Re: Patch to the context lookup change

[Tianjie Xu <xunchang@google.com>]

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Hi there,
I recently made a change in android's selinux repo. The change
implements a new lookup helper function for context matches, and it
aims to skip unnecessary restorecon if the corresponding context
doesn't change.

Quote from the commit message. "We used to hash the file_context and
skip the restorecon on the top level directory if the hash doesn't
change. But the file_context might change after an OTA update; and
some users experienced long restorecon time as they have lots of files
under directories like /data/media. This CL tries to hash all the
partial match entries in the file_context for each directory; and
skips the restorecon if that digest stays the same, regardless of the
changes to the other parts of file_context."
CL in: https://android-review.googlesource.com/c/platform/external/selinux/+/918713

I haven't done the work in upstream code to fully implement this
change. But this CL merges and compiles. It factors out a lookup
helper function and returns an array of matched pointers instead of a
single one. The old loopup_common function is then modified to take
the first element in the array. I wonder if it's possible to merge in
this change to the lookup functions first? It will make the upstream &
android repo more consistent and also save some work for future
merges.

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