Vendored requirements.txt refuses a six (==1.16.0) pin that the inventory and hosted mode accept, because exact pins are read by three grammars #1365

[agent] Filed by the scheduled architecture audit routine (ecosystems and formats). Register: register comment.

Kind: bug. Source: new finding, register E96 (the "duplicated business logic" class; same shape as E91 and E89).

Problem

"Is this requirements.txt line an exact pin of name==version" is decided by three independent grammars. Verified on 9ab72d4. File links below are at that SHA. The line-anchored permalinks are in the first comment, because this tool mangles line anchors in issue bodies.

1. Lock inventory and VEX discovery use utils::requirements::exact_pin (requirements.rs, lines 392-426). Its doc says it is "the ONE exact-pin rule". It accepts whitespace around == and the legacy parenthesised form six (==1.0). It rejects ===, and it returns the version as spelled. Callers: lock_inventory/pypi.rs line 674 and vex/discover/pypi_other.rs line 283.
2. Hosted rewrite uses its own regex and requirement_version (redirect/requirements.rs, lines 146-187 and 198-201). It strips ( … ), accepts === as Arbitrary, and compares == under PEP 440.
3. Vendored planner uses parse_requirement_line (lines 1148-1191) plus scan_pins (lines 78-112) in pypi_requirements.rs. It takes everything after the name and extras as the specifier, strips whitespace and calls pep440::is_exact_pin_of, which needs a leading ==. So (==1.16.0) is a range.

The PEP 508 name-prefix scan ([A-Za-z0-9._-]*) is written out once more in each of these, and also in vendor/common.rs::pep508_name, vendor/pypi_lock.rs::value_identity and vex/discover/pypi_other.rs::pep508_direct_reference.

Proof by execution. A throwaway unit test in vendor::pypi_requirements::tests, run twice on 9ab72d4 and then reverted, fed the same one-line requirements.txt to all three readers for six@1.16.0:

pip reads six (==1.16.0) as the exact pin ==1.16.0: PEP 508's versionspec admits the parenthesised form, and exact_pin's own doc relies on that. So scan offers the package (the inventory finds the pin) and hosted mode wires it, but socket-patch vendor fails with "requirements.txt: six is not pinned to ==1.16.0; pin it exactly …". That message is false.

Symptoms

• Lock-only requirements.txt discovery sends PEP 440-equivalent pins verbatim (six==1.16 → pkg:pypi/six@1.16), so a fresh checkout reports "No patches available" while the same file with a venv is patched #604: lock-only discovery (exact_pin) carries the spelled version (six==1.16 → @1.16), while hosted and vendored compare under PEP 440. This is the same split on the version axis.
• Lockfile-only requirements.txt discovery skips exact pins written with spaces (six == 1.15.0, six (==1.15.0)), so a fresh checkout reports "No patches available" and pip installs the unpatched release #523 (closed) fixed the parenthesised and spaced forms in discovery only. Hosted requirements.txt rewrite skips PEP 440-equivalent pins like six==1.16 for an installed 1.16.0, so scan exits 0 and pip installs the unpatched release (regression from v4.0.0) #475 (closed) fixed PEP 440 equality in hosted and vendored only. Each fix landed in one grammar.

Impact

• A user-visible refusal with a misleading remedy for a valid pip spelling, plus a scan/vendor disagreement of the E91 kind. It is uncommon in hand-written files, but pip-compile users who hand-edit, and older tooling, write name (==x).
• Systemic: every requirements.txt pin rule (PEP 440 equality, ===, parentheses, markers, --hash) has to be fixed in three places, and history shows it is fixed in one at a time.

Proposed change

1. Add one utils::requirements::Requirement parse of a logical line's code part: name as spelled, extras, the specifier split into clauses (parentheses removed), marker, hash options and direct reference. Add Requirement::exact_version(), an exact pin under PEP 440 (== with no wildcard; === reported separately).
2. Make exact_pin a thin wrapper over it (or delete it and move its callers).
3. Delete parse_requirement_line/ParsedRequirement in vendor/pypi_requirements.rs. scan_pins reads Requirement.
4. Delete the hosted name_re regex and requirement_version in patch/redirect/requirements.rs. The hosted rewrite reads Requirement and keeps its own Unpinned/Arbitrary policy on top.
5. Whether vendored should also accept === is a policy choice. Keep today's refusal and say so in a code comment; don't widen it in this change.

Size and scope

• utils/requirements.rs (+120), vendor/pypi_requirements.rs (−60), patch/redirect/requirements.rs (−~50), plus tests. About 300 production lines changed.
• Out of scope: Lock-only requirements.txt discovery sends PEP 440-equivalent pins verbatim (six==1.16 → pkg:pypi/six@1.16), so a fresh checkout reports "No patches available" while the same file with a venv is patched #604's PEP 440 version normalization in the inventory purl. It can land on top of exact_version() as a follow-up, or in the same PR if small. Also out of scope: pep508_name in TOML-based backends, and Hosted requirements.txt rewrite replaces a user's own direct reference (six @ https://mirror/…/six-1.16.0-….whl, file:// fork) with the Socket PyPI build, and rollback then restores six==1.16.0 from PyPI, losing the original source #542 (hosted direct references, PR Fix hosted requirements rewrite of user direct refs (#542) #1333).

Acceptance criteria

• One requirements-line grammar in utils::requirements, used by the inventory, VEX discovery, the hosted rewrite and the vendored planner. parse_requirement_line and the hosted name_re/requirement_version are gone.
• Regression test: a table test over the five lines above, asserting that the inventory, hosted and vendored readers agree on "exact pin of 1.16.0", with vendored wiring six (==1.16.0) and six (== 1.16.0).
• The existing pep440_equivalent_pins_are_rewritten, parse_requirement_line_ignores_path_and_nonname_lines, marker-split (Vendored requirements.txt from uv pip compile --universal refuses a marker-split package (six==1.16.0 ; python < 3.12 + six==1.17.0 ; python >= 3.12) as "not pinned to ==1.16.0", while --dry-run previews would_vendor and hosted / vendored pylock handle the same split #928) and lock-only discovery (Lockfile-only requirements.txt discovery skips exact pins written with spaces (six == 1.15.0, six (==1.15.0)), so a fresh checkout reports "No patches available" and pip installs the unpatched release #523, Lock-only requirements.txt discovery drops a pin whose last line ends in a dangling \ continuation (six==1.16.0 \ at EOF): scan exits 0 with "No patches", but pip installs it #1249) tests stay green.
• cargo test -p socket-patch-core and the pip e2e suites pass.

Dependencies

• None blocking. It touches the same file as PR Fix hosted requirements rewrite of user direct refs (#542) #1333 (Hosted requirements.txt rewrite replaces a user's own direct reference (six @ https://mirror/…/six-1.16.0-….whl, file:// fork) with the Socket PyPI build, and rollback then restores six==1.16.0 from PyPI, losing the original source #542, hosted direct refs). Rebase on whichever lands first.
• Unblocks a one-place fix for Lock-only requirements.txt discovery sends PEP 440-equivalent pins verbatim (six==1.16 → pkg:pypi/six@1.16), so a fresh checkout reports "No patches available" while the same file with a venv is patched #604.