the bar for newcomers
+to begin experimenting with what makes Hurd unique
+without being faced right away with the complexity of
+low-level systems programming.
+
+### Tasks summary
+
+  * Implement Java bindings for Mach
+  * Implement a libports-like library for Java
+  * Modify MIG to output Java code
+  * Implement libfoofs-like Java libraries
+
+### Design principles
+
+The principles I would use to guide the design
+of these Java bindings would be the following ones:
+
+  * The system should be hooked into at a low level,
+    to ensure that Java is a "first class citizen"
+    as far as the access to the Hurd's interfaces is concerned.
+  * At the same time, the memory safety of Java should be maintained
+    and extended to Mach primitives such as port names and
+    out-of-line memory regions.
+  * Higher-level interfaces should be provided as well
+    in order to make translator development
+    as easy as possible.
+  * A minimum amount of JNI code (ie. C code) should be used.
+    Most of the system should be built using Java itself
+    on top of a few low-level primitives.
+  * Hurd objects would map to Java objects.
+  * Using the same interfaces,
+    objects corresponding to local ports would be accessed directly,
+    and remote objects would be accessed over IPC.
+
+One approach used previously to interface programming languages with the Hurd
+has been to create bindings for helper libraries such as libtrivfs. Instead,
+for Java I would like to take a lower-level approach by providing access to
+Mach primitives and extending MIG to generate Java code from the interface
+description files.
+
+This approach would be initially more involved, and would introduces several
+issues related to overcoming the "impedance mismatch" between Java and Mach.
+However, once an initial implementation is done it would be easier to maintain
+in the long run and we would be able to provide Java bindings for a large
+percentage of the Hurd’s interfaces.
+
+### Bindings for Mach system calls
+
+In this low-level approach, my intention is to enable Java code to use Mach
+system calls (in particular, mach_msg) more or less directly. This would
+ensure full access to the system from Java code, but it raises a number of
+issues:
+
+  * the Java code must be able to manipulate Mach-level entities, such as port
+    rights or page-aligned buffers mapped outside of the garbage-collected
+    heap (for out-of-line transfers);
+  * putting together IPC messages requires control of the low-level
+    representation of data.
+
+In order to address these concerns, classes would be encapsulating these
+low-level entities so that they can be referenced through normal, safe objects
+from standard Java code. Bindings for Mach system calls can then be provided
+in terms of these classes. Their implementation would use C code through the
+Java Native Interface (JNI).
+
+More specifically, this functionality would be provided by the `org.gnu.mach`
+package, which would contain at least the following classes:
+
+  * `MachPort` would encapsulate a `mach_port_t`. (Some of) its constructors
+    would act as an interface for the `mach_port_allocate()` system call.
+    `MachPort` objects would also be instantiated from other parts of the JNI
+    C code to represent port rights received through IPC. The `deallocate()`
+    method would call `mach_port_deallocate()` and replace the encapsulated
+    port name with `MACH_PORT_DEAD`. We would recommend that users call it
+    when a port is no longer used, but the finalizer would also deallocate the
+    port when the `MachPort` object is garbage collected.
+  * `Buffer` would represent a page-aligned buffer allocated outside of the
+    Java heap, to be transferred (or having been received) as out-of-line
+    memory. The JNI code would would provide methods to read and write data at
+    an arbitrary offset (but within bounds) and would use `vm_allocate()` and
+    `vm_deallocate()` in the same spirit as for `MachPort` objects.
+  * `Message` would allow Java code to put together Mach messages. The
+    constructor would allocate a `byte[]` member array of a given size.
+    Additional methods would be provided to fill in or query the information
+    in the message header and additional data items, including `MachPort` and
+    `Buffer` objects which would be translated to the corresponding port names
+    and out-of-line pointers.
+    A global map from port names to the corresponding `MachPort` object
+    would probably be needed to ensure that there is a one-to-one
+    correspondence.
