[MM-63556] mmctl: Add compliance export download cmd (#30576)

* add mmctl compliance export download command and tests

- Introduced `ComplianceExportDownloadCmd` to facilitate downloading compliance export files.
- Implemented the `DownloadComplianceExport` method in the Client interface for handling file downloads.
- Added unit tests for the download command, covering successful downloads, error handling for non-existent jobs, and retries on failure.
- Included end-to-end tests to validate the command's functionality.
- Updated documentation to include usage examples and options for the new command.

* don't know why this was left out

* PR comments

* adjust test for new retry logic

* refactored download fn for compliance_export and export

* fix test due to fixed logic

* docs

server/cmd/mmctl/commands/compliance_export.go

@@ -6,8 +6,10 @@ package commands
 import (
 	"context"
 	"fmt"
+	"os"
 
 	"github.com/mattermost/mattermost/server/v8/cmd/mmctl/client"
+	"github.com/mattermost/mattermost/server/v8/cmd/mmctl/printer"
 	"github.com/spf13/cobra"
 )
 
@@ -40,15 +42,26 @@ var ComplianceExportCancelCmd = &cobra.Command{
 	RunE:    withClient(complianceExportCancelCmdF),
 }
 
+var ComplianceExportDownloadCmd = &cobra.Command{
+	Use:     "download [complianceExportJobID] [output filepath (optional)]",
+	Example: "  compliance_export download o98rj3ur83dp5dppfyk5yk6osy",
+	Short:   "Download compliance export file",
+	Args:    cobra.MinimumNArgs(1),
+	RunE:    withClient(complianceExportDownloadCmdF),
+}
+
 func init() {
 	ComplianceExportListCmd.Flags().Int("page", 0, "Page number to fetch for the list of compliance export jobs")
 	ComplianceExportListCmd.Flags().Int("per-page", DefaultPageSize, "Number of compliance export jobs to be fetched")
 	ComplianceExportListCmd.Flags().Bool("all", false, "Fetch all compliance export jobs. --page flag will be ignored if provided")
 
+	ComplianceExportDownloadCmd.Flags().Int("num-retries", 5, "Number of retries if the download fails")
+
 	ComplianceExportCmd.AddCommand(
 		ComplianceExportListCmd,
 		ComplianceExportShowCmd,
 		ComplianceExportCancelCmd,
+		ComplianceExportDownloadCmd,
 	)
 	RootCmd.AddCommand(ComplianceExportCmd)
 }
@@ -75,3 +88,41 @@ func complianceExportCancelCmdF(c client.Client, command *cobra.Command, args []
 
 	return nil
 }
+
+func complianceExportDownloadCmdF(c client.Client, command *cobra.Command, args []string) error {
+	jobID := args[0]
+	var path string
+	if len(args) > 1 {
+		path = args[1]
+	} else {
+		path = jobID + ".zip"
+	}
+
+	retries, _ := command.Flags().GetInt("num-retries")
+
+	downloadFn := func(outFile *os.File) (string, error) {
+		return c.DownloadComplianceExport(context.TODO(), jobID, outFile)
+	}
+
+	suggestedFilename, err := downloadFile(path, downloadFn, retries, "compliance export")
+	if err != nil {
+		return err
+	}
+
+	// If we didn't provide a path and got a suggested filename, rename the file
+	if len(args) == 1 && suggestedFilename != "" && suggestedFilename != path {
+		// If the suggested name already exists, don't overwrite
+		if _, err := os.Stat(suggestedFilename); err == nil {
+			printer.PrintWarning(fmt.Sprintf("File with the server's suggested name %q already exists, keeping %q", suggestedFilename, path))
+		} else {
+			if err := os.Rename(path, suggestedFilename); err != nil {
+				printer.PrintWarning(fmt.Sprintf("Could not rename file to the server's suggested name %q: %v", suggestedFilename, err))
+			} else {
+				path = suggestedFilename
+			}
+		}
+	}
+
+	printer.Print(fmt.Sprintf("Compliance export file downloaded to %q", path))
+	return nil
+}