fix(shell): compare fractional evacuation destination ratios (#11752)

weed/shell/command_volume_server_evacuate.go

@@ -1,6 +1,7 @@
 package shell
 
 import (
+	"cmp"
 	"flag"
 	"fmt"
 	"io"
@@ -273,7 +274,7 @@ func moveAwayOneNormalVolume(commandEnv *CommandEnv, volumeReplicas map[uint32][
 	}
 	// most empty one is in the front
 	slices.SortFunc(otherNodes, func(a, b *Node) int {
-		return int(a.localVolumeRatio(maxVolumeCountFn) - b.localVolumeRatio(maxVolumeCountFn))
+		return cmp.Compare(a.localVolumeRatio(maxVolumeCountFn), b.localVolumeRatio(maxVolumeCountFn))
 	})
 	for i := 0; i < len(otherNodes); i++ {
 		emptyNode := otherNodes[i]

weed/shell/command_volume_server_evacuate_test.go

@@ -5,6 +5,22 @@ import (
 	"testing"
 )
 
+func TestEvacuatePrefersLowerVolumeRatio(t *testing.T) {
+	volumes := mkByteVolumes(1, 1, 1)
+	source := makeByteNode("source", 10, 0, 0, volumes)
+	fuller := makeByteNode("fuller", 10, 0, 0, mkByteVolumes(10, 4, 1))
+	emptier := makeByteNode("emptier", 10, 0, 0, mkByteVolumes(20, 2, 1))
+
+	// Put the 40%-full server first: truncating 0.2 - 0.4 to int makes the ratios tie.
+	moved, err := moveAwayOneNormalVolume(nil, nil, volumes[0], source, []*Node{fuller, emptier}, false)
+	if err != nil || !moved {
+		t.Fatalf("evacuate: moved=%v, err=%v", moved, err)
+	}
+	if _, found := emptier.selectedVolumes[volumes[0].Id]; !found {
+		t.Fatal("expected evacuation to choose the server with lower occupancy")
+	}
+}
+
 func TestVolumeServerEvacuate(t *testing.T) {
 	targetServer := ""
 	volumeRack := "rack1"