+  * `Syscall` would provide static JNI methods for performing system calls not
+    covered by the above classes, such as `mach_msg()` or
+    `mach_thread_self()`.  These methods would accept or return `MachPort`,
+    `Buffer` and `Message` objects when appropriate. The associated C code
+    would access the contents of such objects directly in order to perform the
+    required unsafe operations, such as constructing `MachPort` and `Buffer`
+    objects directly from port names and C pointers.
+
+Note that careful consideration should be given to the interfaces of these
+classes to avoid “safety leaks” which would compromise the safety guarantees
+provided by Java. Potential problematic scenarios include the following
+examples:
+
+  * It must not be possible to write an integer at some position in a
+    `Message` object, and to read it back as a `MachPort` or `Buffer` object,
+    since this would allow unsafe access to arbitrary memory addresses and
+    mach port names.
+  * Providing the `mach_task_self()` system call would also provide access to
+    arbitrary addresses and ports by using the `vm_*` family of RPC operations
+    with the returned `MachPort` object. This means that the relevant task
+    operations should be provided by the `Syscall` class instead.
+
+Finally, access should be provided to the initial ports and file descriptors
+in `_hurd_ports` and provided by the `getdport()` function,
+for instance through static methods such as
+`getCRDir()`, `getCWDir()`, `getProc()`, ... in a dedicated class such as
+`org.gnu.hurd.InitPorts`.
+
+A realistic example of code based on such interfaces would be:
+
+    import org.gnu.mach.MsgType;
+    import org.gnu.mach.MachPort;
+    import org.gnu.mach.Buffer;
+    import org.gnu.mach.Message;
+    import org.gnu.mach.Syscall;
+    import org.gnu.hurd.InitPorts;
+
+    public class Hello
+    {   
+       public static main(String argv[])
+           /* Parent class for all Mach-related exceptions */
+           throws org.gnu.mach.MachException
+       {   
+           /* Allocate a reply port */
+           MachPort reply = new MachPort();
+
+           /* Allocate an out-of-line buffer */
+           Buffer data = new Buffer(MsgType.CHAR, 13);
+           data.writeString(0, "Hello, World!");
+
+           /* Craft an io_write message */
+           Message msg = new Message(1024);
+           msg.setRemotePort(InitPorts.getdport(1));
+           msg.setLocalPort(reply, Message.Type.MAKE_SEND_ONCE);
+           msg.setId(21000);
+           msg.addBuffer(data);
+
+           /* Make the call, MACH_MSG_SEND | MACH_MSG_RECEIVE */
+           Syscall.machMsg(msg, true, true, reply);
+
+           /* Extract the returned value */
+           msg.assertId(21100);
+           int retCode = msg.readInt(0);
+           int amount = msg.readInt(1);
+       }
+    }
+
+Should this paradigm prove insufficient,
+more ideas could be borrowed from the
+[`org.vmmagic`](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.5253&rep=rep1&type=pdf)
+package used by [Jikes RVM](http://jikesrvm.org/),
+a research Java virtual machine itself written in Java.
+
+### Generating Java stubs with MIG
+
+Once the basic machinery is in place to interface with Mach, Java programs
+have more or less equal access to the system functionality without resorting
+to more JNI code. However, as illustrated above, this access is far from
+convenient.
+
+As a solution I would modify MIG to add the option to output Java code. MIG
+would emit a Java interface, a client class able to implement the interface
+given a Mach port send right, an a server class which would be able to handle
+incoming messages. The class diagram below, although it is by no means
+complete or exempt of any problem, illustrates the general idea:
+
+[[gsoc2011_classes.png]]
+
+This structure is somewhat reminiscent of
+[Java RMI](http://en.wikipedia.org/wiki/Java_remote_method_invocation)
+or similar systems,
+which aim to provide more or less transparent access to remote objects.
+The exact way the Java code would be generated still needs to be determined,
+but basically:
+
+  * An interface, corresponding to the header files generated by MIG, would
+    enumerate the operations listed in a given .defs files. Method names would
+    be transformed to adhere to Java conventions (for instance,
+    `some_random_identifier` would become `someRandomIdentifier`).
