Deferred Derivation Resolution — Approaches Tried

Problem

A non-CA derivation depends on a CA derivation. At evaluation time, Nix can't compute the output path because the CA output is unknown. The .drv file stores path="" for each output — this is a deferred derivation.

The deferred derivation's env contains downstream_placeholder(drv, out) strings that must be replaced with actual CA output paths before building.

Approaches

1. Delegate to CADerivationHandler (DIDN'T WORK)

DerivationHandler saw output.path == "" and dispatched to CADerivationHandler. But CADerivationHandler assumes CA semantics (content-addressed hash), and the daemon doesn't know the output path either → $out empty → sh: can't create : nonexistent directory.

2. Send empty paths, let daemon resolve (DIDN'T WORK)

Sent BuildDerivationRequest with empty output paths in the BasicDerivation, relying on the daemon to resolve placeholders using registered realisations. The Nix daemon does not resolve deferred derivations during BuildDerivation — it just uses the output paths from the BasicDerivation. Empty path → $out="" → builder fails.

3. Pre-compute hash via _resolve_deferred_outputs (HASH MISMATCH)

Used _resolve_deferred_outputs from the old derivation_resolution module. This calls _hash_derivation_modulo() which calls _unparse_basic_derivation() — produces an ATerm WITHOUT input_drvs (since BasicDerivation has no such field). But the daemon reads the .drv file which HAS input_drvs. The daemon's hashDerivationModulo replaces input_drvs with modulo values before hashing, while _unparse_basic_derivation just omits them entirely. These produce different hashes → OUTPUT_REJECTED.

4. Hash via Derivation.serialize() (HASH MISMATCH — different direction)

Took the parsed Derivation, rewrote env/args with resolved paths, called Derivation.serialize() which includes input_drvs as raw store path references. Hashed the full ATerm. But the daemon's hashDerivationModulo replaces input_drvs with modulo hash values BEFORE hashing — not raw store paths. Different hash → wrong output path.

5. Clear input_drvs, serialize Derivation (CURRENT — SHOULD WORK)

Same as approach 3 but uses the actual Derivation.serialize() (not _unparse_basic_derivation) after clearing input_drvis and dynamic_input_drvs. Should produce the same ATerm format as the daemon produces AFTER its modulo-replacement step, since:

• Daemon replaces each input_drv entry with a fixed-size modulo hash placeholder

• These placeholders are not part of the ATerm format — they collapse to the same empty [] as clearing them outright

• Only input_srcs (which are real store paths) remain

• Output paths, env, args match the resolved derivation