go: fix golangci-lint issues across cmd/ and mantle/

I ran this through Anthropic/Claude Opus 4.6 and this is what it came up with:

```
Fix 311 golangci-lint issues across 91 files:

- errcheck (166 issues): Add explicit error discards for unchecked return
  values from Close(), os.Setenv(), os.Remove(), os.RemoveAll(),
  os.Unsetenv(), fmt.Fprintf/Fprintln(), w.Flush(), and
  systemdjournal.Print(). For deferred Close() calls, wrap in anonymous
  functions with _ = assignment. For non-deferred calls, add _ = prefix.

- ST1005 (142 issues): Lowercase capitalized error strings per Go
  convention (e.g. "Cannot use" -> "cannot use", "Failed to" ->
  "failed to"). Remove trailing punctuation (periods) and newlines
  from error format strings.

- SA4011 (3 issues): Remove ineffective break statements in switch/
  select cases in mantle/platform/qemu.go.
```

Written-By: <anthropic/claude-opus-4.6>

Author: dustymabe

cmd/coreos-assembler.go

@@ -119,7 +119,7 @@ func initializeGlobalState(argv []string) error {
 	// Set PYTHONUNBUFFERED=1 so that we get unbuffered output. We should
 	// be able to do this on the shebang lines but env doesn't support args
 	// right now. In Fedora we should be able to use the `env -S` option.
-	os.Setenv("PYTHONUNBUFFERED", "1")
+	_ = os.Setenv("PYTHONUNBUFFERED", "1")
 
 	// docker/podman don't run through PAM, but we want this set for the privileged
 	// (non-virtualized) path
@@ -131,7 +131,7 @@ func initializeGlobalState(argv []string) error {
 		} else {
 			user = "cosa"
 		}
-		os.Setenv("USER", user)
+		_ = os.Setenv("USER", user)
 	}
 
 	// https://github.com/containers/libpod/issues/1448
@@ -177,7 +177,7 @@ func initializeGlobalState(argv []string) error {
 		if err != nil {
 			return fmt.Errorf("opening /etc/passwd: %w", err)
 		}
-		defer f.Close()
+		defer func() { _ = f.Close() }()
 		id := os.Getuid()
 		buf := fmt.Sprintf("%s:x:%d:0:%s user:%s:/sbin/nologin\n", user, id, user, home)
 		if _, err = f.WriteString(buf); err != nil {

cmd/remote-session.go

@@ -103,9 +103,9 @@ func envVarIsSet(v string) bool {
 // environment variable error based on the given inputs.
 func envVarError(v string, required bool) error {
 	if required {
-		return fmt.Errorf("The env var %s must be defined and non-empty", v)
+		return fmt.Errorf("the env var %s must be defined and non-empty", v)
 	} else {
-		return fmt.Errorf("The env var %s must not be defined", v)
+		return fmt.Errorf("the env var %s must not be defined", v)
 	}
 }
 
@@ -231,7 +231,7 @@ func runSync(c *cobra.Command, args []string) error {
 		}
 	}
 	if found != 1 {
-		return fmt.Errorf("Must pass in a single arg with `:` prepended")
+		return fmt.Errorf("must pass in a single arg with `:` prepended")
 	}
 	// build command and execute
 	rsyncargs := []string{"-ah", "--no-owner", "--no-group", "--mkpath", "--blocking-io",

mantle/auth/aliyun.go

@@ -51,7 +51,7 @@ func ReadAliyunConfig(path string) (map[string]AliyunProfile, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	var config AliyunConfig
 	if err := json.NewDecoder(f).Decode(&config); err != nil {

mantle/auth/azure.go

@@ -54,7 +54,7 @@ func ReadAzureCredentials(path string) (AzureCredentials, error) {
 	if err != nil {
 		return azCreds, err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	content, err := io.ReadAll(f)
 	if err != nil {

mantle/auth/do.go

@@ -47,7 +47,7 @@ func ReadDOConfig(path string) (map[string]DOProfile, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	var profiles map[string]DOProfile
 	if err := json.NewDecoder(f).Decode(&profiles); err != nil {

mantle/auth/esx.go

@@ -49,7 +49,7 @@ func ReadESXConfig(path string) (map[string]ESXProfile, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	var profiles map[string]ESXProfile
 	if err := json.NewDecoder(f).Decode(&profiles); err != nil {

mantle/cmd/kola/console.go

@@ -81,7 +81,7 @@ func runCheckConsole(cmd *cobra.Command, args []string) error {
 		}
 	}
 	if errorcount > 0 {
-		return errors.New("Errors found on console")
+		return errors.New("errors found on console")
 	}
 	return nil
 }

mantle/cmd/kola/devshell.go

@@ -81,14 +81,14 @@ func runDevShellSSH(ctx context.Context, builder *platform.QemuBuilder, conf *co
 	if err != nil {
 		return err
 	}
-	defer os.RemoveAll(tmpd)
+	defer func() { _ = os.RemoveAll(tmpd) }()
 
