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diff --git a/user/jkoenig/java/proposal.mdwn b/user/jkoenig/java/proposal.mdwn new file mode 100644 index 00000000..4052f455 --- /dev/null +++ b/user/jkoenig/java/proposal.mdwn @@ -0,0 +1,628 @@ + +# 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. + |