add more documentation to the packages in common directory

common/adapter/doc.go

@@ -0,0 +1,8 @@
+/*
+Package adapter helps command line tools connect to the guest via a Packer
+communicator. A typical use is for custom provisioners that wrap command line
+tools. For example, the Ansible provisioner and the Inspec provisioner both
+use this package to proxy communicator calls.
+*/
+
+package adapter

common/bootcommand/doc.go

@@ -0,0 +1,13 @@
+/*
+Package bootcommand generates and sends boot commands to the remote instance.
+
+This package is relevant to people who want to create new builders, particularly
+builders with the capacity to build a VM from an iso.
+
+You can choose between three different drivers to send the command: a vnc
+driver, a usb driver, and a PX-XT keyboard driver. The driver you choose will
+depend on what kind of keyboard codes your hypervisor expects, and how you want
+to implement the connection.
+*/
+
+package bootcommand

common/chroot/doc.go

@@ -0,0 +1,25 @@
+/*
+Package chroot provides convenience tooling specific to chroot builders.
+
+Chroot builders work by creating a new volume from an existing source image and
+attaching it into an already-running instance. Once attached, a chroot is used
+to provision the system within that volume. After provisioning, the volume is
+detached, snapshotted, and a cloud-specific image is made.
+
+Using this process, minutes can be shaved off image build processes because a
+new instance doesn't need to be launched in the cloud before provisioning can
+take place.
+
+There are some restrictions, however. The host instance where the volume is
+attached to must be a similar system (generally the same OS version, kernel
+versions, etc.) as the image being built. Additionally, this process is much
+more expensive because the instance used to perform the build must be kept
+running persistently in order to build images, whereas the other non-chroot
+cloud image builders start instances on-demand to build images as needed.
+
+The HashiCorp-maintained Amazon and Azure builder plugins have chroot builders
+which use this option and can serve as an example for how the chroot steps and
+communicator are used.
+*/
+
+package chroot

common/command.go

@@ -4,8 +4,15 @@ import (
 	"os/exec"
 )
 
-// CommandWrapper is a type that given a command, will possibly modify that
+// CommandWrapper is a type that given a command, will modify that
 // command in-flight. This might return an error.
+// For example, your command could be `foo` and your CommandWrapper could be
+// func(s string) (string, error) {
+//	 return fmt.Sprintf("/bin/sh/ %s", s)
+// }
+// Using the CommandWrapper, you can set environment variables or perform
+// string interpolation once rather than many times, to save some lines of code
+// if similar wrapping needs to be performed many times during a plugin run.
 type CommandWrapper func(string) (string, error)
 
 // ShellCommand takes a command string and returns an *exec.Cmd to execute

common/commonsteps/doc.go

@@ -0,0 +1,16 @@
+/*
+The commonsteps package contains the multistep runner that comprises the main
+architectural convention of Packer builder plugins. It enables builders
+to respect global Packer flags like "on-error" and "debug". It also contains
+a selection of convenience "multistep" steps that perform globally relevant
+tasks that many or most builders will want to implement -- for example,
+launching Packer's internal HTTP server for serving files to the instance.
+
+It also provides step_provision, which contains the hooks necessary for allowing
+provisioners to run inside your builder.
+
+While it is possible to create a simple builder without using the multistep
+runner or step_provision, your builder will lack core Packer functionality.
+*/
+
+package commonsteps

common/filelock/doc.go

@@ -0,0 +1,6 @@
+/*
+Package filelock makes it easy to create and check file locks for concurrent
+processes.
+*/
+
+package filelock

common/guestexec/doc.go

@@ -0,0 +1,12 @@
+/*
+Package guestexec provides a shim for running common operating system commands
+on the guest/remote instance that is being provisioned. It helps provisioners
+which need to perform operating-system specific calls do so in a way that is
+simple and repeatable.
+
+Note that to successfully use this package your provisioner must have knowledge
+of the guest type, which is not information that builders generally collect --
+your provisioner will have to require guest information in its config.
+*/
+
+package guestexec

