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nixpkgs/pkgs/development/libraries/qt-5
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Thomas Watson 8fb885a19e qt{5,6}: disable QML disk cache by default 
Qt QML is a language for designing user interfaces. QML (and related
JavaScript) source files are compiled into bytecode at runtime before
execution. To save time, this bytecode is cached on disk.

The cache is keyed primarily off the application name and mtime of the
source file. Since application names rarely change, source files that
have passed through the store have their mtimes fixed at 1, and
SOURCE_DATE_EPOCH is also set to 1 during builds, compiled bytecode in
this cache for Qt programs built by Nix is rarely or never invalidated.

This results in programs running with QML bytecode that does not match
their source code, which manifests as a constant source of random and
unreproducible glitches, oddities, and crashes in Qt applications when
they are upgraded. For applications like SDDM and Plasma, the user may
be left with a completely broken system, and sometimes even rolling back
doesn't fix the issue.

This patch resolves the issue by simply patching Qt's QML module in all
supported versions to disable the disk cache by default as if the
environment variable QML_DISABLE_DISK_CACHE were always 1. Patching the
code ensures consistent behavior for all Qt applications in all
environments, including non-NixOS uses.

Simple benchmarking suggests an approximate 10% CPU time penalty when
launching QML-heavy applications, and no measurable penalty to booting
into Plasma. This is considerably more benign than the earlier behavior.

For testing or the performance conscious, use of the cache can be
re-enabled after understanding the risks by setting the environment
variable QML_FORCE_DISK_CACHE to 1. This can be done system-wide using
e.g. the `environment.sessionVariables` NixOS option.

Future work could change the cache key through source code changes or
automatic generation of an appropriate SOURCE_DATE_EPOCH. Until then,
this is a simple change which removes a large class of user frustration
and headache for little penalty.
2022-10-20 22:15:04 -05:00
..
5.12 qt{5,6}: disable QML disk cache by default 2022-10-20 22:15:04 -05:00
5.14 qt{5,6}: disable QML disk cache by default 2022-10-20 22:15:04 -05:00
5.15 qt{5,6}: disable QML disk cache by default 2022-10-20 22:15:04 -05:00
hooks
modules treewide: optional -> optionals where the argument is a list 2022-10-10 15:40:21 +03:00
mkDerivation.nix
qt-env.nix
qtModule.nix qtModule: allow shared patches 2022-04-06 08:52:10 -05:00
README.md

README.md

Qt 5 Maintainer's Notes

Minor Updates

Let $major be the major version number, e.g. 5.9.

  1. Change the version number in the $major/fetch.sh.
  2. Run ./maintainers/scripts/fetch-kde-qt.sh pkgs/development/libraries/qt-5/$major from the top of the Nixpkgs tree.

See below if it is necessary to update any patches.

Major Updates

Let $major be the new major version number, e.g. 5.10.

  1. Copy the subdirectory from the previous major version to $major.
  2. Change the version number in $major/fetch.sh.
  3. Run ./maintainers/scripts/fetch-kde-qt.sh pkgs/development/libraries/qt-5/$major from the top of the Nixpkgs tree.
  4. Add a top-level attribute in pkgs/top-level/all-packages.nix for the new major version.
  5. Change the qt5 top-level attribute to point to the new major version.
  6. If the previous major version is not a long-term support release, remove it from Nixpkgs.

See below if it is necessary to update any patches.

Patches

Nixpkgs maintains several patches for Qt which cannot be submitted upstream. To facilitate maintenance, a fork of the upstream repository is created for each patched module:

In each repository, the patches are contained in a branch named nixpkgs/$major for each major version. Please make a pull request to add or update any patch which will be maintained in Nixpkgs.

The forked repository for each module is used to create a single patch in Nixpkgs. To recreate the patch for module $module (e.g. qtbase) at version $version (e.g. 5.9.1) in the branch $major (e.g. 5.9),

  1. Clone the fork for $module from the list above.
  2. Checkout the active branch, git checkout nixpkgs/$major.
  3. Compare the patched branch to the release tag, git diff v$version > $module.patch.
  4. Copy $module.patch into the Nixpkgs tree.

Minor Version Updates

To update module $module to version $version from an older version in the same branch $major,

  1. Clone the fork for $module from the list above.
  2. Checkout the active branch, git checkout nixpkgs/$major.
  3. Merge the new version into the active branch, git merge --no-ff v$version.
  4. Fix any conflicts.
  5. Open a pull request for the changes.
  6. Follow the instructions above to recreate the module patch in Nixpkgs.

Major Version Updates

To update module $module from $oldversion in branch $oldmajor to version $version in branch $major,

  1. Clone the fork for $module from the list above.
  2. Checkout a new branch for the new major version, git checkout -b nixpkgs/$major nixpkgs/$oldmajor.
  3. Rebase the patches from $oldversion onto the new branch, git rebase v$oldversion --onto v$version.
  4. Fix any conflicts.
  5. Open a pull request for the changes.
  6. Follow the instructions above to recreate the module patch in Nixpkgs.