docs: Add comments for improved code readability in main function

This commit enhances the readability of the main function by adding comments to explain each line of code. These comments provide clarity and guidance for beginners, helping them understand the purpose and functionality of each step in setting up the environment for managing Firecracker microVM snapshots.

Signed-off-by: Sagore Sarker <sagoresarker.cse@gmail.com>

Author: sagoresarker

examples/cmd/snapshotting/example_demo.go

@@ -11,6 +11,15 @@
 // express or implied. See the License for the specific language governing
 // permissions and limitations under the License.
 
+// Package main provides a Go program for managing snapshots of Firecracker microVMs.
+// The program utilizes the Firecracker Go SDK for creating and loading snapshots,
+// and it demonstrates how to establish SSH connections to interact with microVMs.
+// Comments are provided to explain each function's purpose and usage.
+
+// In this program, a "snapshot" refers to a point-in-time copy of the state of a Firecracker microVM.
+// Snapshots capture the complete memory and state of the microVM, allowing users to save and restore its exact configuration and execution context.
+// They enable quick deployment and management of microVM instances with pre-defined configurations and states, which is useful for testing, development, and deployment scenarios.
+
 package main
 
 import (
@@ -31,19 +40,21 @@ import (
 	models "github.com/firecracker-microvm/firecracker-go-sdk/client/models"
 )
 
+// Constants for CNI configuration
 const (
 	// Using default cache directory to ensure collision avoidance on IP allocations
-	cniCacheDir = "/var/lib/cni"
-	networkName = "fcnet"
-	ifName      = "veth0"
+	cniCacheDir = "/var/lib/cni" // Default cache directory for IP allocations
+	networkName = "fcnet"        // Name of the network
+	ifName      = "veth0"        // Interface name
 
-	networkMask string = "/24"
-	subnet      string = "10.168.0.0" + networkMask
+	networkMask = "/24" // Subnet mask
+	subnet      = "10.168.0.0" + networkMask
 
-	maxRetries    int           = 10
-	backoffTimeMs time.Duration = 500
+	maxRetries    = 10                     // Maximum number of retries for SSH connection
+	backoffTimeMs = 500 * time.Millisecond // Backoff time for retries
 )
 
+// writeCNIConfWithHostLocalSubnet writes CNI configuration to a file with a host-local subnet
 func writeCNIConfWithHostLocalSubnet(path, networkName, subnet string) error {
 	return ioutil.WriteFile(path, []byte(fmt.Sprintf(`{
 		"cniVersion": "0.3.1",
@@ -63,14 +74,17 @@ func writeCNIConfWithHostLocalSubnet(path, networkName, subnet string) error {
 	  }`, networkName, subnet)), 0644)
 }
 
+// configOpt is a functional option for configuring the Firecracker microVM
 type configOpt func(*sdk.Config)
 
+// withNetworkInterface adds a network interface configuration option to the Firecracker microVM config
 func withNetworkInterface(networkInterface sdk.NetworkInterface) configOpt {
 	return func(c *sdk.Config) {
 		c.NetworkInterfaces = append(c.NetworkInterfaces, networkInterface)
 	}
 }
 
+// createNewConfig creates a new Firecracker microVM configuration
 func createNewConfig(socketPath string, opts ...configOpt) sdk.Config {
 	dir, _ := os.Getwd()
 	fmt.Println(dir)
@@ -110,6 +124,7 @@ func createNewConfig(socketPath string, opts ...configOpt) sdk.Config {
 	return cfg
 }
 
+// connectToVM establishes an SSH connection to the Firecracker microVM
 func connectToVM(m *sdk.Machine, sshKeyPath string) (*ssh.Client, error) {
 	key, err := ioutil.ReadFile(sshKeyPath)
 	if err != nil {
@@ -139,6 +154,7 @@ func connectToVM(m *sdk.Machine, sshKeyPath string) (*ssh.Client, error) {
 	return ssh.Dial("tcp", fmt.Sprintf("%s:22", ip), config)
 }
 