+  * A user class, corresponding to the `*User.c` files,
+    would implement this interface by doing RPC over a given MachPort object.
+  * A server class, corresponding to `*Server.c`, would be able to handle
+    incoming messages using a user-provided implementation of the interface.
+    (Possibly, a skeleton class providing methods which would raise
+    `NotImplementedException`s would be provided as well.
+    Users would derive from this class and override the relevant methods.
+    This would allow them not to implement some operations,
+    and would avoid pre-existing code from breaking when new operations are
+    introduced.)
+
+In order to help with the implementation of servers, some kind of library
+would be needed to associate Mach receive rights with server objects and to
+handle incoming messages on dedicated threads, in the spirit of libports.
+This would probably require support for port sets at the level of the Mach
+primitives described in the previous section.
+
+When possible, operations involving the transmission of send rights
+of some kind would be expressed in terms of the MIG-generated interfaces
+instead of `MachPort` objects.
+Upon reception of a send right,
+a `FooUser` object would be created
+and associated with the corresponding `MachPort` object.
+If the received send right corresponds to a local port
+to which a server object has been associated,
+this object would be used instead.
+This way,
+subsequent operations on the received send right
+would be handled as direct method calls
+instead of going through RPC mechanisms.
+
+Some issues will still need to be solved regarding how MIG will convert
+interface description files to Java interfaces. For instance:
+
+  * `.defs` files are not explicitly associated with a type. For instance in
+    the example above, MIG would have to somehow infer that io_t corresponds
+    to `this` in the `Io` interface.
+  * More generally, a correspondence between MIG and Java types would have
+    to be determined. Ideally this would be automated and not hardcoded
+    too much.
+  * Initially, reply port parameters would be ignored. However they may be
+    needed for some applications.
+
+So the details would need to be flushed out during the community bonding
+period and as the implementation progresses. However I’m confident that a
+satisfactory solution can be designed.
+
+Using these new features, the example above could be rewritten as:
+
+    import org.gnu.hurd.InitPorts;
+    import org.gnu.hurd.Io;
+    import org.gnu.hurd.IoUser;
+
+    class Hello {
+       static void main(String argv[]) throws ...
+       {
+           Io stdout = new IoUser(InitPorts.getdport(1));
+           String hello = “Hello, World!\n”;
+
+           int amount = stdout.write(hello.getBytes(), -1);
+
+           /* (A retCode corresponding to an error
+               would be signalled as an exception.) */
+       }
+    }
+
+An example of server implementation would be:
+
+    import org.gnu.hurd.Io;
+    import java.util.Arrays;
+
+    class HelloIo implements Io {
+       final byte[] contents = “Hello, World!\n”.getBytes();
+
+       int write(byte[] data, int offset) {
+           return SOME_ERROR_CODE;
+       }
+
+       byte[] read(int offset, int amount) {
+           return Arrays.copyOfRange(contents, offset,
+                                     offset + amount - 1);
+       }
+
+       /* ... */
+    }
+
+A new server object could then be created with `new IoServer(new HelloIo())`,
+and associated with some receive right at the level of the ports management
+library.
+
+### Base classes for common types of translators
+
+Once MIG can target Java code, and a libports equivalent is available,
+creating new translators in Java would be greatly facilitated. However,
+we would probably want to introduce basic implementations of file system
+translators in the spirit of libtrivfs or libnetfs. They could take the form
+of base classes implementing the relevant MIG-generated interfaces which
+would then be derived by users,
+or could define a simpler interface
+which would then be used by adapter classes
+to implement the required ones.
+
+I would draw inspiration from libtrivfs and libnetfs
+to design and implement similar solutions for Java.
+
+### Deliverables
+
+  * A hurd-java package would contain the Java code developed
+    in the context of this project.
+  * The Java code would be documented using javadoc
+    and a tutorial for writing translators would be written as well.
+  * Modifications to MIG would be submitted upstream,
+    or a patched MIG package would be made available.
