feat: scheduler (3/): add cycle state implementation (#372)

* Added framework cycle state

* Minor fixes

* Minor fixes

pkg/scheduler/framework/cyclestate.go

@@ -0,0 +1,58 @@
+/*
+Copyright (c) Microsoft Corporation.
+Licensed under the MIT license.
+*/
+
+package framework
+
+import (
+	"fmt"
+	"sync"
+)
+
+// StateKey is the key for a state value stored in a CycleState.
+type StateKey string
+
+// StateValue is the value stored in a CycleState under a specific key.
+type StateValue interface{}
+
+// CycleStatePluginReadWriter is an interface through which plugins can store and retrieve data.
+type CycleStatePluginReadWriter interface {
+	Read(key StateKey) (StateValue, error)
+	Write(key StateKey, val StateValue)
+	Delete(key StateKey)
+}
+
+// CycleState is, similar to its namesake in kube-scheduler, provides a way for plugins to
+// store and retrieve arbitrary data during a scheduling cycle. The scheduler also uses
+// this struct to keep some global states during a scheduling cycle; note that these
+// state are only accessible to the scheduler itself, not to plugins.
+//
+// It uses a sync.Map for concurrency-safe storage.
+type CycleState struct {
+	// store is a concurrency-safe store (a map).
+	store sync.Map
+}
+
+// Read retrieves a value from CycleState by a key.
+func (c *CycleState) Read(key StateKey) (StateValue, error) {
+	if v, ok := c.store.Load(key); ok {
+		return v, nil
+	}
+	return nil, fmt.Errorf("key %s is not found", key)
+}
+
+// Write stores a value in CycleState under a key.
+func (c *CycleState) Write(key StateKey, val StateValue) {
+	c.store.Store(key, val)
+}
+
+// Delete deletes a key from CycleState.
+func (c *CycleState) Delete(key StateKey) {
+	c.store.Delete(key)
+}
+
+// NewCycleState creates a CycleState.
+func NewCycleState() *CycleState {
+	return &CycleState{}
+}

pkg/scheduler/framework/cyclestate_test.go

@@ -0,0 +1,23 @@
+/*
+Copyright (c) Microsoft Corporation.
+Licensed under the MIT license.
+*/
+
+package framework
+
+import "testing"
+
+// TestCycleStateBasicOps tests the basic ops (Read, Write, and Delete) of a CycleState.
+func TestCycleStateBasicOps(t *testing.T) {
+	cs := NewCycleState()
+
+	k, v := "key", "value"
+	cs.Write(StateKey(k), StateValue(v))
+	if out, err := cs.Read("key"); out != "value" || err != nil {
+		t.Fatalf("Read(%v) = %v, %v, want %v, nil", k, out, err, v)
+	}
+	cs.Delete(StateKey(k))
+	if out, err := cs.Read("key"); out != nil || err == nil {
+		t.Fatalf("Read(%v) = %v, %v, want nil, not found error", k, out, err)
+	}
+}