Making IP allocation method configurable

This commit allows IP allocation method to be
changed based on the requirement. For eg if swarm
manager wants IP allocation to be serialized it can
set the serializable option. If docker daemon wants
IP allocation to be first available then that can be
done by set the serial option to false/ignoring setting
an option

Signed-off-by: Abhinandan Prativadi <abhi@docker.com>

bitseq/sequence.go

@@ -199,30 +199,30 @@ func (h *Handle) getCopy() *Handle {
 }
 
 // SetAnyInRange atomically sets the first unset bit in the specified range in the sequence and returns the corresponding ordinal
-func (h *Handle) SetAnyInRange(start, end uint64) (uint64, error) {
+func (h *Handle) SetAnyInRange(start, end uint64, serial bool) (uint64, error) {
 	if end < start || end >= h.bits {
 		return invalidPos, fmt.Errorf("invalid bit range [%d, %d]", start, end)
 	}
 	if h.Unselected() == 0 {
 		return invalidPos, ErrNoBitAvailable
 	}
-	return h.set(0, start, end, true, false)
+	return h.set(0, start, end, true, false, serial)
 }
 
 // SetAny atomically sets the first unset bit in the sequence and returns the corresponding ordinal
-func (h *Handle) SetAny() (uint64, error) {
+func (h *Handle) SetAny(serial bool) (uint64, error) {
 	if h.Unselected() == 0 {
 		return invalidPos, ErrNoBitAvailable
 	}
-	return h.set(0, 0, h.bits-1, true, false)
+	return h.set(0, 0, h.bits-1, true, false, serial)
 }
 
 // Set atomically sets the corresponding bit in the sequence
 func (h *Handle) Set(ordinal uint64) error {
 	if err := h.validateOrdinal(ordinal); err != nil {
 		return err
 	}
-	_, err := h.set(ordinal, 0, 0, false, false)
+	_, err := h.set(ordinal, 0, 0, false, false, false)
 	return err
 }
 
@@ -231,7 +231,7 @@ func (h *Handle) Unset(ordinal uint64) error {
 	if err := h.validateOrdinal(ordinal); err != nil {
 		return err
 	}
-	_, err := h.set(ordinal, 0, 0, false, true)
+	_, err := h.set(ordinal, 0, 0, false, true, false)
 	return err
 }
 
@@ -300,7 +300,7 @@ func (h *Handle) CheckConsistency() error {
 }
 
 // set/reset the bit
-func (h *Handle) set(ordinal, start, end uint64, any bool, release bool) (uint64, error) {
+func (h *Handle) set(ordinal, start, end uint64, any bool, release bool, serial bool) (uint64, error) {
 	var (
 		bitPos  uint64
 		bytePos uint64
@@ -310,6 +310,7 @@ func (h *Handle) set(ordinal, start, end uint64, any bool, release bool) (uint64
 
 	for {
 		var store datastore.DataStore
+		curr := uint64(0)
 		h.Lock()
 		store = h.store
 		h.Unlock()
@@ -320,12 +321,15 @@ func (h *Handle) set(ordinal, start, end uint64, any bool, release bool) (uint64
 		}
 
 		h.Lock()
+		if serial {
+			curr = h.curr
+		}
 		// Get position if available
 		if release {
 			bytePos, bitPos = ordinalToPos(ordinal)
 		} else {
 			if any {
-				bytePos, bitPos, err = getAvailableFromCurrent(h.head, start, h.curr, end)
+				bytePos, bitPos, err = getAvailableFromCurrent(h.head, start, curr, end)
 				ret = posToOrdinal(bytePos, bitPos)
 				if err == nil {
 					h.curr = ret + 1
@@ -517,9 +521,9 @@ func getFirstAvailable(head *sequence, start uint64) (uint64, uint64, error) {
 	return invalidPos, invalidPos, ErrNoBitAvailable
 }
 
-//getAvailableFromCurrent will look for available ordinal from the current ordinal.
+// getAvailableFromCurrent will look for available ordinal from the current ordinal.
 // If none found then it will loop back to the start to check of the available bit.
-//This can be further optimized to check from start till curr in case of a rollover
+// This can be further optimized to check from start till curr in case of a rollover
 func getAvailableFromCurrent(head *sequence, start, curr, end uint64) (uint64, uint64, error) {
 	var bytePos, bitPos uint64
 	if curr != 0 && curr > start {