+
+The Java libraries resulting from this work,
+including any MIG support classes
+as well as the class files built from the MIG-generated code
+for the Mach and Hurd interface definition files,
+would be provided as single `hurd-java` package for
+Debian GNU/Hurd.
+This package would be separate from both Hurd and Mach,
+so as not to impose unreasonable build dependencies on them.
+
+I expect I would be able to act as its maintainer in the foreseeable future, 
+either as an individual or as a part of the Hurd team.
+Hopefully,
+my code would be claimed by the Hurd project as their own,
+and consequently the modifications to MIG
+(which would at least conceptually depend on the Mach Java package)
+could be integrated upstream.
+
+Since by design,
+the Java code would use only a small number of stable interfaces,
+it would not be subject to excessive amounts of bitrot.
+Consequently,
+maintenance would primarily consist in
+fixing bugs as they are reported,
+and adding new features as they are requested.
+A large number of such requests
+would mean the package is useful,
+so I expect that the overall amount of work
+would be correlated with the willingness of more people
+to help with maintenance
+should I become overwhelmed or get hit by a bus.
+
+
+## Timeline
+
+The dates listed are deadlines for the associated tasks.
+
+  * *Community bonding period.*
+    Discuss, refine and complete the design of the Java bindings
+    (in particular the MIG and "libports" parts)
+  * *May 23.*
+    Coding starts.
+  * *May 30.*
+    Finish implementing pthread signal semantics.
+  * *June 5.*
+    Port OpenJDK
+  * *June 12.*
+    Fix the remaining problems with GCJ and/or OpenJDK,
+    possibly port Eclipse or other big Java packages.
+  * *June 19.*
+    Create the bindings for Mach.
+  * *June 26.*
+    Work on some kind of basic Java libports
+    to handle receive rights.
+  * *July 3.*
+    Test, write some documentation and examples.
+  * *July 17 (two weeks).*
+    Add the Java target to MIG.
+  * *July 24.*
+    Test, write some documentation and examples.
+  * *August 7 (two weeks).*
+    Implement a modular libfoofs to help with translator development.
+    Try to write a basic but non-trivial translator
+    to evaluate the performance and ease of use of the result,
+    rectify any rough edges this would uncover.
+  * *August 22. (last two weeks)*
+    Polish the code and packaging,
+    finish writing the documentation.
+
+
+## Conclusion
+
+This project is arguably ambitious.
+However, I have been thinking about it for some time now
+and I'm confident I would be able to accomplish most of it.
+
+In the event multiple language bindings projects
+would be accepted,
+some work could probably be done in common.
+In particular,
+[ArneBab](http://www.bddebian.com/~hurd-web/community/weblogs/ArneBab/2011-04-06-application-pyhurd/)
+seems to favor a low-level approach for his Python bindings as I do for Java,
+and I would be happy to discuss API design and coordinate MIG changes with him.
+I would also have an extra month after the end of the GSoC period
+before I go back to school,
+which I would be able to use to finish the project
+if there is some remaining work.
+(Last year's rewrite of procfs was done during this period.)
+
+As for the project's benefits,
+I believe that good support for Java
+is a must-have for the Hurd.
+Java bindings would also further the Hurd's agenda
+of user freedom by extending this freedom to more people:
+I expect the set of developers
+who would be able to write Java code against a well-written libfoofs
+is much larger than
+those who master the intricacies of low-level systems C programming.
+From a more strategic point of view,
+this would also help recruit new contributors
+by providing an easier path to learning the inner workings of the Hurd.
+
+Further developments
+which would build on the results of this project
+include my planned [[experiment with Joe-E|objcap]]
+(which I would possibly take on as a university project next year).
+Another possibility would be to reimplement some parts
+of the Java standard library
+directly in terms of the Hurd interfaces
+instead of using the POSIX ones through glibc.
+This would possibly improve the performance
+of some Java applications (though probably not by much),
+and would otherwise be a good project
+for someone trying to get acquainted with Hurd.
+
+Overall, I believe this project would be fun, interesting and useful.
+I hope that you will share this sentiment
+and give me the opportunity to spend another summer working on Hurd.
+