Add safe manual instance rebalancing

src/lib/server/hosts/capacity.ts

@@ -11,6 +11,7 @@ const CPU_USABLE_FRACTION = 0.95;
 export class CapacityError extends Error {}
 export interface PlacementOptions {
     hostId?: string; libraryId: string; image: string; bindAddress?: string;
+    excludeHostIds?: string[];
     thorough?: boolean; excludeInstanceId?: string;
     // Execution only: not-yet-submitted launches wait their turn instead of
     // requiring their entire backlog to fit before any one can start.
@@ -100,7 +101,7 @@ function projectedLoad(host: Host, options: PlacementOptions): number {
 export function choosePlacement(options: PlacementOptions): { host: Host; port: number; load: number } {
     const candidates: { host: Host; port: number; load: number }[] = [];
     const reasons: string[] = [];
-    for (const host of listHosts().filter(host => !options.hostId || host.id === options.hostId)) {
+    for (const host of listHosts().filter(host => (!options.hostId || host.id === options.hostId) && !options.excludeHostIds?.includes(host.id))) {
         try {
             requireReadyHost(host.id, { image: options.image, requiredBytes: INSTANCE_STORAGE_BYTES });
             assertReadyLibrary(host.id, options.libraryId);