Add a quick way to check logs from nix build and nix-build

[YellowOnion]

Pain point:

As per the issue below nix build truncates output and gives you a command to copy and paste or type, with a big long hash complicating things.

With nix-build it's possible to blow out your scrollback buffer on occasion, and then you have to google how to check logs in nix, it's not intuitive.

solutions:

A command like nix log --last

or

symlink the drv, so just like ./result we can quickly access the derivation, we can then just issue nix log implying nix log .

issue:
#2420

[Kha]

If nix build was the last thing you did, you don't need to copy the store path, you can just run the same command with build replaced with log (i.e. usually just nix log).

[SuperSandro2000]

Or nix log ./result or you copy the nix log command from the error output.

[YellowOnion]

@Kha that doesn't work:

❯ nix log
warning: Git tree '/mnt/home/daniel/dev/linux/bcachefs-tools' is dirty
error: path '/nix/store/0zy2awdims34v98glmqfwi1a6zsrzjbz-bcachefs-tools-src' is not valid

@SuperSandro2000 did you read what I described? copy-pasting is slow and cumbersome.

Also the ./result link isn't created if it's not built, so it's going to be stale or non-existent, and it's also outputs junk on completed build because we want the <hash>.drv folder, not the "output".

[Kha]

Ah, it doesn't work if it's a dependency that failed as I wrote in #2420 (comment). But the same is true for your second proposed solution, surely result/ should not point to a random failed dependency instead of the thing you actually wanted to build. And I definitely don't see Nix adopt a global(?) "last build" state as in your other proposal.

[mstone]

Thinking about this: how would people feel about, say, a nix.conf option to tell nix to output "error" symlinks, analogous to the "result" symlinks that successful builds produce but here for failed builds, perhaps pointing to the relevant log file or to a shell script that runs the correct nix log invocation?