RFR: 8311302: Allow for jlinking a custom runtime without packaged modules being present [v13]

[Mandy Chung]

On Mon, 11 Dec 2023 16:37:38 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> Please review this patch which adds a jlink mode to the JDK which doesn't need the packaged modules being present. A.k.a run-time image based jlink. Fundamentally this patch adds an option to use `jlink` even though your JDK install might not come with the packaged modules (directory `jmods`). This is particularly useful to further reduce the size of a jlinked runtime. After the removal of the concept of a JRE, a common distribution mechanism is still the full JDK with all modules and packaged modules. However, packaged modules can incur an additional size tax. For example in a container scenario it could be useful to have a base JDK container including all modules, but without also delivering the packaged modules. This comes at a size advantage of `~25%`. Such a base JDK container could then be used to `jlink` application specific runtimes, further reducing the size of the application runtime image (App + JDK runtime; as a single image *or* separate bundles, depending on the app 
 being modularized).
>> 
>> The basic design of this approach is to add a jlink plugin for tracking non-class and non-resource files of a JDK install. I.e. files which aren't present in the jimage (`lib/modules`). This enables producing a `JRTArchive` class which has all the info of what constitutes the final jlinked runtime.
>> 
>> Basic usage example:
>> 
>> $ diff -u <(./bin/java --list-modules --limit-modules java.se) <(../linux-x86_64-server-release/images/jdk/bin/java --list-modules --limit-modules java.se)
>> $ diff -u <(./bin/java --list-modules --limit-modules jdk.jlink) <(../linux-x86_64-server-release/images/jdk/bin/java --list-modules --limit-modules jdk.jlink)
>> $ ls ../linux-x86_64-server-release/images/jdk/jmods
>> java.base.jmod            java.net.http.jmod       java.sql.rowset.jmod      jdk.crypto.ec.jmod         jdk.internal.opt.jmod                     jdk.jdi.jmod         jdk.management.agent.jmod  jdk.security.auth.jmod
>> java.compiler.jmod        java.prefs.jmod          java.transaction.xa.jmod  jdk.dynalink.jmod          jdk.internal.vm.ci.jmod                   jdk.jdwp.agent.jmod  jdk.management.jfr.jmod    jdk.security.jgss.jmod
>> java.datatransfer.jmod    java.rmi.jmod            java.xml.crypto.jmod      jdk.editpad.jmod           jdk.internal.vm.compiler.jmod             jdk.jfr.jmod         jdk.management.jmod        jdk.unsupported.desktop.jmod
>> java.desktop.jmod         java.scripting.jmod      java.xml.jmod             jdk.hotspot.agent.jmod     jdk.i...
>
> Severin Gehwolf has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Disallow packaged modules and run-time image link
>  - Only check for existing path when not a scratch task
>    
>    When using a run-time image link and the initial build was produced with
>    the --keep-packaged-modules option, we don't need to check existing
>    paths to the location where packaged modules need to be copied. This
>    breaks the --verbose output validation.

FWIW.   [f623930](https://github.com/mlchung/jdk/commit/f623930cd529085ddb730a60b7facf106ea01955) for your reference.   I pulled your branch and refactored and made suggestions to the code while I was walking through the code.   Some observations:

The constants such as the pathname of the timestamp file and the internal file listing
per-module non-class non-resource files are part of jlink.  I move the constants to
`JlinkTask`to follow where `OPTIONS_RESOURCE` is defined.

`JRTArchive` scans the class and resource files of a given module from the runtime image.
It should also read `fs_$MODULE_files` to find the list of non-class and non-resource files.

The current implementation checks if a file is modified lazily when `Entry::stream`
is called and so it has to remember if file modification has been checked and
warning has been emitted.   I think doing the file modification check eagerly
when `collectFiles` is called would simplify the code.

For maintainability, better to move the reading of and writing to `fs_$MODULE_files`
together in a single class rather than separated in `JRTArchive` and `JlinkResourcesListPlugin`.
I move them to `JRTArchive.ResourceFileEntry` for now.   There may be a better place.

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PR Comment: https://git.openjdk.org/jdk/pull/14787#issuecomment-1863338821