Configure jenkins with CARGO_HOME in a writeable directory

[teor]

Or develop a workaround in the tor build scripts.

[teor]

weasel has set CARGO_HOME to an empty writeable directory in the tor-rust jenkins builds.

But he says that we should make sure cargo only writes to the build directory, for the sake of Debian, Ubuntu and other distributions.

We should do whatever other Rust packages do.

If there's no clear answer, I suggest that we make the build directory CARGO_HOME if:

• CARGO_HOME is unset or not writeable, and
• HOME is unset or not writeable

[catalyst]

Replying to teor:

If there's no clear answer, I suggest that we make the build directory CARGO_HOME if:

• CARGO_HOME is unset or not writeable, and
• HOME is unset or not writeable

If we build in offline mode, does cargo need to write to CARGO_HOME or HOME at all?

[Hello71]

Replying to teor:

We should do whatever other Rust packages do.

As I said in #26038, "I think most C+Rust projects still use the global cache. I tried searching GitHub (​​https://github.com/search?q=%22CARGO_HOME%22+extension%3Aam&type=Code). I found that tor is the only project that does not. for users who do not care, using the global cache will save download time and bandwidth on repeat builds, and for those who do care, my patch prints a warning so they will know. (maybe it should be downgraded to NOTICE?)". (fwiw, I checked librsvg just now and found that they override CARGO_HOME only for gitlab-ci builds)

If there's no clear answer, I suggest that we make the build directory CARGO_HOME if:

• CARGO_HOME is unset or not writeable, and
• HOME is unset or not writeable

I think this sounds acceptable, but indicates a broken build environment. I know Gentoo sets HOME to a private temporary directory for each package build. I believe all of the presently used standardized Debian build scripts do so as well.

Replying to catalyst:

If we build in offline mode, does cargo need to write to CARGO_HOME or HOME at all?

Not as far as I know. I believe Cargo presently stores only configuration, registry information, and package source in CARGO_HOME. The first one is manually created by the user (another solid reason to not override CARGO_HOME now that I think about it), and the other two are vendored.

[catalyst]

Replying to Hello71:

I believe Cargo presently stores only configuration, registry information, and package source in CARGO_HOME. The first one is manually created by the user (another solid reason to not override CARGO_HOME now that I think about it), and the other two are vendored.

Some quick experimentation seems to confirm this: in a build tree configured to build with Rust but in offline mode, cargo doesn't seem to try writing registry or cache stuff. Let's open a new ticket if we find evidence to the contrary, because the desired Jenkins config change seems to be deployed already.