Delete PatchSources::mem_blobs, the single-variant VendorSource predicates, the redirect-state group-commit capture and the group_commit switch-off oracle

[mikolalysenko]

[agent] Filed by the scheduled architecture audit routine (CLI and core). Register: discussion #560 register.

Kind: refactor (dead code; no behavior change). Source: review 7.4, 7.6 #3, 5.6; register C23.

Problem (verified on 045d7ec)

1. PatchSources::mem_blobs is never Some in production.
   • The field says vendor flows "stage their patch content here". Every production constructor passes mem_blobs: None: commands/vendor.rs (6 sites), vendored_backend/mod.rs:71, repair.rs:413, fetch_stage.rs:39 and vendor/npm_flavor.rs:546,763.
   • Its only readers are the apply branch and vendor/test_support/service_fixture.rs:167.
2. VendorSource has one variant, and its predicates are constant.
   • enum VendorSource { Service }: may_use_service() and requires_service() both return true, and as_tag() always returns "service".
   • The only production branch on it is therefore unconditional, and VendorServiceConfig.source carries no information.
3. Group commit captures a ledger that nothing in its scope writes.
   • LEDGERS includes .socket/vendor/redirect-state.json.
   • v5 never writes that file. save_redirect_state (patch/redirect/state.rs:147-156) is #[doc(hidden)] and test-only.
   • The only deleter, retire_legacy_redirect_ledger in rollback.rs/remove.rs, runs outside any GroupCommit, whose only production scope is the vendor loop.
4. The switched_off("group_commit") oracle keeps the pre-group-commit path alive.
   • vendor.rs:2273-2275 skips GroupCommit::begin under SOCKET_PATCH_SWITCH_OFF=group_commit, a debug-only env var.
   • Only tests/vendor_group_commit_e2e.rs (304 lines) uses it, as an equivalence oracle for a change that has already landed. It is the only switched_off call site.

Not dead (ruled out): the Pypi/LauncherCache update channels in update/channel.rs. They detect pre-v5 pip/gem installs so that --update refuses to swap a package-manager-owned binary and prints a migration hint. That is a live safety refusal, so they stay.

Symptoms / impact

There are no user-visible bugs. The cost falls on readers: two documented mechanisms that do nothing, an unconditional branch dressed as a policy, and a debug env var (SOCKET_PATCH_SWITCH_OFF) that keeps a second code path compiled into debug builds.

Proposed change

Delete:

• the mem_blobs field, its branch in apply.rs, its ~25 mem_blobs: None initializers and the fixture read;
• VendorSource::{may_use_service, requires_service} and the always-true condition at vendor.rs:141. Keep VendorSource::parse and the --vendor-source flag, because accepting service/auto and rejecting build is contract (CLI_CONTRACT "Prebuilt vendor artifacts"). Removing the flag is out of scope (decision C35);
• the redirect-state.json entry from group_commit::LEDGERS, along with its doc line and its test row (group_commit.rs:1071);
• the switched_off("group_commit") guard, failpoint::switched_off if it is then unused, and the oracle test (or turn it into a golden check of the group-commit output).

Size and scope

• Production: about −60 lines. Tests: about −350 lines.
• Files: patch/apply.rs, vendor/{mod,npm_flavor}.rs, vendor/test_support/service_fixture.rs, utils/{group_commit,failpoint}.rs, commands/{vendor,repair,fetch_stage}.rs, commands/vendored_backend/mod.rs, tests/vendor_group_commit_e2e.rs.
• Out of scope: --vendor-source removal, the update channels, the pre-v5 redirect ledger readers (migration).

Acceptance criteria

• grep -rn mem_blobs crates returns nothing.
• VendorSource has no always-true predicate, and --vendor-source service|auto still parses while build is still rejected (the args.rs tests stay green).
• group_commit::LEDGERS lists only .socket/vendor/state.json; the group-commit crash and replay tests stay green.
• No SOCKET_PATCH_SWITCH_OFF reference remains (or one documented user, if a maintainer wants to keep the mechanism).
• cargo test -p socket-patch-core --lib and cargo test -p socket-patch-cli stay green; cargo clippy --workspace --all-features -- -D warnings stays clean.

Dependencies

• Not blocked.
• patch/apply.rs is touched by open PRs Fix agent mode patching linked first-party source (#626) #634, Full Gradle support in agent, hosted and vendored modes #646 and sbt, Mill and scala-cli support in agent, hosted and vendored modes #690; land after them, or keep the apply.rs hunk to the two lines.

[mikolalysenko]

[agent] Triaged: priority:p3 (cross-cutting refactor). Not a duplicate, and no existing fix PR.

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[mikolalysenko]

[agent] Coordination note from the ecosystems audit: #800 removes the rest of the vendored scaffolding near item 2:

• the single-variant PackageSource;
• vend_installed!;
• ServicePolicy::new's unused config;
• the whole SERVICE_ECOSYSTEMS block at commands/vendor.rs:137-150, which can never fire because ecosystem_dir_for_purl returns only those 8 names.

Both issues touch vendor.rs:141. Whichever lands second should drop the remaining block rather than keep !SERVICE_ECOSYSTEMS.contains(&eco) on its own.

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