Replace custom error with protobuf message

Previously, a custom error was returned when attempting to upgrade a platform that was already at the latest available
version. There is dedicated code for handling this specific error.

Now that the function has been changed to return a status.Status instead of error, the previous check for the return
being this error is no longer possible. The capability is restored by replacing the error with a protocol buffer message.

Author: per1234

Diff for: cli/core/upgrade.go

@@ -94,7 +94,7 @@ func runUpgradeCommand(cmd *cobra.Command, args []string) {
 		}
 
 		_, err := core.PlatformUpgrade(context.Background(), r, output.ProgressBar(), output.TaskProgress())
-		if err.Message() == core.ErrAlreadyLatest.Error() {
+		if d := err.Details(); len(d) > 0 && d[0].(*rpc.AlreadyAtLatestVersionError) != nil {
 			feedback.Printf("Platform %s is already at the latest version", platformRef)
 		} else if err != nil {
 			feedback.Errorf("Error during upgrade: %v", err)

Diff for: commands/core/upgrade.go

@@ -17,8 +17,6 @@ package core
 
 import (
 	"context"
-	"errors"
-	"fmt"
 
 	"github.com/arduino/arduino-cli/arduino/cores/packagemanager"
 	"github.com/arduino/arduino-cli/commands"
@@ -27,12 +25,6 @@ import (
 	"google.golang.org/grpc/status"
 )
 
-var (
-	// ErrAlreadyLatest is returned when an upgrade is not possible because
-	// already at latest version.
-	ErrAlreadyLatest = errors.New("platform already at latest version")
-)
-
 // PlatformUpgrade FIXMEDOC
 func PlatformUpgrade(ctx context.Context, req *rpc.PlatformUpgradeRequest,
 	downloadCB commands.DownloadProgressCB, taskCB commands.TaskProgressCB) (*rpc.PlatformUpgradeResponse, *status.Status) {
@@ -48,7 +40,7 @@ func PlatformUpgrade(ctx context.Context, req *rpc.PlatformUpgradeRequest,
 		PlatformArchitecture: req.Architecture,
 	}
 	if err := upgradePlatform(pm, ref, downloadCB, taskCB, req.GetSkipPostInstall()); err != nil {
-		return nil, status.Convert(err)
+		return nil, err
 	}
 
 	status := commands.Init(&rpc.InitRequest{Instance: req.Instance}, nil)
@@ -61,33 +53,37 @@ func PlatformUpgrade(ctx context.Context, req *rpc.PlatformUpgradeRequest,
 
 func upgradePlatform(pm *packagemanager.PackageManager, platformRef *packagemanager.PlatformReference,
 	downloadCB commands.DownloadProgressCB, taskCB commands.TaskProgressCB,
-	skipPostInstall bool) error {
+	skipPostInstall bool) *status.Status {
 	if platformRef.PlatformVersion != nil {
-		return fmt.Errorf("upgrade doesn't accept parameters with version")
+		return status.New(codes.InvalidArgument, "upgrade doesn't accept parameters with version")
 	}
 
 	// Search the latest version for all specified platforms
 	platform := pm.FindPlatform(platformRef)
 	if platform == nil {
-		return fmt.Errorf("platform %s not found", platformRef)
+		return status.Newf(codes.InvalidArgument, "platform %s not found", platformRef)
 	}
 	installed := pm.GetInstalledPlatformRelease(platform)
 	if installed == nil {
-		return fmt.Errorf("platform %s is not installed", platformRef)
+		return status.Newf(codes.InvalidArgument, "platform %s is not installed", platformRef)
 	}
 	latest := platform.GetLatestRelease()
 	if !latest.Version.GreaterThan(installed.Version) {
-		return ErrAlreadyLatest
+		status, e := status.New(codes.AlreadyExists, "platform already at latest version").WithDetails(&rpc.AlreadyAtLatestVersionError{})
+		if e != nil { // should never happen
+			panic(e)
+		}
+		return status
 	}
 	platformRef.PlatformVersion = latest.Version
 
 	platformRelease, tools, err := pm.FindPlatformReleaseDependencies(platformRef)
 	if err != nil {
-		return fmt.Errorf("platform %s is not installed", platformRef)
+		return status.Newf(codes.FailedPrecondition, "platform %s is not installed", platformRef)
 	}
 	err = installPlatform(pm, platformRelease, tools, downloadCB, taskCB, skipPostInstall)
 	if err != nil {
-		return err
+		return status.Convert(err)
 	}
 
 	return nil