Backport of easltic-agent PR #3682 (#40704)

This is a manual backport of elastic/elastic-agent#3682
Only the essentials have been backported ignoring features like parsed
agent version or download retries which are part of other PRs that have
not been backported onto elastic-agent 7.17

x-pack/elastic-agent/pkg/agent/application/upgrade/step_download.go

@@ -24,22 +24,53 @@ const (
 	defaultUpgradeFallbackPGP = "https://artifacts.elastic.co/GPG-KEY-elastic-agent"
 )
 
+type downloaderFactory func(string, *logger.Logger, *artifact.Config) (download.Downloader, error)
+
 func (u *Upgrader) downloadArtifact(ctx context.Context, version, sourceURI string, skipVerifyOverride bool, pgpBytes ...string) (string, error) {
 	// do not update source config
 	settings := *u.settings
+
+	var factory downloaderFactory
+	var verifier download.Verifier
+	var err error
 	if sourceURI != "" {
 		if strings.HasPrefix(sourceURI, "file://") {
 			// update the DropPath so the fs.Downloader can download from this
 			// path instead of looking into the installed downloads directory
 			settings.DropPath = strings.TrimPrefix(sourceURI, "file://")
+
+			// set specific downloader, local file just uses the fs.NewDownloader
+			// no fallback is allowed because it was requested that this specific source be used
+			factory = func(version string, l *logger.Logger, config *artifact.Config) (download.Downloader, error) {
+				return fs.NewDownloader(config), nil
+			}
+
+			// set specific verifier, local file verifies locally only
+			allowEmptyPgp, pgp := release.PGP()
+			verifier, err = fs.NewVerifier(&settings, allowEmptyPgp, pgp)
+			if err != nil {
+				return "", errors.New(err, "initiating verifier")
+			}
+			// log that a local upgrade artifact is being used
+			u.log.Infow("Using local upgrade artifact", "version", version,
+				"drop_path", settings.DropPath,
+				"target_path", settings.TargetDirectory, "install_path", settings.InstallPath)
 		} else {
 			settings.SourceURI = sourceURI
 		}
 	}
 
+	if factory == nil {
+		// set the factory to the newDownloader factory
+		factory = newDownloader
+		u.log.Infow("Downloading upgrade artifact", "version", version,
+			"source_uri", settings.SourceURI, "drop_path", settings.DropPath,
+			"target_path", settings.TargetDirectory, "install_path", settings.InstallPath)
+	}
+
 	pgpBytes = appendFallbackPGP(pgpBytes)
 
-	fetcher, err := newDownloader(version, u.log, &settings)
+	fetcher, err := factory(version, u.log, &settings)
 	if err != nil {
 		return "", errors.New(err, "initiating fetcher")
 	}
@@ -53,9 +84,11 @@ func (u *Upgrader) downloadArtifact(ctx context.Context, version, sourceURI stri
 		return path, nil
 	}
 
-	verifier, err := newVerifier(version, u.log, &settings)
-	if err != nil {
-		return "", errors.New(err, "initiating verifier")
+	if verifier == nil {
+		verifier, err = newVerifier(version, u.log, &settings)
+		if err != nil {
+			return "", errors.New(err, "initiating verifier")
+		}
 	}
 
 	matches, err := verifier.Verify(agentSpec, version, true, pgpBytes...)