LMDB: add utilities that allow to explore LMDB contents

[marisn]

As LMDB stores a binary blob, there are no CLI tools to explore database contents. These two utilities allow to look inside a LMDB cache by listing all keys stored in it and extracting data by key value.

Usage examples

Find out how many tiles are stored in the database:

$ mapcache_lmdb_list -d o2016/ -s
Total keys found: 1296

List all 1296 tile keys (be ware of printing keys for large databases!)

$ mapcache_lmdb_list -d o2016/
orto2016-L-{dim}-0-0-0.xxx
orto2016-L-{dim}-0-0-1.xxx
...

List all 1296 keys and additionally print tile timestamp and size. Size 4 indicates that it is a blank tile (just RGBA value).

$ mapcache_lmdb_list -d o2016/ -e
orto2016-L-{dim}-0-2-1.xxx,2025-09-02T13:41:35Z,4
orto2016-L-{dim}-0-2-2.xxx,2025-09-02T13:41:35Z,1727
...

Extract one tile to a file

$ mapcache_lmdb_get -d o2016/ -k orto2016-L-{dim}-0-2-1.xxx -o my_tile.png
Key found. Blank tile, color #ffffffff. Writing description to "my_tile.png"
Timestamp: Tue, 02 Sep 2025 13:41:35 GMT
$ mapcache_lmdb_get -d o2016/ -k orto2016-L-{dim}-0-2-2.xxx -o my_tile.png
Key found. Writing 1727 image bytes to "my_tile.png"
Timestamp: Tue, 02 Sep 2025 13:41:35 GMT

[jmckenna]

thanks @marisn !

[marisn]

What wasn't discussed if all functionality should be wrapped in a single utility or kept as two separate. Both are valid options. Yes, thinking is not my strong point.

[jmckenna]

I personally prefer 2 separate utilities, as you have done.