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+.. _mozbuild_files_metadata:
+
+==============
+Files Metadata
+==============
+
+:ref:`mozbuild-files` provide a mechanism for attaching metadata to
+files. Essentially, you define some flags to set on a file or file
+pattern. Later, some tool or process queries for metadata attached to a
+file of interest and it does something intelligent with that data.
+
+Defining Metadata
+=================
+
+Files metadata is defined by using the
+:ref:`Files Sub-Context <mozbuild_subcontext_Files>` in ``moz.build``
+files. e.g.::
+
+    with Files('**/Makefile.in'):
+        BUG_COMPONENT = ('Core', 'Build Config')
+
+This working example says, *for all Makefile.in files in every directory
+underneath this one - including this directory - set the Bugzilla
+component to Core :: Build Config*.
+
+For more info, read the
+:ref:`docs on Files <mozbuild_subcontext_Files>`.
+
+How Metadata is Read
+====================
+
+``Files`` metadata is extracted in :ref:`mozbuild_fs_reading_mode`.
+
+Reading starts by specifying a set of files whose metadata you are
+interested in. For each file, the filesystem is walked to the root
+of the source directory. Any ``moz.build`` encountered during this
+walking are marked as relevant to the file.
+
+Let's say you have the following filesystem content::
+
+   /moz.build
+   /root_file
+   /dir1/moz.build
+   /dir1/foo
+   /dir1/subdir1/foo
+   /dir2/foo
+
+For ``/root_file``, the relevant ``moz.build`` files are just
+``/moz.build``.
+
+For ``/dir1/foo`` and ``/dir1/subdir1/foo``, the relevant files are
+``/moz.build`` and ``/dir1/moz.build``.
+
+For ``/dir2``, the relevant file is just ``/moz.build``.
+
+Once the list of relevant ``moz.build`` files is obtained, each
+``moz.build`` file is evaluated. Root ``moz.build`` file first,
+leaf-most files last. This follows the rules of
+:ref:`mozbuild_fs_reading_mode`, with the set of evaluated ``moz.build``
+files being controlled by filesystem content, not ``DIRS`` variables.
+
+The file whose metadata is being resolved maps to a set of ``moz.build``
+files which in turn evaluates to a list of contexts. For file metadata,
+we only care about one of these contexts:
+:ref:`Files <mozbuild_subcontext_Files>`.
+
+We start with an empty ``Files`` instance to represent the file. As
+we encounter a *files sub-context*, we see if it is appropriate to
+this file. If it is, we apply its values. This process is repeated
+until all *files sub-contexts* have been applied or skipped. The final
+state of the ``Files`` instance is used to represent the metadata for
+this particular file.
+
+It may help to visualize this. Say we have 2 ``moz.build`` files::
+
+    # /moz.build
+    with Files('*.cpp'):
+        BUG_COMPONENT = ('Core', 'XPCOM')
+
+    with Files('**/*.js'):
+        BUG_COMPONENT = ('Firefox', 'General')
+
+    # /foo/moz.build
+    with Files('*.js'):
+        BUG_COMPONENT = ('Another', 'Component')
+
+Querying for metadata for the file ``/foo/test.js`` will reveal 3
+relevant ``Files`` sub-contexts. They are evaluated as follows:
+
+1. ``/moz.build - Files('*.cpp')``. Does ``/*.cpp`` match
+   ``/foo/test.js``? **No**. Ignore this context.
+2. ``/moz.build - Files('**/*.js')``. Does ``/**/*.js`` match
+   ``/foo/test.js``? **Yes**. Apply ``BUG_COMPONENT = ('Firefox', 'General')``
+   to us.
+3. ``/foo/moz.build - Files('*.js')``. Does ``/foo/*.js`` match
+   ``/foo/test.js``? **Yes**. Apply
+   ``BUG_COMPONENT = ('Another', 'Component')``.
+
+At the end of execution, we have
+``BUG_COMPONENT = ('Another', 'Component')`` as the metadata for
+``/foo/test.js``.
+
+One way to look at file metadata is as a stack of data structures.
+Each ``Files`` sub-context relevant to a given file is applied on top
+of the previous state, starting from an empty state. The final state
+wins.
+
+.. _mozbuild_files_metadata_finalizing:
+
+Finalizing Values
+=================
+
+The default behavior of ``Files`` sub-context evaluation is to apply new
+values on top of old. In most circumstances, this results in desired
+behavior. However, there are circumstances where this may not be
+desired. There is thus a mechanism to *finalize* or *freeze* values.
+
+Finalizing values is useful for scenarios where you want to prevent
+wildcard matches from overwriting previously-set values. This is useful
+for one-off files.
+
+Let's take ``Makefile.in`` files as an example. The build system module
+policy dictates that ``Makefile.in`` files are part of the ``Build
+Config`` module and should be reviewed by peers of that module. However,
+there exist ``Makefile.in`` files in many directories in the source
+tree. Without finalization, a ``*`` or ``**`` wildcard matching rule
+would match ``Makefile.in`` files and overwrite their metadata.
+
+Finalizing of values is performed by setting the ``FINAL`` variable
+on ``Files`` sub-contexts. See the
+:ref:`Files documentation <mozbuild_subcontext_Files>` for more.
+
+Here is an example with ``Makefile.in`` files, showing how it is
+possible to finalize the ``BUG_COMPONENT`` value.::
+
+    # /moz.build
+    with Files('**/Makefile.in'):
+        BUG_COMPONENT = ('Core', 'Build Config')
+        FINAL = True
+
+    # /foo/moz.build
+    with Files('**'):
+        BUG_COMPONENT = ('Another', 'Component')
+
+If we query for metadata of ``/foo/Makefile.in``, both ``Files``
+sub-contexts match the file pattern. However, since ``BUG_COMPONENT`` is
+marked as finalized by ``/moz.build``, the assignment from
+``/foo/moz.build`` is ignored. The final value for ``BUG_COMPONENT``
+is ``('Core', 'Build Config')``.
+
+Here is another example::
+
+    with Files('*.cpp'):
+        BUG_COMPONENT = ('One-Off', 'For C++')
+        FINAL = True
+
+    with Files('**'):
+        BUG_COMPONENT = ('Regular', 'Component')
+
+For every files except ``foo.cpp``, the bug component will be resolved
+as ``Regular :: Component``. However, ``foo.cpp`` has its value of
+``One-Off :: For C++`` preserved because it is finalized.
+
+.. important::
+
+   ``FINAL`` only applied to variables defined in a context.
+
+   If you want to mark one variable as finalized but want to leave
+   another mutable, you'll need to use 2 ``Files`` contexts.
+
+Guidelines for Defining Metadata
+================================
+
+In general, values defined towards the root of the source tree are
+generic and become more specific towards the leaves. For example,
+the ``BUG_COMPONENT`` for ``/browser`` might be ``Firefox :: General``
+whereas ``/browser/components/preferences`` would list
+``Firefox :: Preferences``.