+// createSnapshotSSH creates a snapshot of a Firecracker microVM and returns the IP of the VM
 func createSnapshotSSH(ctx context.Context, socketPath, memPath, snapPath string) string {
 	dir, err := os.Getwd()
 	if err != nil {
@@ -246,6 +262,7 @@ func createSnapshotSSH(ctx context.Context, socketPath, memPath, snapPath string
 	return vmIP
 }
 
+// loadSnapshotSSH loads a snapshot of the Firecracker microVM using SSH
 func loadSnapshotSSH(ctx context.Context, socketPath, memPath, snapPath, ipToRestore string) {
 	var ipFreed bool = false
 	var err error
@@ -383,48 +400,57 @@ func loadSnapshotSSH(ctx context.Context, socketPath, memPath, snapPath, ipToRes
 
 func main() {
 	// Check for kvm and root access
-	err := unix.Access("/dev/kvm", unix.W_OK)
-	if err != nil {
+	err := unix.Access("/dev/kvm", unix.W_OK) // Check if the program has write access to /dev/kvm
+	if err != nil {                           // If there's an error (e.g., access denied), log and exit
 		log.Fatal(err)
 	}
 
+	// Check if the program is running with root privileges
 	if x, y := 0, os.Getuid(); x != y {
 		log.Fatal("Root acccess denied")
 	}
 
+	// Get the current working directory
 	dir, err := os.Getwd()
-	if err != nil {
+	if err != nil { // If there's an error getting the working directory, log and exit
 		log.Fatal(err)
 	}
 
+	// Create a directory for CNI configuration files
 	cniConfDir := filepath.Join(dir, "cni.conf")
-	err = os.Mkdir(cniConfDir, 0777)
-	if err != nil {
+	err = os.Mkdir(cniConfDir, 0777) // Create the directory with full permissions
+	if err != nil {                  // If there's an error creating the directory, log and exit
 		log.Fatal(err)
 	}
-	defer os.Remove(cniConfDir)
+	defer os.Remove(cniConfDir) // Remove the directory when main function exits
 
-	// Setup socket and snapshot + memory paths
-	tempdir, err := ioutil.TempDir("", "FCGoSDKSnapshotExample")
-	if err != nil {
+	// Setup temporary directory and paths for socket, snapshot, and memory files
+	tempdir, err := ioutil.TempDir("", "FCGoSDKSnapshotExample") // Create a temporary directory
+	if err != nil {                                              // If there's an error creating the temporary directory, log and exit
 		log.Fatal(err)
 	}
-	defer os.Remove(tempdir)
-	socketPath := filepath.Join(tempdir, "snapshotssh")
+	defer os.Remove(tempdir)                            // Remove the temporary directory when main function exits
+	socketPath := filepath.Join(tempdir, "snapshotssh") // Create a socket path within the temporary directory
 
+	// Create a directory for snapshot and memory files
 	snapshotsshPath := filepath.Join(dir, "snapshotssh")
-	err = os.Mkdir(snapshotsshPath, 0777)
-	if err != nil && !errors.Is(err, os.ErrExist) {
+	err = os.Mkdir(snapshotsshPath, 0777)           // Create the directory with full permissions
+	if err != nil && !errors.Is(err, os.ErrExist) { // If there's an error creating the directory and it's not already exist, log and exit
 		log.Fatal(err)
 	}
-	defer os.RemoveAll(snapshotsshPath)
+	defer os.RemoveAll(snapshotsshPath) // Remove the directory and its contents when main function exits
 
+	// Set paths for snapshot and memory files
 	snapPath := filepath.Join(snapshotsshPath, "SnapFile")
 	memPath := filepath.Join(snapshotsshPath, "MemFile")
 
+	// Create a background context
 	ctx := context.Background()
 
+	// Create a snapshot of the Firecracker microVM and get the IP address to restore
 	ipToRestore := createSnapshotSSH(ctx, socketPath, memPath, snapPath)
 	fmt.Println()
+
+	// Load the snapshot of the Firecracker microVM
 	loadSnapshotSSH(ctx, socketPath, memPath, snapPath, ipToRestore)
 }