don't include derivation name in temporary build directories

[edolstra]

Motivation

With the migration to /nix/var/nix/builds we now have failing builds when the derivation name is too long.
This change removes the derivation name from the temporary build to have a predictable prefix length:

Also see: NixOS/infra#764
for context.

(cherry picked from commit 725a2f3) (cherry picked from commit 7c3fd50)

Context

Summary by CodeRabbit

• Bug Fixes

  • Temporary build directories no longer include derivation names, reducing path-length-related build failures and providing predictable nix-* prefixes under /nix/var/nix/builds.

• Documentation

  • Added a page explaining the new temporary build directory naming and its implications, with references for further details.

• Tests

  • Updated functional and NixOS tests to reflect the new nix-* build directory patterns for improved reliability.

[coderabbitai]

Walkthrough

Temporary build directory naming is changed from including derivation names to a generic “nix” prefix. Corresponding tests update their glob patterns to match the new naming, and documentation adds a page describing the change and rationale. No public APIs are modified.

Changes

Cohort / File(s)	Summary
Build temp dir naming src/libstore/unix/build/derivation-builder.cc	Changes createTempDir top-level prefix from "nix-build-" to "nix", affecting the generated temporary build directory path.
Tests: glob updates for build dirs tests/functional/check.sh, tests/nixos/user-sandboxing/default.nix	Updates glob patterns and path expectations from nix-build-* to nix-*, adjusting messages and directory extraction accordingly.
Docs: new page on shorter build dir names doc/manual/rl-next/shorter-build-dir-names.md	Adds documentation describing that temporary build directories no longer include derivation names; notes predictable prefix lengths under /nix/var/nix/builds.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant DB as DerivationBuilder
  participant FS as Filesystem
  participant Env as Build Env

  DB->>FS: createTempDir(prefix="nix")
  FS-->>DB: returns /nix/var/nix/builds/nix-XXXX
  DB->>Env: setBuildTmpDir(/nix/.../nix-XXXX)
  note over DB,Env: Subsequent build steps use the generic-prefixed temp dir

Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I hop through builds with lighter names,
No sprawling tags, no suffix games.
A neat small “nix” is all I need—
To burrow, build, and then proceed.
Tests nod along, docs hum a tune—
Shorter paths, my lucky rune. 🐇✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The title "don't include derivation name in temporary build directories" succinctly and accurately summarizes the primary change: removing derivation names from temporary build-directory names to avoid failures when names are too long. It is a single clear sentence that matches the PR objectives and the code, test, and documentation diffs.

✨ Finishing touches

• 📝 Generate Docstrings

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch shorter-tmpdirs

---

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[github-actions]

🚀 Deployed on https://68caf6f8ba54691e8a2cede5--determinate-nix.netlify.app

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (4)

tests/functional/check.sh (1)

55-56: Drop redundant count check and rely on the array length.

The array length check below already validates cardinality; the count call adds complexity and can misbehave without nullglob.

-  [[ 1 == "$(count "$customBuildDir/nix-"*)" ]]
-  local buildDir=("$customBuildDir/nix-"*)
+  local buildDir=("$customBuildDir/nix-"*)

tests/nixos/user-sandboxing/default.nix (2)

107-109: Prefer selecting the newest nix- dir to avoid multi-match flakiness.*

Using a time-sorted ls reduces risk if multiple builds are present.

-          machine.wait_until_succeeds("stat /nix/var/nix/builds/nix-*/build/syncPoint")
-          dir = machine.succeed("ls -d /nix/var/nix/builds/nix-*").strip()
+          machine.wait_until_succeeds("stat /nix/var/nix/builds/nix-*/build/syncPoint")
+          dir = machine.succeed("ls -dt /nix/var/nix/builds/nix-* | head -n1").strip()

---

128-130: Same here: pick the most recent directory.

-          machine.wait_until_succeeds("stat /nix/var/nix/builds/nix-*/build/syncPoint")
-          dir = machine.succeed("ls -d /nix/var/nix/builds/nix-*").strip()
+          machine.wait_until_succeeds("stat /nix/var/nix/builds/nix-*/build/syncPoint")
+          dir = machine.succeed("ls -dt /nix/var/nix/builds/nix-* | head -n1").strip()

doc/manual/rl-next/shorter-build-dir-names.md (1)

1-6: Add a short migration note for downstream scripts.

Call out the glob change explicitly to help users updating tests/tooling.

 Temporary build directories created during derivation builds no longer include the derivation name in their path to avoid build failures when the derivation name is too long. This change ensures predictable prefix lengths for build directories under `/nix/var/nix/builds`.
+ 
+ Migration: if you relied on matching `/nix/var/nix/builds/nix-build-*`, update globs to `/nix/var/nix/builds/nix-*`.

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 9f8a904 and 3a3b2fd.

📒 Files selected for processing (4)

• doc/manual/rl-next/shorter-build-dir-names.md (1 hunks)
• src/libstore/unix/build/derivation-builder.cc (1 hunks)
• tests/functional/check.sh (1 hunks)
• tests/nixos/user-sandboxing/default.nix (2 hunks)

🧰 Additional context used

🧬 Code graph analysis (1)

src/libstore/unix/build/derivation-builder.cc (2)

src/libutil/include/nix/util/file-system.hh (1)

• createTempDir (330-330)

src/libutil/file-system.cc (2)

• createTempDir (677-707)
• createTempDir (677-677)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)

• GitHub Check: build_aarch64-darwin / build

🔇 Additional comments (3)

tests/functional/check.sh (1)

58-58: Good: updated error message matches new glob.

doc/manual/rl-next/shorter-build-dir-names.md (1)

3-3: Confirm PR reference metadata.

Is prs: [13839] intentional for this repo? If you want to cross-link this PR as well, add it here per your docs conventions.

src/libstore/unix/build/derivation-builder.cc (1)

720-720: Shorter tmpdir prefix LGTM — remaining "nix-build-" usages found; verify or update.

• src/libstore/unix/build/linux-derivation-builder.cc: cgroup names use "nix-build-uid-"/"nix-build-pid-" (lines ~235–236).
• tests/nixos/cgroups/default.nix: looks for "nix-build-uid-*" (lines 33, 38).
• src/libstore/include/nix/store/globals.hh: example tmpDir/topTmpDir contain "nix-build-..." (lines ~1450, 1452).
• doc/manual/source/advanced-topics/diff-hook.md: references '/tmp/nix-build-...' (line 94).
• tests/functional/nix-build.sh: test invocations reference nix-build (lines 37–46).

Confirm these are intentionally unchanged or update tests/docs/cgroup naming to match the new "nix" tmpdir prefix.