add support for increment/decrement values in fly scale count (#3891)

* add support for increment/decrement values in `fly scale count`
Use `fly scale count +n` or `fly scale count -- -n` to
increment or decrement count relative to the current count.

internal/command/scale/count.go

@@ -93,30 +93,44 @@ func runScaleCount(ctx context.Context) error {
 	return runMachinesScaleCount(ctx, appName, appConfig, groups, maxPerRegion)
 }
 
-func parseGroupCounts(args []string, defaultGroupName string) (map[string]int, error) {
-	groups := make(map[string]int)
+type groupCount struct{ absolute, relative int }
+type groupCounts map[string]groupCount
+
+func parseGroupCounts(args []string, defaultGroupName string) (groupCounts, error) {
+	groups := make(groupCounts)
+	apply := func(group string, countStr string) error {
+		var count groupCount
+		countNum, err := strconv.Atoi(countStr)
+		if err != nil {
+			return err
+		}
+		if strings.HasPrefix(countStr, "+") || strings.HasPrefix(countStr, "-") {
+			if countNum == 0 {
+				return fmt.Errorf("invalid relative scale count: %v", countStr)
+			}
+			count.relative = countNum
+		} else {
+			count.absolute = countNum
+		}
+		groups[group] = count
+		return nil
+	}
 
 	// single numeric arg: fly scale count 3
 	if len(args) == 1 {
-		count, err := strconv.Atoi(args[0])
-		if err == nil {
-			groups[defaultGroupName] = count
-		}
+		_ = apply(defaultGroupName, args[0]) // fall-through on error
 	}
 
-	// group labels: fly scale web=X worker=Y
+	// group labels: fly scale count web=X worker=Y
 	if len(groups) < 1 {
 		for _, arg := range args {
 			parts := strings.Split(arg, "=")
 			if len(parts) != 2 {
 				return nil, fmt.Errorf("'%s' is not a valid process=count option", arg)
 			}
-			count, err := strconv.Atoi(parts[1])
-			if err != nil {
+			if err := apply(parts[0], parts[1]); err != nil {
 				return nil, err
 			}
-
-			groups[parts[0]] = count
 		}
 	}
 

internal/command/scale/count_machines.go

@@ -20,7 +20,7 @@ import (
 	"github.com/superfly/flyctl/iostreams"
 )
 
-func runMachinesScaleCount(ctx context.Context, appName string, appConfig *appconfig.Config, expectedGroupCounts map[string]int, maxPerRegion int) error {
+func runMachinesScaleCount(ctx context.Context, appName string, appConfig *appconfig.Config, expectedGroupCounts groupCounts, maxPerRegion int) error {
 	io := iostreams.FromContext(ctx)
 	flapsClient := flapsutil.ClientFromContext(ctx)
 	ctx = appconfig.WithConfig(ctx, appConfig)
@@ -262,15 +262,22 @@ func (pi *planItem) MachineSize() string {
 	return ""
 }
 
-func computeActions(machines []*fly.Machine, expectedGroupCounts map[string]int, regions []string, maxPerRegion int, defaults *defaultValues) ([]*planItem, error) {
+func computeActions(machines []*fly.Machine, expectedGroupCounts groupCounts, regions []string, maxPerRegion int, defaults *defaultValues) ([]*planItem, error) {
 	actions := make([]*planItem, 0)
 	seenGroups := make(map[string]bool)
 	machineGroups := lo.GroupBy(machines, func(m *fly.Machine) string {
 		return m.ProcessGroup()
 	})
+	expectedCounts := lo.MapValues(expectedGroupCounts, func(c groupCount, group string) int {
+		count := c.absolute
+		if c.relative != 0 {
+			count = len(machineGroups[group]) + c.relative
+		}
+		return max(count, 0)
+	})
 
 	for groupName, groupMachines := range machineGroups {
-		expected, ok := expectedGroupCounts[groupName]
+		expected, ok := expectedCounts[groupName]
 		// Ignore the group if it is not expected to change
 		if !ok {
 			continue
@@ -309,7 +316,7 @@ func computeActions(machines []*fly.Machine, expectedGroupCounts map[string]int,
 	}
 
 	// Fill in the groups without existing machines
-	for groupName, expected := range expectedGroupCounts {
+	for groupName, expected := range expectedCounts {
 		if seenGroups[groupName] {
 			continue
 		}