SRE-637: Fix false positive stale approval dismissal on base-branch-only updates

[TimDiekmann]

Summary

Fixes false positive stale approval dismissals when only the base branch advances (e.g., squash-merge of a stacked PR).

Problem

When a stacked PR's base gets squash-merged, the base branch advances but the PR branch is untouched. The shifted merge-base causes git range-diff to report false differences:

main:       A ─── B ─────────── S (squash merge of featureA)
                   \
featureA:           C (gets squash-merged)
                     \
featureB:             D (approved, untouched)

Before squash-merge (previous run):

• PREV_BASE_SHA = C, PREV_HEAD_SHA = D
• merge-base(C, D) = C → range: C..D (just featureB's commit)

After squash-merge (current run):

• BASE_SHA = S, HEAD_SHA = D (unchanged!)
• merge-base(S, D) = B (shifted!) → range: B..D (featureA + featureB)

range-diff C..D vs B..D:
  -:  ------- > 1:  92ee94d featureA commit    ← false positive!
  1:  404d18d = 2:  404d18d featureB commit

Fix

Extracts the range-diff logic into a testable function run_check_range_diff_stale in range-diff-stale.sh. The function checks if HEAD_SHA == PREV_HEAD_SHA before running range-diff — same HEAD SHA = same git content by definition, so the review can't be stale.

The fix lives in the same code that action.yml calls, not as an untestable YAML short-circuit.

Tests

• Parser test (test_base_branch_advance_is_not_stale): verifies the range-diff parser reports stale=true for the shifted merge-base, documenting the false positive
• E2E test (test_base_branch_advance_e2e_is_not_stale): reproduces the full stacked PR squash-merge scenario, calls run_check_range_diff_stale directly, asserts stale=false
  • Fails without the fix (range-diff false positive → stale=true)
  • Passes with the fix (HEAD unchanged → stale=false)

Related

• SRE-637
• Observed on internal-infra PR #187

[augmentcode]

🤖 Augment PR Summary

Summary: Prevents false stale-approval dismissals when a PR’s base branch advances but the PR head commit remains unchanged.

Changes: Adds an early exit in the range-diff step when HEAD_SHA == PREV_HEAD_SHA, skipping range-diff entirely for base-only updates.

Tests: Adds a repo-based reproduction of the shifted merge-base range-diff false positive, plus a structural assertion that the short-circuit exists in action.yml.

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