Change jsinspector back to a shared library in the CMake build #42144
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…ook#42144) Summary: D51895785 changed several CMake libraries from shared to static, including `jsinspector`. This happens to be semantically incorrect in the case of `jsinspector`, as the library contains singletons which can be inadvertently duplicated due to static linking. As a result, different parts of the code can end up accessing different instances of a supposed singleton, leading to bugs. Here we revert the change to `jsinspector` (only) and add an explanatory comment to signpost this for future readers. ## More context & general principle While nothing is broken today, allowing static libraries to contain global state is brittle and breaks in surprising ways: * The upcoming diff D52231237 introduces a new dependency on `jsinspector` which builds cleanly, but causes debugging to stop working because of the duplicated singleton. * The only reason debugging currently works in the CMake build of Bridgeless is by a happy accident: the shared library `hermesinstancejni` depends on `reactnativejni` through a chain of three other libraries unrelated to debugging, and as a result, can access `reactnativejni`'s copy of `jsinspector` (see graph). {F1237835169} It seems that the safest rule of thumb, given the way React Native is currently structured, is that **singletons should live in their own shared libraries** so no call site can cause them to be duplicated through static linking. (It's reasonable to revisit this guidance if we manage to consolidate React Native into one monolithic shared library, eliminating the footgun at the source.) Changelog: [Internal] [Changed] - Change jsinspector back to a shared library in the CMake build. Reviewed By: cortinico, NickGerleman Differential Revision: D52541488
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This pull request was exported from Phabricator. Differential Revision: D52541488 |
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This pull request was successfully merged by @motiz88 in 9ba3bc9. When will my fix make it into a release? | Upcoming Releases |
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…ook#42144) Summary: Pull Request resolved: facebook#42144 D51895785 changed several CMake libraries from shared to static, including `jsinspector`. This happens to be semantically incorrect in the case of `jsinspector`, as the library contains singletons which can be inadvertently duplicated due to static linking. As a result, different parts of the code can end up accessing different instances of a supposed singleton, leading to bugs. Here we revert the change to `jsinspector` (only) and add an explanatory comment to signpost this for future readers. ## More context & general principle While nothing is broken today, allowing static libraries to contain global state is brittle and breaks in surprising ways: * The upcoming diff D52231237 introduces a new dependency on `jsinspector` which builds cleanly, but causes debugging to stop working because of the duplicated singleton. * The only reason debugging currently works in the CMake build of Bridgeless is by a happy accident: the shared library `hermesinstancejni` depends on `reactnativejni` through a chain of three other libraries unrelated to debugging, and as a result, can access `reactnativejni`'s copy of `jsinspector` (see graph). {F1237835169} It seems that the safest rule of thumb, given the way React Native is currently structured, is that **singletons should live in their own shared libraries** so no call site can cause them to be duplicated through static linking. (It's reasonable to revisit this guidance if we manage to consolidate React Native into one monolithic shared library, eliminating the footgun at the source.) Changelog: [Internal] [Changed] - Change jsinspector back to a shared library in the CMake build. Reviewed By: cortinico, NickGerleman Differential Revision: D52541488 fbshipit-source-id: 502210add0b734a9bbc470bdf38fb70a41e149a9
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Summary:
D51895785 changed several CMake libraries from shared to static, including
jsinspector. This happens to be semantically incorrect in the case ofjsinspector, as the library contains singletons which can be inadvertently duplicated due to static linking. As a result, different parts of the code can end up accessing different instances of the supposed singleton, leading to bugs.Here we revert the change to
jsinspector(only) and add an explanatory comment to signpost this for future readers.More context & general principle
While nothing is broken today, allowing static libraries to contain global state is brittle and breaks in surprising ways:
jsinspectorwhich builds cleanly, but causes debugging to stop working because of the duplicated singleton.hermesinstancejnidepends onreactnativejnithrough a chain of three other libraries unrelated to debugging, and as a result, can accessreactnativejni's copy ofjsinspector(see graph).{F1237835169}
It seems that the safest rule of thumb, given the way React Native is currently structured, is that singletons should live in their own shared libraries so no call site can cause them to be duplicated through static linking. (It's reasonable to revisit this guidance if we manage to consolidate React Native into one monolithic shared library, eliminating the footgun at the source.)
Changelog:
[Internal] [Changed] - Change jsinspector back to a shared library in the CMake build.
Differential Revision: D52541488