Map Blast Radius

Map the Blast Radius

You are working on a change to gastownhall/gascity and need to know what it ripples to before you commit. A narrow-looking change can run on a config reload, leak a goroutine in a CLI path, or read stale config — the failures a maintainer's adoption review catches. This skill maps the surface so you catch them first.

Run it from your local checkout. It uses only git, grep, and gh — no special index required. (If you've run codegraph init in the repo, its codegraph_callers / codegraph_impact are faster than grep for caller traces; fall back to grep when it returns nothing or the call is dynamic.)

main below means the upstream main you're targeting — origin/main if origin is gastownhall/gascity, else upstream/main.

Step 1 — Enumerate touched functions

git diff main...HEAD --stat
git diff main...HEAD -- '*.go' | grep '^[+-]func ' | grep -v _test.go

Record each new/modified function: package, name, exported/unexported.

Step 2 — Map callers (upward)

For each touched function, find who calls it:

grep -rn 'FunctionName(' cmd/gc/ internal/ --include='*.go' | grep -v _test.go

Classify every caller into an execution context — the context determines the risk:

Step 3 — Map callees (downward)

Read each modified function body and trace what it calls. Flag:

• cs.cfg reads inside anything called from update() → stale-config risk (B21)
• store.SetMetadata* / store.Close / store.Create → errors must propagate (B12)
• shellCommand / exec.Command → is the timeout shared with another subsystem? (B13)
• go func() → is the caller a CLI path? Then it needs a completion signal (B17)

Step 4 — Config-field impact

If the change adds or modifies a config field, verify it appears in all of these — a missing one is a silent drop:

grep -rn 'FieldName' internal/config/ cmd/gc/ --include='*.go' | grep -v _test.go

• config.Agent struct
• config.AgentPatch struct
• config.AgentOverride struct
• applyAgentPatch()
• applyAgentOverride()
• poolAgents() deep-copy in cmd/gc/pool.go
• ApplyAgentDefaults() if there's a city-wide default
• Effective*() accessor if the nil/empty distinction matters (B11)

TestAgentFieldSync catches the struct definitions only — the apply functions and the pool deep-copy must be checked by hand.

Step 5 — Domain-boundary crossings

Check whether the change crosses any of these boundaries:

Step 6 — Concurrency

If the change involves goroutines, semaphores, or shared state:

git diff main...HEAD -- '*.go' | grep -E 'go func|\.Lock\(|\.RLock\(|<-|chan '

• New goroutine in a CLI path → does it return a completion signal? (B17)
• New semaphore → does it bound exactly the right set of operations? (B13)
• Package-level mutable state written by reload → atomic.* / mutex? (B30)
• Shared state guarded by the right mutex (e.g. serviceStateMu)?

Step 7 — Cross-repo impact

Gas City extracts subsystems from gastownhall/gastown, depends on gastownhall/beads, and uses dolthub/dolt as its storage engine. If your change touches a contract that spans repos, check it:

• Beads interface — if you touch bead-store operations (Create, SetMetadata, Query, Close), confirm against gastownhall/beads that the interface contract still holds. Browse the repo on GitHub or gh search code --repo gastownhall/beads '<symbol>'.
• Gastown original behavior — for extracted code, check how gastownhall/gastown handled the same path. Classify a divergence as [extraction bug] (gascity broke something gastown gets right), [inherited] (gastown has the same limitation), or [intentional divergence] (document why).
• Upstream drift — has gastown/beads/dolt changed recently in this area? gh search commits --repo gastownhall/gastown '<area-keyword>'. Flag if gascity may need to pull in an upstream fix.

Step 8 — Report

Blast Radius — <branch>

Changed functions:
  <pkg>.<Func> — <one-line summary>

Execution contexts:
  <Func> called from: startup / tick / reload / CLI / API / shutdown

Upward callers:
  <Func> ← <caller1> ← <caller2> (context: reload path)

Downward callees:
  <Func> → store.SetMetadata (must propagate error)
  <Func> → go func() (CLI path — needs completion gate)

Config field chain: complete / MISSING <which step>

Domain boundary crossings:
  Pool ↔ named-session    : safe / RISK: <detail>
  User ↔ infra agents     : safe / RISK: <detail>
  Startup ↔ reload        : safe / RISK: <detail>
  CLI ↔ server            : safe / RISK: <detail>
  Snapshot ↔ live config  : safe / RISK: <detail>
  Tick ↔ background        : safe / RISK: <detail>

Concurrency:
  Goroutines: none / N spawned, completion signals: yes/no
  Semaphores: none / scoped to: X subsystem only
  Shared state: none / protected by: <mutex>

Cross-repo:
  Beads interface : compatible / DIVERGES: <detail>
  Gastown original: matches / [extraction bug] / [inherited] / [intentional divergence]
  Upstream drift  : none recent / NEEDS SYNC: <detail>

Risk summary:
  HIGH: <anything a maintainer would call a "major">
  MED:  <anything a maintainer would call a "minor">
  LOW:  <nits, style>

Recommendation: PROCEED / FIX BEFORE COMMIT

Keep findings to one line each, with a file:line ref. The HIGH/MED items feed the Risks section of your plan-implementation plan and the audit in review.

When to use

This skill is dual-use — a phase of a parent skill AND a standalone utility:

• As Phase 2 of plan-implementation, before you write code.
• Standalone, for ad-hoc impact mapping — before committing a change that touches the reconciler, controller, or lifecycle code; when a change spans multiple subsystems (config + runtime, beads + dispatch); when you're unsure whether a function runs on startup only or also on reload; when scoping a refactor or mapping a change for someone else.