feat: add --write-output flag to helmfile fetch for air-gapped environments (#2572)

* feat: add --write-output flag to helmfile fetch for air-gapped environments

Add --write-output flag to helmfile fetch that outputs a modified
helmfile.yaml with chart references updated to point to downloaded
local chart paths. Combined with --output-dir, this enables preparing
all charts for deployment in air-gapped environments.

Usage:
  helmfile fetch --output-dir ./charts --write-output > helmfile-airgapped.yaml

Fixes #2571

Signed-off-by: yxxhero <yxxhero@users.noreply.github.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

* fix: update fetch-write-output integration test grep to match YAML list item chart field

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* fix: send status messages to stderr and enforce sequential processing in --write-output mode

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* fix: restore SequentialHelmfiles after Fetch and use %s for YAML string formatting

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* test: add test for SequentialHelmfiles restore after Fetch with --write-output

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* fix: disable live output on --write-output and fix shell quoting/portability in integration test

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* fix: use unquoted ${helmfile} variable to allow word splitting for EXTRA_HELMFILE_FLAGS

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* fix: restore helm.EnableLiveOutput after Fetch --write-output via defer

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* test: strengthen enableLiveOutput restore assertion with non-trivial initial value

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* feat: restrict --write-output to a single helmfile state file with clear error

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* refactor: apply code review suggestions for variable and test naming

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* fix: replace naked return with explicit return ok, errs to fix nakedret lint error

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* fix: buffer YAML output and update --write-output flag description

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* fix: shorten --write-output flag description, move detail to Long help

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---------

Signed-off-by: yxxhero <yxxhero@users.noreply.github.com>
Signed-off-by: yxxhero <aiopsclub@163.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

cmd/fetch.go

@@ -15,6 +15,14 @@ func NewFetchCmd(globalCfg *config.GlobalImpl) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "fetch",
 		Short: "Fetch charts from state file",
+		Long: `Fetch downloads all charts referenced in the Helmfile state.
+
+Useful for air-gapped environments: download charts with --output-dir and --write-output,
+then transfer the output directory and the generated helmfile.yaml to the air-gapped environment.
+
+The --write-output flag requires a single helmfile state file specified with -f.
+It fails if the input resolves to multiple state files (e.g. a directory or a helmfile
+with nested helmfiles: entries).`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			fetchImpl := config.NewFetchImpl(globalCfg, fetchOptions)
 			err := config.NewCLIConfigImpl(fetchImpl.GlobalImpl)
@@ -35,6 +43,7 @@ func NewFetchCmd(globalCfg *config.GlobalImpl) *cobra.Command {
 	f.IntVar(&fetchOptions.Concurrency, "concurrency", 0, "maximum number of concurrent helm processes to run, 0 is unlimited")
 	f.StringVar(&fetchOptions.OutputDir, "output-dir", "", "directory to store charts (default: temporary directory which is deleted when the command terminates)")
 	f.StringVar(&fetchOptions.OutputDirTemplate, "output-dir-template", state.DefaultFetchOutputDirTemplate, "go text template for generating the output directory. Available fields: {{ .OutputDir }}, {{ .ChartName }}, {{ .Release.* }}, {{ .Environment.Name }}, {{ .Environment.KubeContext }}, {{ .Environment.Values.* }}")
+	f.BoolVar(&fetchOptions.WriteOutput, "write-output", false, "write a helmfile.yaml to stdout with chart references updated to local chart paths; requires --output-dir and a single helmfile (use -f)")
 
 	return cmd
 }