8fb885a19e
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.
13 lines
532 B
Diff
13 lines
532 B
Diff
diff --git a/src/qml/qml/qqmltypeloader.cpp b/src/qml/qml/qqmltypeloader.cpp
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index 1d66e75..827567a 100644
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--- a/src/qml/qml/qqmltypeloader.cpp
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+++ b/src/qml/qml/qqmltypeloader.cpp
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@@ -727,7 +727,7 @@ bool QQmlTypeLoader::Blob::isDebugging() const
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bool QQmlTypeLoader::Blob::diskCacheEnabled() const
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{
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- return (!disableDiskCache() && !isDebugging()) || forceDiskCache();
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+ return forceDiskCache();
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}
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bool QQmlTypeLoader::Blob::qmldirDataAvailable(const QQmlRefPointer<QQmlQmldirData> &data, QList<QQmlError> *errors)
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