Fix link failure when existing PR in stack is queued

[skarim]

• Fixes gh stack link fails when existing stack contains queued PRs #170

gh stack link refused to add PRs to a stack whose bottom PR was already in the merge queue. Because linking is additive — every existing stack PR must be listed again or the update is rejected for dropping them — including the queued PR triggered an eligibility failure, while omitting it triggered the drop guard:

✗ PR #439655 cannot be added to a stack: it is queued for merge

PR #439655 was already part of the target stack, so being in the merge queue should not have blocked adding new PRs on top of it. The eligibility check ran before link fetched the repository's stacks, so validatePREligibility had no way to know which PRs were already stacked and rejected every queued PR unconditionally. The same catch-22 applied to merged, closed, and auto-merge-enabled PRs already in the stack.

Behavior

link now resolves the target stack before validating eligibility and exempts PRs that are already members of it:

• Already in the target stack — eligibility checks (queued, auto-merge, merged, closed) are skipped, since the PR is only being re-listed, not added. This lets new PRs be linked on top of a stack whose existing PRs are queued.
• Not in the target stack — the previous strict checks still apply, so a queued or auto-merge PR that is not yet stacked is still rejected.
• Creating a new stack — unchanged; with no target stack, every PR is validated as before.

Changes

cmd/link.go:

• runLink: fetch the stacks and resolve the target stack (findMatchingStack) before eligibility validation instead of after, and pass it down.
• validatePREligibility: takes the target stack and skips all eligibility checks for any PR already in it.
• prevalidateStack: takes the already-resolved target stack instead of looking it up a second time.

cmd/link_test.go — four new tests:

• TestLink_AllowsQueuedPRAlreadyInStack: a queued PR already in the stack no longer blocks adding new PRs; the stack is updated with the full ordered list.
• TestLink_AllowsMergedPRAlreadyInStack / TestLink_AllowsAutoMergePRAlreadyInStack: the same exemption for merged and auto-merge-enabled members.
• TestLink_RejectsQueuedPRNotInStack_WhenAddingToExistingStack: a queued PR not yet part of the target stack is still rejected.

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Pull request overview

This PR fixes a gh stack link failure mode when the target stack already contains ineligible PR states (queued for merge, merged, closed, or auto-merge-enabled). The fix ensures link resolves the target stack before running eligibility checks, and exempts PRs that are already members of that target stack from those checks—allowing additive updates without getting stuck in the “must re-list but can’t validate” loop.

Changes:

• Resolve the target stack earlier in runLink, then pass it into validation/prevalidation so existing stacked PRs can be exempted appropriately.
• Update validatePREligibility to skip queued/merged/closed/auto-merge checks for PRs already present in the resolved target stack.
• Add regression tests covering queued/merged/auto-merge exemptions for already-stacked PRs, plus a negative test ensuring queued PRs not in the stack are still rejected.

Show a summary per file

File	Description
cmd/link.go	Reorders stack resolution ahead of eligibility checks; threads the resolved stack into validation and simplifies prevalidation to use the already-matched stack.
cmd/link_test.go	Adds targeted regression tests to ensure queued/merged/auto-merge PRs already in the target stack don’t block linking new PRs, while preserving rejection for queued PRs not yet in the stack.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Low