build-system: use better default for DLCACHE_DIR

[maribu]

Contribution description

We already have $(BUILD_DIR) for RIOT cache's and is used for caching package source and STM32 vendor header files. To follow suit, this sets the default to $(BUILD_DIR)/dlcache.

In addition, this allows building on a read-only source directories.

Testing procedure

With master and RIOT mounted read-only into /data/ext/RIOT, not setting DLCACHE_DIR by hand, and having BUILD_DIR mounted read-write into the container:

/data/ext/RIOT/dist/tools/dlcache/dlcache.sh: line 34: can't create /data/ext/RIOT/.dlcache/c25519-2017-10-05.zip.locked: Read-only file system
read EC key
make[1]: *** [Makefile:29: /build/pkg/c25519-2017-10-05.zip] Error 2
writing EC key
/data/ext/RIOT/dist/tools/dlcache/dlcache.sh: line 34: can't create /data/ext/RIOT/.dlcache/c25519-2017-10-05.zip.locked: Read-only file system

Same situation with this PR:

/data/ext/RIOT/dist/tools/dlcache/dlcache.sh: downloading "https://www.dlbeer.co.nz/downloads/c25519-2017-10-05.zip"...
Connecting to www.dlbeer.co.nz (120.138.18.251:443)
saving to '/build/dlcache/c25519-2017-10-05.zip'
c25519-2017-10-05.zi  47% |***************                 | 32457  0:00:01 ETA
c25519-2017-10-05.zi 100% |********************************| 68419  0:00:00 ETA
'/build/dlcache/c25519-2017-10-05.zip' saved
/data/ext/RIOT/dist/tools/dlcache/dlcache.sh: done downloading "https://www.dlbeer.co.nz/downloads/c25519-2017-10-05.zip"

Issues/PRs references

None

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• none

[riot-ci]

Murdock results

✔️ PASSED

5f5d6dd build-system: use better default for DLCACHE_DIR

Success	Failures	Total	Runtime
11124	0	11124	12m:05s

Artifacts

• Documentation preview

[crasbe]

Wait that's weird... the DLCACHE is supposed to be at home at ~/.dlcache.

I guess the way it works is that the DLCACHE_DIR path doesn't exist and therefore dist/tools/dlcache/dlcache.sh chooses ~/.dlcache and creates the folder if it doesn't exist?

RIOT/dist/tools/dlcache/dlcache.sh

Lines 6 to 8 in fa7ffa1

	DLCACHE_DIR=${DLCACHE_DIR:-~/.dlcache}
	mkdir -p "$DLCACHE_DIR"

[crasbe]

Okay fascinating... in a Docker container, the local folder is used and without the Docker container, the folder in HOME is used.

That's not ideal I think 🤔

[maribu]

The KEY=${KEY:-<DEFAULT>} syntax is to make sure that if $KEY is not provided by the environment, it does have a sane default (here ~/.dlcache).

I'd argue that both ~/.dlcache and $(RIOTBASE)/.dlcache are poor defaults. Ideally, one would use subfolder of "${XDG_CACHE_DIR:-$(xdg-user-dir)/.cache}" by default, as that would be the canonical place as far as I know.

But given that we do actually have $(BUILD_DIR) to control the RIOT cache (and many RIOT users do set it to point to "${XDG_CACHE_DIR:-$(xdg-user-dir)/.cache}/RIOT" via ~/zshrc/~/.profile/...), I'd argue that would make the better default here.

[crasbe]

What would a .cache folder in the $(RIOTBASE) be for? I don't have that.
I have a .cache folder in $HOME, but there's nothing in it.

The point of having .dlcache in the $HOME directory is that the stuff downloaded rarely changes, similarly to .gitcache, which also defaults to $HOME.

I'm not aware of any documentation to setup a .cache folder for RIOT stuff 🤔

[maribu]

What would a .cache folder in the $(RIOTBASE) be for? I don't have that.

I agree on that. "${XDG_CACHE_DIR:-$(xdg-user-dir)/.cache}/RIOT" would evaluate to ~/.cache/RIOT on most systems and IMO that would be a better default for $(BUILD_DIR) than the current $(RIOT_BASE)/build, especially when working with multiple work trees.

But the problem at hand is that while most caches in RIOT do use $(BUILD_DIR), $(DLCACHE_DIR) is not. It IMO definitely should. Especially given:

cat ~/.cache/RIOT/CACHEDIR.TAG
Signature: 8a477f597d28d172789f06886806bc55
# This folder contains RIOT's build cache

The creation of that CACHEDIR.TAG and the comment make it pretty clear that we intend this to be the base folder for all RIOT caches.