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platform_build/tools/fs_config/README.md

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FS Config Generator

The fs_config_generator.py tool uses the platform android_filesystem_config.h and the TARGET_FS_CONFIG_GEN files to generate the following:

  • fs_config_dirs and fs_config_files files for each partition
  • passwd and group files for each partition
  • The generated_oem_aid.h header

Outputs

fs_config_dirs and fs_config_files

The fs_config_dirs and fs_config_files binary files are interpreted by the libcutils fs_config() function, along with the built-in defaults, to serve as overrides to complete the results. The Target files are used by filesystem and adb tools to ensure that the file and directory properties are preserved during runtime operations. The host files in the $OUT directory are used in the final stages when building the filesystem images to set the file and directory properties.

See ./fs_config_generator.py fsconfig --help for how these files are generated.

passwd and group files

The passwd and group files are formatted as documented in man pages passwd(5) and group(5) and used by bionic for implementing getpwnam() and related functions.

See ./fs_config_generator.py passwd --help and ./fs_config_generator.py group --help for how these files are generated.

The generated_oem_aid.h header

The generated_oem_aid.h creates identifiers for non-platform AIDs for developers wishing to use them in their native code. To do so, include the oemaids_headers header library in the corresponding makefile and #include "generated_oem_aid.h" in the code wishing to use these identifiers.

See ./fs_config_generator.py oemaid --help for how this file is generated.

Parsing

See the documentation on source.android.com for details and examples.

Ordering

Ordering within the TARGET_FS_CONFIG_GEN files is not relevant. The paths for files are sorted like so within their respective array definition:

  • specified path before prefix match
    • for example: foo before f*
  • lexicographical less than before other
    • for example: boo before foo

Given these paths:

paths=['ac', 'a', 'acd', 'an', 'a*', 'aa', 'ac*']

The sort order would be:

paths=['a', 'aa', 'ac', 'acd', 'an', 'ac*', 'a*']

Thus the fs_config tools will match on specified paths before attempting prefix, and match on the longest matching prefix.

The declared AIDs are sorted in ascending numerical order based on the option "value". The string representation of value is preserved. Both choices were made for maximum readability of the generated file and to line up files. Sync lines are placed with the source file as comments in the generated header file.

Unit Tests

From within the fs_config directory, unit tests can be executed like so:

$ python -m unittest test_fs_config_generator.Tests
.............
----------------------------------------------------------------------
Ran 13 tests in 0.004s

OK

One could also use nose if they would like:

$ nose2

To add new tests, simply add a test_<xxx> method to the test class. It will automatically get picked up and added to the test suite.