Git dependencies don't work due to global lock

[aidanhs]

Dec 11 07:55:53.321 INFO testing https://github.com/servo/servo#22472b4db4564f7bc274eb01447e4f04fd7e7205 against master#cf30759a8409bee031ac252ee207452ab4804467 for pr-45002-2
Dec 11 07:55:53.321 INFO running: cargo +cf30759a8409bee031ac252ee207452ab4804467 build --frozen
Dec 11 07:55:53.322 INFO creating container for: cargo +cf30759a8409bee031ac252ee207452ab4804467 build --frozen
Dec 11 07:55:53.322 INFO running `"docker" "create" "-v" "/home/ec2-user/cargobomb/./work/local/test-source/pr-45002-2/master#cf30759a8409bee031ac252ee207452ab4804467:/source:ro,Z" "-v" "/home/ec2-user/cargobomb/./work/local/target-dirs/pr-45002-2/master#cf30759a8409bee031ac252ee207452ab4804467:/target:rw,Z" "-v" "/home/ec2-user/cargobomb/./work/local/cargo-home:/cargo-home:ro,Z" "-v" "/home/ec2-user/cargobomb/./work/local/rustup-home:/rustup-home:ro,Z" "-e" "USER_ID=500" "-e" "CMD=cargo +cf30759a8409bee031ac252ee207452ab4804467 build --frozen" "crater"`
Dec 11 07:55:53.450 INFO blam! 746e3c86c1b5c73cbe3b5bb9c690d06d0277b4bd0f6d2943c10274aee3b36336
Dec 11 07:55:53.451 INFO running `"docker" "start" "-a" "746e3c86c1b5c73cbe3b5bb9c690d06d0277b4bd0f6d2943c10274aee3b36336"`
Dec 11 07:55:53.974 INFO kablam! error: failed to load source for a dependency on `browserhtml`
Dec 11 07:55:53.974 INFO kablam! 
Dec 11 07:55:53.974 INFO kablam! Caused by:
Dec 11 07:55:53.974 INFO kablam!   Unable to update https://github.com/browserhtml/browserhtml?branch=crate#84913eaf
Dec 11 07:55:53.974 INFO kablam! 
Dec 11 07:55:53.974 INFO kablam! Caused by:
Dec 11 07:55:53.974 INFO kablam!   failed to open: /cargo-home/git/.cargo-lock-git
Dec 11 07:55:53.974 INFO kablam! 
Dec 11 07:55:53.974 INFO kablam! Caused by:
Dec 11 07:55:53.974 INFO kablam!   Read-only file system (os error 30)
Dec 11 07:55:53.976 INFO kablam! su: No module specific data is present
Dec 11 07:55:54.337 INFO running `"docker" "rm" "-f" "746e3c86c1b5c73cbe3b5bb9c690d06d0277b4bd0f6d2943c10274aee3b36336"`
Dec 11 07:55:54.359 INFO blam! 746e3c86c1b5c73cbe3b5bb9c690d06d0277b4bd0f6d2943c10274aee3b36336

/cargo-home/git/.cargo-lock-git is on a read only mountpoint, but I guess it gets locked even in frozen mode to stop it changing from under the feet of cargo.

[SimonSapin]

@alexcrichton, in what cases does Cargo need write access to $CARGO_HOME?

[alexcrichton]

Oh this might be a bug in Cargo, I think we specifically fixed this for the registries where if we fail to acquire a write lock we fall back to a read lock, maybe the same fix just needs to be for git?

[alexcrichton]

In theory though if everything is checked out Cargo doesn't actually need write access.

[SimonSapin]

@alexcrichton Do you remember what was in the commit message for that change? I searched for "lock" in Cargo’s git log but did not find it in 2017.

[alexcrichton]

I believe I was thinking of rust-lang/cargo#3259, although it's not quite the same solution. I suspect a solution would be along the lines though of "handle the error" rather than "unconditionally propagate it outwards"