Diff first progress projection

SKILL: Diff-First Progress Projection

Domain: extensions-mcp
Trigger: Background marketplace reconciliation needs to show progress before the clone/install work begins (before reconcileMarketplaces). Source Pattern: Distilled from reviewed extension lifecycle and source-reconciliation patterns.

Core Method

Run the marketplace diff (diffMarketplaces) before any long-running work so you can enumerate missing and sourceChanged entries. Collapse those names into pendingNames and immediately setAppState with plugins.installationStatus.marketplaces = pendingNames.map(name => ({ name, status: 'pending' })) before calling reconcileMarketplaces. That makes the UI show the right number of pending spinners even while git clones are still starting.

Key Rules

• Compute declared and materialized marketplaces and pass them through diffMarketplaces once, capturing missing plus sourceChanged.map(c => c.name) for the pending set.
• Initialize installationStatus.marketplaces with the pending list; leave installationStatus.plugins empty unless a plugin-level hook has more granular progress.
• Write the pending state before reconcileMarketplaces so that any UI watching plugins.installationStatus.marketplaces can render spinners before the expensive work begins.
• Keep the status field locked to 'pending' until the onProgress bridge reports a transition; do not assume any installs succeeded until they emit events.

Example Application

performBackgroundPluginInstallations computes pendingNames and immediately updates AppState with a queue of 'pending' marketplaces. Even before any git clones happen, a marketplace status line renders with a spinner, which later echos the installing/installed lifecycle when the onProgress bridge fires.

Anti-Patterns (What NOT to do)

• Do not call reconcileMarketplaces before the diff – you lose the chance to know how many spinners the UI must draw and the install list might appear empty during the initial load.
• Do not skip seeding installationStatus.marketplaces in setAppState, or the UI will never show any pending state and users will assume nothing is happening.
• Do not mix plugin-level progress into installationStatus.plugins until you actually collect plugin-specific events; that list is for instrumentation that doesn't exist in this path.