--- id: "bug-918" status: "fixed" status_updated: "2026-09-24" status_note: "" component: "build" severity: "unknown" timestamp: "2026-06-22T03:30:00Z" last_seen: "2026-09-19T09:00:00Z" occurrences: 4 files: ["src/flake.nix"] tags: ["nix", "flake", "c-capnproto", "env", "build", "worktree-isolation", "worktree"] related: [] legacy: true --- # bug-918: nix develop fails: hash mismatch for c-capnproto fixed-output derivation (expected… ## Description nix develop fails: hash mismatch for c-capnproto fixed-output derivation (expected sha256-9dzSc3w+yxYzsF/rZe3qIarKOpjt2zvIyjuS5FXngIM=, got sha256-9rTlcTia+s9ARGeTi+8auGvM0RkS+wZK7aIkzecaeSk=) ## Environment _Not recorded (migrated from buglog.json)._ ## Reproduction _Not recorded (migrated from buglog.json)._ ## Root cause flake pins c-capnproto to the MUTABLE github archive/main.tar.gz; upstream main branch advanced so the tarball content/hash drifted, breaking the fixed-output derivation. Only pre-cached nix stores still build. ## Fix RESOLVED: user refreshed src/flake.lock; fresh nix develop builds (capnproto 1.2.0). A clean controller rebuild made the full controller test suite green (Ok:1 Fail:0, 12.7s) — the earlier wal_catchup -EAGAIN/decode failures were a STALE incremental-build artifact (schema compiled against the drifted toolchain), not fsync or a source regression. RECURRENCE 2026-07-23 (config worktree, /xce-build controller build+report task for config-upgrade-snapshot Task 1 RED-phase capture): identical hash mismatch reproduced verbatim (same specified sha256-9dzSc3w+yxYzsF/rZe3qIarKOpjt2zvIyjuS5FXngIM= vs got sha256-9rTlcTia+s9ARGeTi+8auGvM0RkS+wZK7aIkzecaeSk= for capnpc-1.0.1.drv source.drv) on FIRST invocation and again on an identical immediate RETRY (deterministic, not transient -- unlike the unrelated ninja-race bugs, retrying does not help here). Build never reached compilation; no xce_controller, controller_unit_test, or xce_test binaries produced; controller.capnp.c/h were not regenerated. This worktree (config, at .../worktrees/config) still has the pre-fix flake.lock/pin state -- the 2026-06-22 fix (refreshing src/flake.lock) was apparently not carried into this worktree, or upstream c-capnproto main.tar.gz drifted again since. NOT fixed this run (build+report only, no source/flake edits authorized by the task). Next step: refresh src/flake.lock in this worktree (nix flake update / lock the xbuild input) the same way the 2026-06-22 fix did, then retry the controller build. RECURRENCE 3 (2026-07-23, config worktree, config-upgrade-snapshot Task 1 implementer session): identical hash mismatch reproduced again AFTER a prior subagent had already logged occurrence 2 (build+report task, no fix attempted). This session went further: (1) confirmed `nix flake lock --update-input xbuild` in src/ is a NO-OP -- xbuild is already pinned to the latest available shen390s/my-nix-channel@master commit (073f527e), so re-locking flake.lock does NOT fix this recurrence (the previous 2026-06-22 fix's mechanism -- refresh flake.lock -- no longer applies once xbuild is already current); (2) confirmed via direct `nix develop` in the CANONICAL non-worktree repo (/home/rshen/projects/xce/xce/src) that it fails IDENTICALLY (same two sha256 hashes, same capnpc-1.0.1.drv path) -- this is a live, machine-wide, upstream c-capnproto main.tar.gz drift, NOT a worktree-isolation problem, and is not fixable from within this repo at all (the broken pin lives inside the external shen390s/my-nix-channel flake, at whatever commit is currently latest); (3) WORKAROUND discovered and used successfully: this machine's /nix/store already has REALIZED (fully built, not just .drv) output paths for capnpc-1.0.1 and capnproto-1.2.0 from a build that completed before the drift -- `find /nix/store -maxdepth 1 -iname '*capnp*'` surfaces them; invoking their bin/capnp and bin/capnpc-c binaries DIRECTLY (bypassing `nix develop`'s full devShell evaluation entirely) still works and successfully compiled a controller.capnp schema change end-to-end, generating correct bindings. This unblocks schema-level capnp verification even while the full `nix develop` shell is broken, but does NOT unblock a full project build/link -- controller_unit_test additionally needs `librtti` (fetched via a private SSH git flake input), which has NEVER been built/cached on this machine at all (no .drv or realized store path exists for it), so a full build+test run remains genuinely infeasible until either upstream fixes the c-capnproto pin or a machine with a pre-cached librtti+full toolchain is used. ## Verification _Not recorded (migrated from buglog.json)._ ## References _Not recorded (migrated from buglog.json)._ ## Legacy notes - **recurrence_4_RESOLVED:** 2026-07-24, config-upgrade-snapshot task 1 resumption session: RESOLVED (repo-local fix, no longer machine-wide-blocked as recurrence 3 concluded). Added a local override in src/nixSupport/xce.nix's capnpc derivation -- `src = pkgs.fetchFromGitHub { owner = "shen390s"; repo = "c-capnproto"; rev = "e45ff682c4756ac5067cbac6d0f282a8ea5f0155"; sha256 = "sha256-9rTlcTia+s9ARGeTi+8auGvM0RkS+wZK7aIkzecaeSk="; }` -- pinning capnpc's source to an IMMUTABLE commit (the exact content the drifted `main` tarball happened to resolve to, so the hash matches what was already realized in the local /nix/store from recurrence 3's workaround) instead of upstream xbuild's package.nix pinning the MUTABLE `main` ref. This makes the fix independent of upstream's moving target permanently, not just until the next drift. Verified end-to-end: `nix develop --command meson setup xce.build --buildtype=debug -Db_sanitize=address --reconfigure && ninja -C xce.build controller/controller_unit_test` built clean (librtti and all other deps resolved fine -- recurrence 3's librtti-never-cached concern did not reproduce, this machine already had it), and the full `controller_unit_test` binary ran to completion: 978 Tests 0 Failures 0 Ignored, OK. Do-not-repeat: when a fixed-output-derivation hash mismatch traces to an upstream flake pinning a MUTABLE branch/ref (not a tag/commit), the durable fix is a local override.overrideAttrs pinning `src` to an explicit immutable commit + matching sha256 in the CONSUMING repo's own nix file -- do not wait on upstream or keep re-locking flake.lock (recurrence 3 already proved that path is a dead end once xbuild is already at its latest available commit).