Refactor upgrade and check upgrade cmdline options

The main() function is growing too big (142 lines as of the date of this
commit), so this attempts to extract some functionality out of there and
into their own functions to make it easier to reason about them and keep
functions short and concise.

cmd/syncthing/main.go

@@ -305,43 +305,14 @@ func main() {
 		return
 	}
 
-	if doUpgrade || doUpgradeCheck {
-		releasesURL := "https://api.github.com/repos/syncthing/syncthing/releases?per_page=30"
-		if cfg, _, err := loadConfig(locations[locConfigFile]); err == nil {
-			releasesURL = cfg.Options().ReleasesURL
-		}
-		rel, err := upgrade.LatestRelease(releasesURL, Version)
-		if err != nil {
-			l.Fatalln("Upgrade:", err) // exits 1
-		}
-
-		if upgrade.CompareVersions(rel.Tag, Version) <= 0 {
-			l.Infof("No upgrade available (current %q >= latest %q).", Version, rel.Tag)
-			os.Exit(exitNoUpgradeAvailable)
-		}
-
-		l.Infof("Upgrade available (current %q < latest %q)", Version, rel.Tag)
-
-		if doUpgrade {
-			// Use leveldb database locks to protect against concurrent upgrades
-			_, err = db.Open(locations[locDatabase])
-			if err != nil {
-				l.Infoln("Attempting upgrade through running Syncthing...")
-				err = upgradeViaRest()
-				if err != nil {
-					l.Fatalln("Upgrade:", err)
-				}
-				l.Okln("Syncthing upgrading")
-				return
-			}
-
-			err = upgrade.To(rel)
-			if err != nil {
-				l.Fatalln("Upgrade:", err) // exits 1
-			}
-			l.Okf("Upgraded to %q", rel.Tag)
-		}
+	if doUpgradeCheck {
+		checkUpgrade()
+		return
+	}
 
+	if doUpgrade {
+		release := checkUpgrade()
+		performUpgrade(release)
 		return
 	}
 
@@ -429,6 +400,46 @@ func debugFacilities() string {
 	return b.String()
 }
 
+func checkUpgrade() upgrade.Release {
+	releasesURL := "https://api.github.com/repos/syncthing/syncthing/releases?per_page=30"
+	if cfg, _, err := loadConfig(locations[locConfigFile]); err == nil {
+		releasesURL = cfg.Options().ReleasesURL
+	}
+	release, err := upgrade.LatestRelease(releasesURL, Version)
+	if err != nil {
+		l.Fatalln("Upgrade:", err)
+	}
+
+	if upgrade.CompareVersions(release.Tag, Version) <= 0 {
+		noUpgradeMessage := "No upgrade available (current %q >= latest %q)."
+		l.Infof(noUpgradeMessage, Version, release.Tag)
+		os.Exit(exitNoUpgradeAvailable)
+	}
+
+	l.Infof("Upgrade available (current %q < latest %q)", Version, release.Tag)
+	return release
+}
+
+func performUpgrade(release upgrade.Release) {
+	// Use leveldb database locks to protect against concurrent upgrades
+	_, err := db.Open(locations[locDatabase])
+	if err == nil {
+		err = upgrade.To(release)
+		if err != nil {
+			l.Fatalln("Upgrade:", err)
+		}
+		l.Okf("Upgraded to %q", release.Tag)
+	} else {
+		l.Infoln("Attempting upgrade through running Syncthing...")
+		err = upgradeViaRest()
+		if err != nil {
+			l.Fatalln("Upgrade:", err)
+		}
+		l.Okln("Syncthing upgrading")
+		os.Exit(exitUpgrading)
+	}
+}
+
 func upgradeViaRest() error {
 	cfg, err := config.Load(locations[locConfigFile], protocol.LocalDeviceID)
 	if err != nil {