fix(hardlink): name the mutable set directly, under both build-dir layouts #16

Merged

claude merged 4 commits from fix/layout-v2-selection into main 2026-08-27 20:03:41 +00:00

Conversation 2 Commits 4 Files Changed 4

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4 Commits

Author	SHA1	Message	Date
claude	4bb880b7a7	chore(ci): trigger CI after un-WIP	2026-08-27 14:52:11 -05:00
claude	fb3c72aa28	docs(hardlink): withdraw the nlink claim, which asserted more than was measured ... The comments said hardlink count at link time was ruled out as the discriminator between a rewritten executable and an intact one, citing a lib+bin crate whose two test binaries were both `nlink == 1` and appeared to behave differently. Re-checked on review: the intact one had not been rebuilt at all — same content, same inode — so it demonstrated nothing, and forcing both to rebuild rewrote both. What was actually observed is narrower and now says so: every executable measured intact had an uplift hardlink twin Cargo must re-create anyway, every one measured rewritten had none, and whether the twin is the mechanism or a correlate was not determined. The rule does not rest on the answer — exempting twinned executables would recover none of the bytes this change newly copies. This PR exists because a claim outlived its evidence; it should not ship one.	2026-08-27 14:50:53 -05:00
claude	0553a6b956	fix(hardlink): resolve an ambiguous out/ toward unsharing, and pin the linked-output scenario on a shape that exhibits it ... Two review findings on #16. The `out/` discriminator keyed on Cargo's record of a build-script execution, which Cargo writes only AFTER the script exits successfully. A build script that populates OUT_DIR and then fails leaves a unit with no record at all, so its OUT_DIR read as a compile unit's artifact directory and stayed shared — a regression against the old `-name build` selection, which real-copied that state by construction. Reproduced on cargo 1.93.1 stable. An `out` directory now stays shared only when two independent signals agree: it holds an `.rlib`/`.rmeta` of its own, and its unit carries no execution record. Either one missing real-copies it. The cost is unchanged to the byte — the newly-unshared directories hold only executables and `*.d`, both already privately owned by the file rules. The live linked-test-binary scenario was built on the lib+bin probe crate, whose test binaries relink to a fresh inode — a shape gitdan-actions#17 records as measured safe. Both halves passed green against the unfixed selection on dep-info mutations the previous scenario already covers. It now builds a bin-only crate with a unit test, reads only executables, and skips loudly with a warning rather than passing quietly if the toolchain does not exhibit the rewrite at all. Also: drop a clause asserting the linker writes in place "whenever the path has no other hard link", which this change's own evidence denies; move the load-bearing comment block back above `unshare_mutable_paths`; correct a superseded 99.998% figure; and record both cost rows in the README rather than only the flattering whole-tree one.	2026-08-27 14:39:29 -05:00
claude	e5b26a9368	fix(hardlink): name the mutable set directly, under both build-dir layouts ... `unshare_mutable_paths` selected `.fingerprint` and `build` directories. Under Cargo's build-dir layout v2 the first clause matches nothing and the second matches the whole tree, because v2 regroups artifacts under `build/` alongside the metadata. Measured on one scratch crate: 39.3% of the tree real-copied under v1, 99.996% under v2. The selection now names the mutable set rather than the container it used to live in: fingerprint directories under either spelling, layout v2's `run/` directories, layout v1's loose build-script run metadata, and the `out` directories that are a build script's OUT_DIR rather than a compile unit's artifact directory. The two are told apart structurally, by Cargo's record of the build-script execution sitting beside the OUT_DIR and nowhere else. Verifying that turned up a second, layout-independent hazard: a linked executable is written through whatever inode is already at its path, so a `cargo test --no-run` inside a `cp -al` clone rewrites the source's own test binary. Reproduced on cargo 1.93.1 stable, 1.96.0-nightly, 1.98.0-nightly and 1.100.0-nightly, under both layouts. Every executable is now real-copied; `.rlib`, `.rmeta` and `incremental/` are what stay shared. hardlink-clone-selftest.sh gains two file-only layout fixtures that pin the partition in both directions without a compiler, and a live scenario that relinks a test binary.	2026-08-27 14:12:01 -05:00