 	// Define SSH key
 	agent, err := network.NewSSHAgent(network.NewRetryDialer())
 	if err != nil {
 		return err
 	}
-	defer agent.Close()
+	defer func() { _ = agent.Close() }()
 
 	keys, err := agent.List()
 	if err != nil {
@@ -352,7 +352,7 @@ func (se systemdEventMessage) message() (string, error) {
 	} else if s, ok := se.Message.(string); ok {
 		return s, nil
 	} else {
-		return "", fmt.Errorf("Unhandled systemd json message of type %T", se.Message)
+		return "", fmt.Errorf("unhandled systemd json message of type %T", se.Message)
 	}
 }
 

mantle/cmd/kola/kola.go

@@ -289,7 +289,7 @@ func writeProps() error {
 	if err != nil {
 		return err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	enc := json.NewEncoder(f)
 	enc.SetIndent("", "    ")
@@ -447,12 +447,12 @@ func runList(cmd *cobra.Command, args []string) error {
 	if !listJSON {
 		var w = tabwriter.NewWriter(os.Stdout, 0, 8, 0, '\t', 0)
 
-		fmt.Fprintln(w, "Test Name\tPlatforms\tArchitectures\tDistributions\tTags")
-		fmt.Fprintln(w, "\t")
+		_, _ = fmt.Fprintln(w, "Test Name\tPlatforms\tArchitectures\tDistributions\tTags")
+		_, _ = fmt.Fprintln(w, "\t")
 		for _, item := range newtestlist {
-			fmt.Fprintf(w, "%v\n", item)
+			_, _ = fmt.Fprintf(w, "%v\n", item)
 		}
-		w.Flush()
+		_ = w.Flush()
 	} else {
 		out, err := json.MarshalIndent(newtestlist, "", "\t")
 		if err != nil {
@@ -521,7 +521,7 @@ func runHTTPServer(cmd *cobra.Command, args []string) error {
 
 	http.Handle("/", http.FileServer(http.Dir(directory)))
 
-	fmt.Fprintf(os.Stdout, "Serving HTTP on port: %d\n", httpPort)
+	_, _ = fmt.Fprintf(os.Stdout, "Serving HTTP on port: %d\n", httpPort)
 	return http.ListenAndServe(fmt.Sprintf(":%d", httpPort), nil)
 }
 
@@ -553,7 +553,7 @@ func preRunUpgrade(cmd *cobra.Command, args []string) error {
 
 func runUpgradeCleanup() {
 	if qemuImageDir != "" && qemuImageDirIsTemp {
-		os.RemoveAll(qemuImageDir)
+		_ = os.RemoveAll(qemuImageDir)
 	}
 }
 
@@ -621,7 +621,7 @@ func syncFindParentImageOptions() error {
 			qemuImageDirIsTemp = true
 		}
 		if parentCosaBuild.BuildArtifacts.Qemu == nil {
-			return fmt.Errorf("No QEMU in parent meta.json")
+			return fmt.Errorf("no QEMU in parent meta.json")
 		}
 		qcowURL := parentBaseURL + parentCosaBuild.BuildArtifacts.Qemu.Path
 		qcowLocal := filepath.Join(qemuImageDir, parentCosaBuild.BuildArtifacts.Qemu.Path)

mantle/cmd/kola/options.go

@@ -441,7 +441,7 @@ func syncStreamOptions() error {
 	var artifacts *stream.Arch
 	switch kola.Options.Distribution {
 	case "":
-		return fmt.Errorf("Must specify -b/--distro with --stream")
+		return fmt.Errorf("must specify -b/--distro with --stream")
 	case "fcos":
 		artifacts, err = fcos.FetchCanonicalStreamArtifacts(kola.Options.Stream, kola.Options.CosaBuildArch)
 		if err != nil {
@@ -453,7 +453,7 @@ func syncStreamOptions() error {
 			return errors.Wrapf(err, "failed to fetch stream")
 		}
 	default:
-		return fmt.Errorf("Unhandled stream for distribution %s", kola.Options.Distribution)
+		return fmt.Errorf("unhandled stream for distribution %s", kola.Options.Distribution)
 	}
 
 	release := ""
@@ -472,7 +472,7 @@ func syncStreamOptions() error {
 		kola.GCPOptions.Image = fmt.Sprintf("projects/%s/global/images/%s", imageproject, imagename)
 		extra = fmt.Sprintf("(image project: %s, image name: %s)", imageproject, imagename)
 	default:
-		return fmt.Errorf("Unhandled platform %s for stream", kolaPlatform)
+		return fmt.Errorf("unhandled platform %s for stream", kolaPlatform)
 	}
 
 	fmt.Printf("Resolved distro=%s stream=%s platform=%s arch=%s to release=%s %s\n",