When the native arch (e.g. `x86_64`) `APKINDEX` files are outdated, and
`pmbootstrap` gets instructed to build a linux package for a foreign
arch, then the `APKINDEX` cache did not get used anymore for the
current session. This means that every lookup of a package in an
`APKINDEX` caused the whole `APKINDEX` file to get parsed again instead
of using the cached version. This slowed it down so much that it felt
like `pmbootstrap` was looping forever.
How this happens in detail:
* Whenever pmbootstrap parses an `APKINDEX`, it fills up the
`args.cache["apkindex"]` dict with the parsed information and the
last modified date of the file.
* `pmbootstrap` checks the last modified date of the `APKINDEX` files
and updates them if they are older than 4 hours.
* When the bug appeared, then the cache was already filled up, then an
update happened and then `pmbootstrap` tried to read from the cache.
* So when reading from the `APKINDEX`, the cache gets ignored because
the last modified date is different.
* Up to this commit, the cache does not get deleted and filled up
again!
How to test:
Try these commands once without this commit, and then with this commit
applied:
```
$ sudo touch -m -t 201801010000 \
~/.local/var/pmbootstrap/cache_apk_x86_64/APKINDEX.*
$ pmbootstrap -v build linux-postmarketos-mainline --arch=armhf
```
Without the patch, you can see in `pmbootstrap log` that it is
resolving the dependencies properly, but very slowly. With the patch
the resolving happens almost instantly.
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