Attempt to use the scalset API and caching

On github, attempt to use the scaleset API to list all runners without
pagination. This will avoid missing runners and accidentally removing them.

Fall back to paginated API if we can't use the scaleset API.

Add ability to retrieve all instances from cache, for an entity.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

runner/pool/cache.go

@@ -0,0 +1,75 @@
+package pool
+
+import (
+	"sort"
+	"strings"
+	"sync"
+	"sync/atomic"
+
+	runnerErrors "github.com/cloudbase/garm-provider-common/errors"
+	"github.com/cloudbase/garm/params"
+)
+
+type poolCacheStore interface {
+	Next() (params.Pool, error)
+	Reset()
+	Len() int
+}
+
+type poolRoundRobin struct {
+	pools []params.Pool
+	next  uint32
+}
+
+func (p *poolRoundRobin) Next() (params.Pool, error) {
+	if len(p.pools) == 0 {
+		return params.Pool{}, runnerErrors.ErrNoPoolsAvailable
+	}
+
+	n := atomic.AddUint32(&p.next, 1)
+	return p.pools[(int(n)-1)%len(p.pools)], nil
+}
+
+func (p *poolRoundRobin) Len() int {
+	return len(p.pools)
+}
+
+func (p *poolRoundRobin) Reset() {
+	atomic.StoreUint32(&p.next, 0)
+}
+
+type poolsForTags struct {
+	pools         sync.Map
+	poolCacheType params.PoolBalancerType
+}
+
+func (p *poolsForTags) Get(tags []string) (poolCacheStore, bool) {
+	sort.Strings(tags)
+	key := strings.Join(tags, "^")
+
+	v, ok := p.pools.Load(key)
+	if !ok {
+		return nil, false
+	}
+	poolCache := v.(*poolRoundRobin)
+	if p.poolCacheType == params.PoolBalancerTypePack {
+		// When we service a list of jobs, we want to try each pool in turn
+		// for each job. Pools are sorted by priority so we always start from the
+		// highest priority pool and move on to the next if the first one is full.
+		poolCache.Reset()
+	}
+	return poolCache, true
+}
+
+func (p *poolsForTags) Add(tags []string, pools []params.Pool) poolCacheStore {
+	sort.Slice(pools, func(i, j int) bool {
+		return pools[i].Priority > pools[j].Priority
+	})
+
+	sort.Strings(tags)
+	key := strings.Join(tags, "^")
+
+	poolRR := &poolRoundRobin{pools: pools}
+	v, _ := p.pools.LoadOrStore(key, poolRR)
+	return v.(*poolRoundRobin)
+}