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Historical revision — this record as it stood on 20 May 2026, not the current version. View the current record →

Per-User Build Cache Discipline — Preventing Cross-User Cargo Races

Cargo's incremental build cache lives under target/. By default that is a per-crate directory inside each crate. The workspace overrides this via CARGO_TARGET_DIR=/srv/foundry/cargo-target/ so the build cache is shared across all crates in the monorepo — saving disk space and rebuild time.

This works for a single developer. With two developers in the workspace group it becomes a race. Cargo's target/.cargo-lock serialises full-tree builds, so two cargo build invocations on different crates block one another. Worse, mixed file ownership — build artefacts written by one user interleaved with another user's — causes sporadic Permission denied errors during incremental rebuild, even when the crates appear to be unrelated.

The fix: /etc/profile.d/foundry-cargo.sh exports CARGO_TARGET_DIR=/srv/foundry/cargo-target/$USER/. Each user gets a private subdirectory. The build cache stays shared within a user — the original optimisation is preserved — but no longer crosses users. The environment variable takes precedence over ~/.cargo/config.toml's target-dir setting, so the change takes effect on any new shell without per-user dotfile edits.

The migration involved moving the existing 3.3 GiB cache from /srv/foundry/cargo-target/{release,debug,cxxbridge,tmp} into one user's subdirectory. That user's incremental state is preserved; the other developer starts with an empty cache that fills on first build.

Service accounts in the workspace group (local-doorman, local-design, and similar) do not run Cargo and do not receive a per-user cache directory. The profile.d script only exports when /srv/foundry/cargo-target/ exists, so non-workspace contexts are unaffected.

The lesson generalises: any group-writable shared workspace path needs explicit design around which operations are safe to share (reads, content appends) and which require per-user partition (build state, lock files, temp). Cargo is the prominent example; the same logic applies to anything with mutable state under a shared root.

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