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| developer-doc | Native Image | infrastructure |
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Native Image
NativeImage defines a task that is used for
compiling a project into a native binary using Graal's Native Image. It compiles
the project and runs the Native Image tool which builds the image. Currently,
Native Image is used for building the Launcher.
Requirements
Native Image Component
The Native Image component has to be installed within the used GraalVM
distribution. It can be installed by running
<path-to-graal-home>/bin/gu install native-image.
Additional Linux Dependencies
To be able to link statically on Linux, we need to link
against a libc implementation. The default glibc contains
a bug that would cause
crashes when downloading files form the internet, which is a crucial Launcher
functionality. Instead, musl implementation is
suggested by Graal as an alternative. The sbt task automatically downloads a
bundle containing all requirements for a static build with musl. It only
requires a tar command to be available to extract the bundle.
Currently, to use musl, the --libc=musl option has to be added to the build
and x86_64-linux-musl-gcc must be available in the system PATH for the
native-image. In the future it is possible that a different option will be used
or that the bundle will not be required anymore if it became prepackaged. This
task may thus need an update when moving to a newer version of Graal. More
information may be found in
the Native Image documentation.
To make the bundle work correctly with GraalVM 20.2, a shell script called
x86_64-linux-musl-gcc which loads the bundle's configuration is created by the
task and the paths starting with /build/bundle in musl-gcc.specs are
replaced with absolute paths to the bundle location.
Static Builds
The task is parametrized with staticOnLinux parameter which if set to true,
will statically link the built binary, to ensure portability between Linux
distributions. For Windows and MacOS, the binaries should generally be portable,
as described in Launcher Portability.
No Cross-Compilation
As Native Image does not support cross-compilation, the native binaries can only be built for the platform and architecture that the build is running on.
Configuration
As the Native Image builds a native binary, certain capabilities, like
reflection,
may be limited. The build system tries to automatically detect some reflective
accesses, but it cannot detect everything. It is possible for the built binary
to fail with java.lang.ClassNotFoundException or the following error:
java.lang.InstantiationException: Type `XYZ` can not be instantiated reflectively as it does not have a no-parameter constructor or the no-parameter constructor has not been added explicitly to the native image.`
To avoid such issues, additional configuration has to be added to the Native Image build so that it can include the missing constructors.
This can be done manually by creating a file reflect-config.json. The build
task looks for the configuration files in every subdirectory of
META-INF/native-image on the project classpath.
Creating the configuration manually may be tedious and error-prone, so GraalVM includes a tool for assisted configuration. The link describes in detail how the tool can be used. The gist is, the JVM version of the application can be run with a special agentlib in order to trace reflective accesses and save the generated configuration to the provided directory. To run the tool it is easiest to assemble the application into a JAR and run it with the following command:
java \
-agentlib:native-image-agent=config-merge-dir=/path/to/native-image-config \
-jar /path/to/application.jar \
<application arguments>
For example, to update settings for the Launcher:
java -agentlib:native-image-agent=config-merge-dir=engine/launcher/src/main/resources/META-INF/native-image/org/enso/launcher -jar launcher.jar <arguments>
The command may need to be re-run with different arguments to ensure that all execution paths that use reflection are covered. The configuration files between consecutive runs will be merged (a warning may be issued for the first run if the configuration files did not exist, this is not a problem).
It is possible that different classes are reflectively accessed on different
platforms. In that case it may be necessary to run the agent on multiple
platforms and merge the configs. If the conflicts were conflicting (i.e. some
reflectively accessed classes existed only on one platform), it may be necessary
to maintain separate configs for each platform. Currently, the
native-image-agent is not available on Windows, so Windows-specific reflective
accesses may have to be gathered manually. For some types of accesses it may be
possible to force the Windows-specific code paths to run on Linux and gather
these accesses semi-automatically.
After updating the Native Image configuration, make sure to clean it by running
cd tools/native-image-config-cleanup && npm install && npm start
Launcher Configuration
In case of the launcher, to gather the relevant reflective accesses one wants to test as many execution paths as possible, especially the ones that are likely to use reflection. One of these areas is HTTP support and archive extraction.
To trace this accesses, it is good to run at least
... launcher.jar install engine which will trigger HTTP downloads and archive
extraction.
Currently, archive-related accesses are platform dependent - Linux launcher only
uses .tar.gz and Windows uses .zip. While the Linux launcher never unpacks
ZIP files, we can manually force it to do so, to register the reflection
configuration that will than be used on Windows to enable ZIP extraction.
To force the launcher to extract a ZIP on Linux, one can add the following code
snippet (with the necessary imports) to org.enso.launcher.cli.Main.main:
Archive.extractArchive(Path.of("enso-engine-windows.zip"), Path.of("somewhere"), None)
With this snippet, launcher.jar should be built using the
launcher / assembly task, and the tracing tool should be re-run as shown
above.
Moreover, some reflective accesses may not be detected by the tool
automatically, so they may need to be added manually. One of them is an access
to the class [B when using Akka, so it would require manually adding it to the
reflect-config.json. This strange looking access is most likely reflective
access to an array of bytes. To make it easier, a package akka-native has been
created that gathers workarounds required to be able to build native images
using Akka, so it is enough to just add it as a dependency. It does not handle
other reflective accesses that are related to Akka, because the ones that are
needed are gathered automatically using the tool described above.
Project Manager Configuration
Configuring the Native Image for the Project Manager goes similarly as with the
launcher. You need to build the JAR with project-manager/assembly and execute
the test scenarios by starting it with:
java -agentlib:native-image-agent=config-merge-dir=lib/scala/project-manager/src/main/resources/META-INF/native-image/org/enso/projectmanager -jar project-manager.jar
To trace relevant reflection paths, the primary scenario is to start the Project Manager and connect an IDE to it. Since the Project Manager is able to install engine versions, similar steps should be taken to force it to extract a zip archive, as described in Launcher Configuration above. If necessary, other scenarios, like project renaming may be covered.
Remember to run the cleanup script as described above, as tracing the Project
Manager seems to find recursive accesses of some ephemeral-like classes named
Foo/0x00001234.... This classes are not accessible when building the Native
Image and they lead to warnings. For now no clues have been found that ignoring
these classes would impact the native build, it seems that they can be ignored
safely.