Build failures executing Cargo concurrently using a local registry

[hugwijst]

Problem

There is a race condition when building multiple projects concurrently that depend on the same crates. When extracting crates from a local registry, the crate is opened as read-only with shared locks (https://github.com/rust-lang/cargo/blob/master/src/cargo/sources/registry/local.rs#L74). Unpacking the crate is assumed to be safe due to the tar file being locked (https://github.com/rust-lang/cargo/blob/master/src/cargo/sources/registry/mod.rs#L428).

Output of build failure:

   Unpacking form v0.4.0 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking svd2rust v0.14.0 (registry `file:///home/builduser/cargo_registry/latest`)
  Installing svd2rust v0.14.0
  Installing form v0.4.0
   Unpacking log v0.4.6 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking cast v0.2.2 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking getopts v0.2.18 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking svd-parser v0.6.0 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking failure v0.1.3 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking quote v0.3.15 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking env_logger v0.5.13 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking clap v2.32.0 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking syn v0.11.11 (registry `file:///home/builduser/cargo_registry/latest`)
   Unpacking syn v0.11.11 (registry `file:///home/builduser/cargo_registry/latest`)
error: failed to compile `form v0.4.0`, intermediate artifacts can be found at `/tmp/cargo-installKeMHuZ`

Caused by:
  unable to get packages from source

Caused by:
  failed to download replaced source registry `https://github.com/rust-lang/crates.io-index`

Caused by:
  failed to unpack package `syn v0.11.11 (registry `file:///home/builduser/cargo_registry/latest`)`

Caused by:
  failed to unpack entry at `syn-0.11.11/src/aster/mod.rs`

Caused by:
  failed to unpack `/home/builduser/.cargo/registry/src/-39fe0c9a09214cd4/syn-0.11.11/src/aster/mod.rs`

Steps

1. Create a local registry.
2. Clear the local crate source cache.
3. Execute two builds at the same time.

Possible Solution(s)

• Lock the crate file on the local registry. This is not ideal, as the local registry might be read-only or an NFS mount.
• Copy the crate file to the registry cache in $CARGO_HOME, exclusively locking that copy.
• Locking an empty file in $CARGO_HOME.

Notes

Output of cargo version:
cargo 1.29.0 (524a578d7 2018-08-05)

[alexcrichton]

Thanks for the report, definitely seems like a bug! We'll want to handle this by acquiring a read-write lock on the destination we're writing into. If we're extracting a tarball we're already writing files there and we rely on the atomicity of the destination, not the source.

In theory shouldn't be too hard to do internally in Cargo!

[hugwijst]

Do you prefer copying the crate file or locking an empty file? I might have some time today to create a PR.