Getting a live view of environment variables (Gradle and JDK 9)

[David Holmes]

Hi Cedric,

Starting a new sub-thread on this as I want to back up on something.

Can you elaborate as to why specifying the "big kill switch" 
--permit-illegal-access is not viable? Specifically if you use:

-XX:+IgnoreUnrecognizedVMOptions --permit-illegal-access

it should work on 9 and be ignored on earlier releases.

Thanks,
David

On 11/05/2017 7:37 AM, Cédric Champeau wrote:
> Hi all,
>
> I'm writing this on behalf of the Gradle team. This email is closely
> related to the other thread just posted today, but just a timeline
> coincidence (just like the email exchange I had today about this with Alan
> Bateman ;)) and not exactly the same issue.
>
> We are in the process of making sure Gradle runs properly on JDK 9, but
> there's an issue which is unresolved so far, and probably requires a new
> API. It's described at [1], and I have discussed this at Devoxx France with
> Rémi Forax who suggested to post something here.
>
> In short, Gradle is a build tool which supports building a variety of
> different things, from Java to C++. The JVM happens to be its runtime
> environment, and Gradle has what we call the Gradle Daemon [2] which is a
> long running process, benefiting from the JIT, aimed at effectively running
> builds. When the build starts, a client connects to the daemon and sends a
> "build request". A daemon will run a single build at a time, and there are
> several cases which would trigger a new daemon to spawn (the daemon JVM
> arguments are one) but the environment variables are *not*. Since the
> daemon is a long running process, it is possible that the environment
> variables are mutated between the moment the daemon was spawned (in a
> previous build) and the moment the build is executed.
>
> What we do, now, is to send the environment variables of the client to the
> daemon, which *mutates* the existing environment variables map provided by
> System.getenv. This is exactly what is described in [1] as being sneaky (it
> is) and broken in JDK 9 (since the underlying map doesn't exist anymore).
> However, there are valid use cases for this:
>
>    - in practice, environment variables are not immutable. It is especially
>    true for long running process.
>    - native programs can mutate the environment variables. Even if it's not
>    recommended, it is possible and legal.
>    - Gradle runs in a "blackbox": we don't know what plugins are doing.
>    Even if we provide an API which gives access to "environment variables",
>    and that those environment variables are not the ones returned by
>    System.getenv, plugin authors would have to use this new API to get
>    correct information. However, they may use libraries which access
>    System.getenv directly, or use native APIs which would get out-of-sync
>    information.
>    - we need to propagate the environment to forked process (typically,
>    forked compilers and worker daemons)
>
> This means that today, we use JNI to effectively mutate the environment
> variables of running process (that’s one of the purposes of the
> native-platform project). Then, we mutate the backing map of the JDK to
> reflect those changes, otherwise the mutation is not visible from Java code.
>
> What can we do now?
>
>    - Have the JDK honor the fact that environment variables *can* be
>    mutated, because it just happens. In short, don't create an immutable copy
>    of environment variables at startup, but provide a live view of the
>    environment variables (using the existing APIs, System.getenv, would be the
>    best thing because it would be immediately visible to all consumers,
>    including 3rd party code run in plugins). In addition (but not mandatory),
>    you could provide us with an API to set environment variables directly from
>    Java. This would avoid JNI calls to do this. However, it’s not mandatory,
>    because the live view of environment variables would just work in this case.
>    - Last, but we would really, really avoid to do this, spawn a new daemon
>    if we detect that the environment variables have changed (diff between what
>    the client has and the daemon sees). The major drawback of this approach is
>    that it kills performance, since a new daemon would have to be spawned. And
>    it is likely to do so each time something (through native code, for
>    example), mutates environment variables. A very simple example is
> the PWD environment
>    variables on Linux which contains the working directory. Basically changing
>    the directory would be enough to spawn a new daemon. Another example is the
>    TERM_SESSION_ID one, which means that 2 different terminals would force
>    us to spawn 2 different Gradle daemons. We could, of course, have a list of
>    “blessed” environments variables that we don’t trust, but it’s very easily
>    broken, and no good design. That’s why, even if it’s possible, we don’t
>    consider this a solution.
>
> Thanks for considering our request, which is currently a blocker for us
> (understand, have Gradle running properly under JDK 9).
>
> [1] https://github.com/adammurdoch/native-platform/issues/16
> [2] https://docs.gradle.org/current/userguide/gradle_daemon.html
>