common/net/configure_port.go

@@ -17,11 +17,11 @@ import (
 
 var _ net.Listener = &Listener{}
 
-// Listener wraps a net.Lister with some magic packer capabilies. For example
-// until you call Listener.Close, any call to ListenRangeConfig.Listen cannot
-// bind to Port. Packer tries tells moving parts which port they can use, but
-// often the port has to be released before a 3rd party is started, like a VNC
-// server.
+// Listener wraps a net.Lister with some Packer-specific capabilies. For
+// example, until you call Listener.Close, any call to ListenRangeConfig.Listen
+// cannot bind to a Port. Packer tries to tell moving parts which port they can
+// use, but often the port has to be released before a 3rd party is started,
+// like a VNC server.
 type Listener struct {
 	// Listener can be closed but Port will be file locked by packer until
 	// Close is called.

common/packer_config.go

@@ -2,7 +2,8 @@ package common
 
 // PackerConfig is a struct that contains the configuration keys that
 // are sent by packer, properly tagged already so mapstructure can load
-// them. Embed this structure into your configuration class to get it.
+// them. Embed this structure into your configuration class to get access to
+// this information from the Packer Core.
 type PackerConfig struct {
 	PackerBuildName     string            `mapstructure:"packer_build_name"`
 	PackerBuilderType   string            `mapstructure:"packer_builder_type"`

common/packerbuilderdata/generated_data.go

@@ -7,9 +7,11 @@ import "github.com/hashicorp/packer/helper/multistep"
 const PlaceholderMsg = "To set this dynamically in the Packer template, " +
 	"you must use the `build` function"
 
-// GeneratedData manages variables exported by a builder after
-// it started. It uses the builder's multistep.StateBag internally, make sure it
-// is not nil before calling any functions.
+// GeneratedData manages variables created and exported by a builder after
+// it starts, so that provisioners and post-processors can have access to
+// build data generated at runtime -- for example, instance ID or instance IP
+// address. Internally, it uses the builder's multistep.StateBag. The user
+// must make sure that the State field is not is not nil before calling Put().
 type GeneratedData struct {
 	// The builder's StateBag
 	State multistep.StateBag

common/random/string.go

@@ -19,11 +19,23 @@ var (
 
 var rnd = rand.New(rand.NewSource(time.Now().UnixNano() + int64(os.Getpid())))
 
+// Numbers returns a random numeric string of the given length
 func Numbers(length int) string { return String(PossibleNumbers, length) }
+
+// AlphaNum returns a random alphanumeric string of the given length. The
+// returned string can contain both uppercase and lowercase letters.
 func AlphaNum(length int) string { return String(PossibleAlphaNum, length) }
+
+// AlphaNumLower returns a random alphanumeric string of the given length. The
+// returned string can contain lowercase letters, but not uppercase.
 func AlphaNumLower(length int) string { return String(PossibleAlphaNumLower, length) }
+
+// AlphaNumUpper returns a random alphanumeric string of the given length. The
+// returned string can contain uppercase letters, but not lowercase.
 func AlphaNumUpper(length int) string { return String(PossibleAlphaNumUpper, length) }
 
+// String returns a random string of the given length, using only the component
+// characters provided in the "chooseFrom" string.
 func String(chooseFrom string, length int) (randomString string) {
 	cflen := len(chooseFrom)
 	bytes := make([]byte, length)

common/retry/retry.go

@@ -36,7 +36,16 @@ func (err *RetryExhaustedError) Error() string {
 	return fmt.Sprintf("retry count exhausted. Last err: %s", err.Err)
 }
 
-// Run fn until context is cancelled up until StartTimeout time has passed.
+// Run will repeatedly retry the proivided fn within the constraints set in the
+// retry Config. It will retry until one of the following conditions is met:
+// - The provided context is cancelled.
+// - The Config.StartTimeout time has passed.
+// - The function returns without an error.
+// - The maximum number of tries, Config.Tries is exceeded.
+// - The function returns with an error that does not satisfy conditions
+//   set in the Config.ShouldRetry function.
+// If the given function (fn) does not return an error, then Run will return
+// nil. Otherwise, