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

[Cédric Champeau]

Thanks Peter for your thoughts, but I don't think it's as simple as that.
As I explained in my original email, there are multiple ways the
environment variables can be mutated, and it can be done _externally_.
Typically, during a task execution, a JNI call performed by a random native
tool could mutate the environment. That's something we, as a build tool,
have to consider as a use case. It's very unlikely but it can happen. This
means that for the same classloader, the environment can change. And for
performance reasons, we cache classloaders between builds, and reuse them
as much as possible (because classloading is far from being cheap).

2017-05-14 19:51 GMT+02:00 Peter Levart <peter.levart at gmail.com>:

> Hi Cedric,
>
> Chiming in with my thoughts...
>
> It's unfortunate that Gradle plugins or libraries used by plugins use
> environment variables at all. I wonder who was the first? Did Gradle
> introduce the feature of passing client environment to the daemon and
> establishing the view of System.getenv for plugins 1st or did libraries
> used by plugins happen to use environment variables using System.getenv and
> Gradle was just kind enough to provide them a dynamic view controlled by
> client?
>
> In the end it doesn't matter. The fact is that System.getenv is part of
> standard Java API and anyone using it should be aware that by doing so,
> they are limiting their software to be (re)configured only by spawning new
> process(es). UNIX environment was not designed to be mutated during the
> course of a long-running process. Maybe just at process startup/setup when
> conditions are under control (i.e. a single running thread) but later, the
> environment is meant to be read only.
>
> Maybe there is a solution for Gradle and othert though. But that solution,
> I think, is not in exposing a "live" view of process environment through
> System.getenv or new methods to "refresh" the view as you are proposing,
> since that would only encourage people to mutate the process environment
> which, as was established on this thread, is not safe in a long-running
> multi-threaded process, which Java processes usually are. Perhaps the
> solution is in extending the System.getenv API with ways to provide
> "custom" views of variables for code that runs in a "container".
>
> Gradle is, among other things, a container for plugins. Is Gradle running
> plugins in a separate ClassLoader? Does it use a separate ClassLoader for
> each "build" which might bring with it a new set of environment variables
> from the client? In such a setup, one could provide a separate set of
> environment variables for each ClassLoader, simply by passing them to the
> ClassLoader constructor. System.getenv would need to be a @CallerSensitive
> method which would return caller's ClassLoader view of variables, if any
> such view was established, or simply the view inherited from the parent
> ClassLoader.
>
> Such would be a "functional" approach, which would keep environment
> variables immutable, but allow child "contexts" to have different views of
> them. Such approach would also play well with idioms like:
>
> class AbcPlugin {
>     static final String SOME_SETTING = System.getenv("SOME_SETTING");
>
> ...and would also help other containers (such as app servers) support
> existing libraries that use environment variables to be configured
> differently in different apps deployed in the same VM process.
>
> Just a thought.
>
> Regards, Peter
>
>
>
> On 05/11/2017 09:02 AM, Cédric Champeau wrote:
>
> Thanks for the answers, folks, but I think they are kind of missing the
> point. The fact is that environment variables *are* mutable. Java happens
> to return an immutable view of the environment variables when the VM was
> started, which is not the reality. We cannot trust what `System.geteenv`
> returns. The Javadocs say "current system environment" but it's just not
> true. The method should be called `getEnvWhenTheVMWasStarted`.
>
> We also have the problem that the environment of the client and the daemon
> differ over time, as I explained in the previous email. And it is pretty
> easy to understand that _when the build runs_, we need to get the
> environment variables of the _client_, not the daemon. Imagine something as
> simple as this:
>
> String toolPath = System.getenv('SOMETOOL_HOME')
>
> and imagine that this code runs in the daemon. When the daemon is started,
> there's absolutely no guarantee that this variable is going to exist.
> However, we know that when we're going to execute the build, this variable
> *has* to be defined. Obviously, it's going to be done from the CLI:
>
> $ export SOMETOOL_HOME = ...
> $ ./gradlew run ---> connects to the daemon, passes environment variables,
> mutates those of the daemon, which then executes the build
>
> Similarly, from a *single* CLI/terminal, it's very common to change the
> values of environment variables (because, hey, they are mutable!):
>
> $ export SOMETOOL_HOME = another path I want to try out
> $ ./gradlew run --> ... must update env vars again
>
> So, to work around the problem that Java doesn't provide a way to mutate
> the current environment variables, we perform a JNI call to do it. *Then*
> we need to mutate the "immutable view" that Java provides, or all Java code
> is going to get wrong information, and we would never succeed. Note that
> having to resort on JNI to mutate the environment is not the problem. We
> can live with that. Once can think it's a bad idea to allow mutation, in
> practice, it is necessary, but I reckon it could be a bad idea to have an
> API for this. The *real* issue is that `System.getenv` does *not* return
> the real values, because it's an immutable view of what existed at the VM
> startup.
>
> Note that it's not recommended to mutate environment variables in a
> multi-threaded environment. But in practice, you can assimilate what we do
> to the VM startup: we get environment variables, mutate them, _then_ the
> build runs (and only at that moment we would effectively be
> multi-threaded). We can live with potential issues of mutating the
> environment: the benefits outperforms the drawbacks by orders of magnitude.
> When the daemon is activated, we're not talking about 10% faster builds.
> They can effectively be 3, 5 or 10x faster!
>
> Now, back to the problem with JDK 9:
>
> - first, the implementation has changed. But we could adapt our code.
> - second, calling `setAccessible` is not allowed anymore. And this is a
> MAJOR pita. The problem is that we're trying to access the java base
> module, and reflection on that module is not allowed anymore. There are no
> good solutions for this: we could write a JVM agent, like you suggested,
> that rewrites bytecode. But since we're trying to rewrite a core class, it
> would have to be found on the invocation of `java` command itself, and
> cannot be dynamically loaded. In addition, we would have to add a flag to
> open core classes to rewrite. There are multiple problems with this
> approach: first, we don't know on which environment we run before we're
> started. Gradle can run on JDK 7, 8, 9, ... and we cannot have a startup
> script which tries to infer the version from whatever it finds, this is not
> realistic and would add unacceptable latency on the client side (plus, a
> lot of headaches to support the various environments we support: Linux,
> Mac, Windows, ...). Second, we may not have a hand on the CLI at all. For
> example, if the build runs in Travis CI, Amazon, or whatever cloudish thing
> that spawns its own containers, we cannot tweak the VM arguments. We just
> have to use whatever is there. Last, rewriting bytecode has a cost, that
> you pay at startup.
>
> Again, what we need is that Java just returns the live view of the
> environment variables. Allowing *mutation* is not necessary, technically
> speaking, because we can rely on JNI. However, I reckon that returning an
> immutable view is done for performance reasons. That's why adding a way to
> mutate "the view" would be an acceptable thing I think. No reflection, no
> bytecode rewriting, just give a way to mutate the map that
> `ProcessEnvironment` retains. The advantage of this is that you would not
> effectively mutate the environment variables, so you, on the JVM side,
> would be safe. What you would mutate is the view you are retaining.
> Alternatively, provide us with an API to "refresh" the view. Like,
> `System.getenv(true)`. The benefit would be that you would only have to get
> the new view of environment variables if the user asks for it. And,
> subsequent callers would get the refreshed view, so people using `gettenv`
> in their code would be up-to-date.
>
> I'm really concerned that this problem is not taken seriously. It's a major
> performance problem for the most widely used build tool out there
> (considering all users, from native to Java and Android). Just saying
> "you're doing something nasty" is not a valid answer: we have to do it, and
> do it for good reasons.
>
>
> 2017-05-11 4:50 GMT+02:00 Martin Buchholz <martinrb at google.com> <martinrb at google.com>:
>
>
> Since you're OK with doing brain surgery on the JDK and you control the
> entire process, consider controlling your daemon with a bytecode rewriting
> agent (e.g. byteman) that changes the definition of System.getenv etc.
>
> (Whose side are you on Martin?! ...  I confess I also wish for a faster
> gradle ...)
>
>
>
>