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Add documentation for <uses-library> checks. 

Bug: 132357300
Test: m nothing
Test: check that markdown looks ok in gitiles on gerrit
Change-Id: I2019f65cc69b467889f8dda720b2a4264eae36d7
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Ulya Trafimovich 2021-03-31 11:49:53 +01:00
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# Build System Changes for Android.mk Writers
## Dexpreopt starts enforcing `<uses-library>` checks (for Java modules)
In order to construct correct class loader context for dexpreopt, build system
needs to know about the shared library dependencies of Java modules listed in
the `<uses-library>` tags in the manifest. Since the build system does not have
access to the manifest contents, that information must be present in the build
files. In simple cases Soong is able to infer it from its knowledge of Java SDK
libraries and the `libs` property in Android.bp, but in more complex cases it is
necessary to add the missing information in Android.bp/Android.mk manually.
To specify a list of libraries for a given modules, use:
* Android.bp properties: `uses_libs`, `optional_uses_libs`
* Android.mk variables: `LOCAL_USES_LIBRARIES`, `LOCAL_OPTIONAL_USES_LIBRARIES`
If a library is in `libs`, it usually should *not* be added to the above
properties, and Soong should be able to infer the `<uses-library>` tag. But
sometimes a library also needs additional information in its
Android.bp/Android.mk file (e.g. when it is a `java_library` rather than a
`java_sdk_library`, or when the library name is different from its module name,
or when the module is defined in Android.mk rather than Android.bp). In such
cases it is possible to tell the build system that the library provides a
`<uses-library>` with a given name (however, this is discouraged and will be
deprecated in the future, and it is recommended to fix the underlying problem):
* Android.bp property: `provides_uses_lib`
* Android.mk variable: `LOCAL_PROVIDES_USES_LIBRARY`
It is possible to disable the check on a per-module basis. When doing that it is
also recommended to disable dexpreopt, as disabling a failed check will result
in incorrect class loader context recorded in the .odex file, which will cause
class loader context mismatch and dexopt at first boot.
* Android.bp property: `enforce_uses_lib`
* Android.mk variable: `LOCAL_ENFORCE_USES_LIBRARIES`
Finally, it is possible to globally disable the check:
* For a given product: `PRODUCT_BROKEN_VERIFY_USES_LIBRARIES := true`
* On the command line: `RELAX_USES_LIBRARY_CHECK=true`
The environment variable overrides the product variable, so it is possible to
disable the check for a product, but quickly re-enable it for a local build.
## `LOCAL_REQUIRED_MODULES` requires listed modules to exist {#BUILD_BROKEN_MISSING_REQUIRED_MODULES}
Modules listed in `LOCAL_REQUIRED_MODULES`, `LOCAL_HOST_REQUIRED_MODULES` and