tree c8faa1d4536126830cd5b129546e8ad66cf137d6
parent ec2403eb16ca158ecc60b958033c6cb6a7cb932b
author arteghem <52428902+oliviernotteghem@users.noreply.github.com> 1783411839 -0700
committer GitHub <noreply@github.com> 1783411839 +0100
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Normalize KSP2 output jar timestamps for build determinism (#1657)

Summary:
KSP2 output jars (ksp-kt-gensrc.jar, ksp-genclasses.jar) were
  non-deterministic across builds: every entry was stamped with the
  wall-clock time at the moment of execution, even with identical inputs.

  Root cause: packageDirectoriesToJar creates JarEntry objects and calls
  putNextEntry() without ever calling setTime(). Java's JarOutputStream
  then defaults each entry's timestamp to System.currentTimeMillis(). A
  second issue is the JarOutputStream(stream, manifest) constructor, which
  internally writes META-INF/MANIFEST.MF with the same wall-clock stamp
  before the caller has any opportunity to intervene.

  Because Bazel's action cache key is computed from the hashes of all
  action inputs, a target whose KSP output jars change every build will
  always miss cache for its downstream KotlinCompile action on the next
  build — even with no source changes. This cascaded through the entire
  reverse dependency graph of any target using KSP processors, causing
  hundreds of unnecessary recompilations per build.

  Fix:
  - Add a FIXED_JAR_TIMESTAMP constant (1980-01-01 00:00:00 UTC),
    matching the epoch used by Bazel's JarHelper.DEFAULT_TIMESTAMP for
    reproducible builds.
  - Add a jarEntry() helper that creates a JarEntry with the fixed
    timestamp already set, used at every putNextEntry() call site.
  - Switch from JarOutputStream(stream, manifest) to JarOutputStream(stream)
    and write META-INF/MANIFEST.MF manually via jarEntry(), so the manifest
    entry is also stamped with the fixed epoch rather than wall-clock time.

JIRA Issues: MOBDROID-2959

Differential Revision: https://code.uberinternal.com/D25376885