fs_config: Update docs

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The fs_config_generator.py tool uses the platform android_filesystem_config.h and the

TARGET_FS_CONFIG_GEN files to generate the fs_config_dirs and fs_config_files files for each

partition, as well as passwd and group files, and the generated_oem_aid.h header.

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.

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 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.

The parsing of the TARGET_FS_CONFIG_GEN files follows the Python ConfigParser specification, with

the sections and fields as defined below. There are two types of sections, both sections require all

options to be specified. The first section type is the "caps" section.

The "caps" section follows the following syntax:

[path]

mode: Octal file mode

user: AID_<user>

group: AID_<group>

caps: cap*

Where:

[path]

  The filesystem path to configure. A path ending in / is considered a dir,

  else its a file.

mode:

  A valid octal file mode of at least 3 digits. If 3 is specified, it is

  prefixed with a 0, else mode is used as is.

user:

  Either the C define for a valid AID or the friendly name. For instance both

  AID_RADIO and radio are acceptable. Note custom AIDs can be defined in the

  AID section documented below.

group:

  Same as user.

caps:

  The name as declared in

  system/core/include/private/android_filesystem_capability.h without the

  leading CAP_. Mixed case is allowed. Caps can also be the raw:

   * binary (0b0101)

   * octal (0455)

   * int (42)

   * hex (0xFF)

  For multiple caps, just separate by whitespace.

It is an error to specify multiple sections with the same [path] in different

files. Note that the same file may contain sections that override the previous

section in Python versions <= 3.2. In Python 3.2 it's set to strict mode.

The next section type is the "AID" section, for specifying OEM specific AIDS.

The AID section follows the following syntax:

[AID_<name>]

value: <number>

Where:

[AID_<name>]

  The <name> can contain characters in the set uppercase, numbers

  and underscores.

value:

  A valid C style number string. Hex, octal, binary and decimal are supported.

  See "caps" above for more details on number formatting.

It is an error to specify multiple sections with the same [AID_<name>]. With

the same constraints as [path] described above. It is also an error to specify

multiple sections with the same value option. It is also an error to specify a

value that is outside of the inclusive OEM ranges:

 * AID_OEM_RESERVED_START(2900) - AID_OEM_RESERVED_END(2999)

 * AID_OEM_RESERVED_2_START(5000) - AID_OEM_RESERVED_2_END(5999)

as defined by system/core/include/private/android_filesystem_config.h.

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

 ** ie foo before f*

 * lexicographical less than before other

 ** ie 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.

# 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](https://source.android.com/devices/tech/config/filesystem#configuring-aids) 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.