feat: download-genesis command (backport #2791) (#2795)

This is an automatic backport of pull request #2791 done by
[Mergify](https://mergify.com).
Cherry-pick of c34a93d has failed:
```
On branch mergify/bp/v1.x/pr-2791
Your branch is up to date with 'origin/v1.x'.

You are currently cherry-picking commit c34a93d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   cmd/celestia-appd/cmd/download-genesis.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cmd/celestia-appd/cmd/root.go

```


To fix up this pull request, you can check it out locally. See
documentation:
https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---


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</details>

---------

Co-authored-by: Rootul P <rootulp@gmail.com>

cmd/celestia-appd/cmd/download-genesis.go

@@ -0,0 +1,128 @@
+package cmd
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+
+	"github.com/cosmos/cosmos-sdk/server"
+	"github.com/spf13/cobra"
+)
+
+var chainIDToSha256 = map[string]string{
+	"celestia":   "9727aac9bbfb021ce7fc695a92f901986421283a891b89e0af97bc9fad187793",
+	"mocha-4":    "0846b99099271b240b638a94e17a6301423b5e4047f6558df543d6e91db7e575",
+	"arabica-10": "fad0a187669f7a2c11bb07f9dc27140d66d2448b7193e186312713856f28e3e1",
+}
+
+func downloadGenesisCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "download-genesis [chain-id]",
+		Short: "Download genesis file from https://github.com/celestiaorg/networks",
+		Long: "Download genesis file from https://github.com/celestiaorg/networks.\n" +
+			"The first argument should be a known chain-id. Ex. celestia, mocha-4, or arabica-10.\n" +
+			"If no argument is provided, defaults to celestia.\n",
+		Args: cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			chainID := getChainIDOrDefault(args)
+			if !isKnownChainID(chainID) {
+				return fmt.Errorf("unknown chain-id: %s. Must be: celestia, mocha-4, or arabica-10", chainID)
+			}
+			outputFile := server.GetServerContextFromCmd(cmd).Config.GenesisFile()
+			fmt.Printf("Downloading genesis file for %s to %s\n", chainID, outputFile)
+
+			url := fmt.Sprintf("https://raw.githubusercontent.com/celestiaorg/networks/master/%s/genesis.json", chainID)
+			if err := downloadFile(outputFile, url); err != nil {
+				return fmt.Errorf("error downloading / persisting the genesis file: %s", err)
+			}
+			fmt.Printf("Downloaded genesis file for %s to %s\n", chainID, outputFile)
+
+			// Compute SHA-256 hash of the downloaded file
+			hash, err := computeSha256(outputFile)
+			if err != nil {
+				return fmt.Errorf("error computing sha256 hash: %s", err)
+			}
+
+			// Compare computed hash against known hash
+			knownHash, ok := chainIDToSha256[chainID]
+			if !ok {
+				return fmt.Errorf("unknown chain-id: %s", chainID)
+			}
+
+			if hash != knownHash {
+				return fmt.Errorf("sha256 hash mismatch: got %s, expected %s", hash, knownHash)
+			}
+
+			fmt.Printf("SHA-256 hash verified for %s\n", chainID)
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+// getChainIDOrDefault returns the chainID from the command line arguments. If
+// none is provided, defaults to celestia (mainnet).
+func getChainIDOrDefault(args []string) string {
+	if len(args) == 1 {
+		return args[0]
+	}
+	return "celestia"
+}
+
+// isKnownChainID returns true if the chainID is known.
+func isKnownChainID(chainID string) bool {
+	knownChainIDs := []string{
+		"arabica-10", // testnet
+		"mocha-4",    // testnet
+		"celestia",   // mainnet
+	}
+	return contains(knownChainIDs, chainID)
+}
+
+// contains checks if a string is present in a slice.
+func contains(slice []string, s string) bool {
+	for _, v := range slice {
+		if v == s {
+			return true
+		}
+	}
+	return false
+}
+
+// downloadFile will download a URL to a local file.
+func downloadFile(filepath string, url string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	out, err := os.Create(filepath)
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	_, err = io.Copy(out, resp.Body)
+	return err
+}
+
+// computeSha256 computes the SHA-256 hash of a file.
+func computeSha256(filepath string) (string, error) {
+	f, err := os.Open(filepath)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	hasher := sha256.New()
+	if _, err := io.Copy(hasher, f); err != nil {
+		return "", err
+	}
+
+	return hex.EncodeToString(hasher.Sum(nil)), nil
+}

cmd/celestia-appd/cmd/root.go

@@ -128,6 +128,7 @@ func initRootCmd(rootCmd *cobra.Command, encodingConfig encoding.Config) {
 		debugCmd,
 		config.Cmd(),
 		commands.CompactGoLevelDBCmd,
+		downloadGenesisCommand(),
 	)
 
 	server.AddCommands(rootCmd, app.DefaultNodeHome, NewAppServer, createAppAndExport, addModuleInitFlags)