Disable grab download resume

This is to mitigate cases like #4296

By default grab tries to resume the download if the file name determined from either the url or from content-type headers already exists. This makes things go side ways, if the existing file is smaller than the new one, the old content would still be there and only the "extra" new bytes would get written. I.e. the download would be "resumed". 🤦

This is probably not a fix for the root cause in #4296 as the only way I've been able to make grab fail with `bad content length` is by crafting a custom http server that maliciously borks `Content-Length` header.

This is a minimal possible fix that we can easily backport. @twz123 is already working on bigger refactoring of autopilot download functionality that gets rid of grab. Grab seems to bring more (bad) surprises than real benefits. In the end, we just download files and we should pretty much always just replace them. No need for full library dependecy for that.

Signed-off-by: Jussi Nummelin <jnummelin@mirantis.com>

pkg/autopilot/controller/signal/common/download.go

@@ -32,6 +32,8 @@ type DownloadManifest struct {
 	apdl.Config
 
 	SuccessState string
+	// A hook that will be called after the download itself is done succesfully
+	AfterTransferSuccess func() error
 }
 
 type DownloadManifestBuilder interface {
@@ -90,7 +92,12 @@ func (r *downloadController) Reconcile(ctx context.Context, req cr.Request) (cr.
 
 	} else {
 		logger.Infof("Download of '%s' successful", manifest.URL)
-
+		// When download is successful, run the post-download hook
+		if manifest.AfterTransferSuccess != nil {
+			if err := manifest.AfterTransferSuccess(); err != nil {
+				return cr.Result{}, fmt.Errorf("failed to run post-download hook: %w", err)
+			}
+		}
 		// When the download is complete move the status to the success state
 		signalData.Status = apsigv2.NewStatus(manifest.SuccessState)
 	}

pkg/autopilot/controller/signal/k0s/download.go

@@ -16,6 +16,8 @@ package k0s
 
 import (
 	"crypto/sha256"
+	"os"
+	"path/filepath"
 
 	apcomm "github.com/k0sproject/k0s/pkg/autopilot/common"
 	apdel "github.com/k0sproject/k0s/pkg/autopilot/controller/delegate"
@@ -88,6 +90,13 @@ func (b downloadManifestBuilderK0s) Build(signalNode crcli.Object, signalData ap
 			ExpectedHash: signalData.Command.K0sUpdate.Sha256,
 			Hasher:       sha256.New(),
 			DownloadDir:  b.k0sBinaryDir,
+			Filename:     filepath.Join(b.k0sBinaryDir, "k0s.tmp"),
+		},
+		// After the download is done, we need to rename the file to the correct name
+		AfterTransferSuccess: func() error {
+			src := filepath.Join(b.k0sBinaryDir, "k0s.tmp")
+			dst := filepath.Join(b.k0sBinaryDir, "k0s")
+			return os.Rename(src, dst)
 		},
 		SuccessState: Cordoning,
 	}

pkg/autopilot/download/downloader.go

@@ -34,6 +34,7 @@ type Config struct {
 	ExpectedHash string
 	Hasher       hash.Hash
 	DownloadDir  string
+	Filename     string
 }
 
 type downloader struct {
@@ -72,6 +73,15 @@ func (d *downloader) Download(ctx context.Context) error {
 		dlreq.SetChecksum(d.config.Hasher, expectedHash, true)
 	}
 
+	// We're never really resuming downloads, so disable this feature.
+	// This also allows to re-download the file if it's already present.
+	dlreq.NoResume = true
+
+	if d.config.Filename != "" {
+		d.logger.Infof("Setting filename to %s", d.config.Filename)
+		dlreq.Filename = d.config.Filename
+	}
+
 	client := grab.NewClient()
 	// Set user agent to mitigate 403 errors from GitHub
 	// See https://github.com/cavaliergopher/grab/issues/104