[Bug 2144934] [NEW] Selfbuild autopkgtest is flaky due to excessive resource usage

[Max Gilmour]

Public bug reported:

The large resource usage of the rustc selfbuild autopkgtest not only causes
a large, unnecessary load on our autopkgtest infrastructure, but the tests can
also sometimes run out of resources, making the autopkgtests flaky. This
flakiness problem is extra bad for tests which can take upwards of 2 hours to
run.

Currently, Rust toolchains before rustc-1.93 run the entire bootstrap process
during the selfbuild autopkgtest, including thousands of unit tests. This is
unnecessary, as we already run the unit tests during the build process itself.

Additionally, many settings which are important for optimizing the toolchain
are left on for the selfbuild autopkgtest. The performance of the toolchain
built by the autopkgtest is irrelevant, and the settings increase resource
usage, so they should be disabled as well.

Luckily, as stated above, rustc-1.93 has solved this problem by disabling
certain resource-intensive bootstrap optimizations and only running a
subset of unit tests to verify that the new toolchain is capable of testing
other toolchain builds.

The solution is to cherry-pick these fixes from rustc-1.93 in order to make
older toolchains' autopkgtests more reliable.

** Affects: rustc-1.85 (Ubuntu)
     Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
         Status: Triaged

** Affects: rustc-1.85 (Ubuntu Noble)
     Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
         Status: Triaged

** Affects: rustc-1.85 (Ubuntu Questing)
     Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
         Status: Triaged

** Affects: rustc-1.85 (Ubuntu Resolute)
     Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
         Status: Triaged

** Also affects: rustc-1.85 (Ubuntu Noble)
   Importance: Undecided
       Status: New

** Also affects: rustc-1.85 (Ubuntu Questing)
   Importance: Undecided
       Status: New

** Changed in: rustc-1.85 (Ubuntu Noble)
     Assignee: (unassigned) => Max Gilmour (maxgmr)

** Changed in: rustc-1.85 (Ubuntu Questing)
     Assignee: (unassigned) => Max Gilmour (maxgmr)

** Also affects: rustc-1.85 (Ubuntu Resolute)
   Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
       Status: New

** Changed in: rustc-1.85 (Ubuntu Noble)
       Status: New => Triaged

** Changed in: rustc-1.85 (Ubuntu Questing)
       Status: New => Triaged

** Changed in: rustc-1.85 (Ubuntu Resolute)
       Status: New => Triaged

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https://bugs.launchpad.net/bugs/2144934

Title:
  Selfbuild autopkgtest is flaky due to excessive resource usage

Status in rustc-1.85 package in Ubuntu:
  Triaged
Status in rustc-1.85 source package in Noble:
  Triaged
Status in rustc-1.85 source package in Questing:
  Triaged
Status in rustc-1.85 source package in Resolute:
  Triaged

Bug description:
  The large resource usage of the rustc selfbuild autopkgtest not only causes
  a large, unnecessary load on our autopkgtest infrastructure, but the tests can
  also sometimes run out of resources, making the autopkgtests flaky. This
  flakiness problem is extra bad for tests which can take upwards of 2 hours to
  run.

  Currently, Rust toolchains before rustc-1.93 run the entire bootstrap process
  during the selfbuild autopkgtest, including thousands of unit tests. This is
  unnecessary, as we already run the unit tests during the build process itself.

  Additionally, many settings which are important for optimizing the toolchain
  are left on for the selfbuild autopkgtest. The performance of the toolchain
  built by the autopkgtest is irrelevant, and the settings increase resource
  usage, so they should be disabled as well.

  Luckily, as stated above, rustc-1.93 has solved this problem by disabling
  certain resource-intensive bootstrap optimizations and only running a
  subset of unit tests to verify that the new toolchain is capable of testing
  other toolchain builds.

  The solution is to cherry-pick these fixes from rustc-1.93 in order to make
  older toolchains' autopkgtests more reliable.

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