fix(scripts): surface git-diff failure in selfhost pre-deploy summary

[jaytbarimbao-collab]

Closes #7775

scripts/selfhost-pre-deploy-summary.sh scanned for historically-sensitive deploy paths by piping git diff --name-only "$range" into a loop via a process substitution (done < <(git diff ...)). A process substitution's exit status is invisible to set -euo pipefail, so if that git diff failed (bad ref, shallow clone, detached history), the loop simply saw zero lines and the script printed "no historically-sensitive paths touched" — a false all-clear on what is meant to be a deploy-safety advisory.

Fix: capture the diff through a checked command substitution and error out loudly if it fails, instead of silently proceeding:

if ! diff_output="$(git diff --name-only "$range")"; then
  echo "error: 'git diff --name-only $range' failed; cannot assess historically-sensitive paths" >&2
  exit 1
fi
while IFS= read -r file; do
  ...
done <<< "$diff_output"

The success path is unchanged (all existing behavior tests still pass).

Test: added a regression test to test/unit/selfhost-pre-deploy-summary-script.test.ts that shadows git with a fake failing only git diff --name-only (delegating fetch and git diff --stat to real git), then asserts the script exits non-zero with the "cannot assess historically-sensitive paths" error rather than the false all-clear. scripts/** is outside the Codecov coverage.include, so no patch-coverage gate applies.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.57%. Comparing base (0709850) to head (ccadf32).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7787   +/-   ##
=======================================
  Coverage   88.57%   88.57%           
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  Files         725      725           
  Lines       76410    76410           
  Branches    22762    22760    -2     
=======================================
  Hits        67683    67683           
  Misses       7681     7681           
  Partials     1046     1046           

Flag	Coverage Δ
shard-1	55.40% <ø> (ø)
shard-2	52.57% <ø> (-0.01%)	⬇️
shard-3	47.38% <ø> (ø)

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✅ LoopOver review result - approve/merge recommended

_{Review updated: 2026-07-21 13:00:16 UTC}

2 files · 1 AI reviewer · no blockers · readiness 98/100 · CI green · clean

✅ Suggested Action - Approve/Merge

• safe to merge

Review summary
This is a narrow, correct fix for #7775: swapping `done < <(git diff ...)` for a checked command substitution (`diff_output="$(...)"`) closes the exact hole where a process-substitution failure was invisible to `set -euo pipefail`, previously producing a false 'no historically-sensitive paths touched' all-clear. The success path (`<<< "$diff_output"` replacing `< <(...)`) is behaviorally equivalent, and the added test genuinely shadows only `git diff --name-only` while delegating fetch and `git diff --stat` to real git, exercising the actual failure path rather than fabricating an unreachable state. Scope is tight and matches the linked issue.

Nits — 4 non-blocking

• The inline comment block above the fix (scripts/selfhost-pre-deploy-summary.sh:118-122) duplicates most of the PR description almost verbatim — could be trimmed to a shorter pointer to selfhost-pre-deploy-summary.sh gives a false all-clear if its own git diff command fails #7775 to avoid comment bloat next to the existing large sensitive-paths comment block.
• The fake git shim in the test (test/unit/selfhost-pre-deploy-summary-script.test.ts) hardcodes `$1`/`$2` positional checks for `diff --name-only`; if the script's flag order ever changes this test would silently stop exercising the failure path — a minor coupling worth a one-line comment noting the dependency.
• Consider shortening the new script comment to one or two lines referencing selfhost-pre-deploy-summary.sh gives a false all-clear if its own git diff command fails #7775 instead of restating the full rationale already covered in the PR description.
• Could add a similarly-checked substitution note/reference near `git rev-list --count` and `git diff --stat` calls if those are ever suspected of similar silent-failure risk, though that's out of scope for this PR.

Decision drivers

• ✅ Code review — No blockers (1 reviewer)
• ✅ Gate result — Passing (No configured blocker found.)

Context & advisory signals — never blocks the verdict

Signal	Result	Evidence
Linked issue	✅ Linked	#7775
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	✅ 20/20	Low review scope from cached public metadata (1 linked issue).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 151 registered-repo PR(s), 73 merged, 16 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	jaytbarimbao-collab; Gittensor profile; 151 PR(s), 16 issue(s).
Improvement	ℹ️ Insufficient signal	risk: clean · value: insufficient-signal

Linked issue satisfaction

Addressed
The diff replaces the unchecked process substitution with a checked command substitution that exits non-zero and prints an explicit error when git diff fails, exactly matching the requested fix pattern, and adds a regression test simulating the diff failure.

Review context

• Author: jaytbarimbao-collab
• Role context: outside_contributor
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: Rust
• Official Gittensor activity: 151 PR(s), 16 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Keep the PR focused and include validation evidence before maintainer review.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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LoopOver approves — the gate is satisfied and CI is green.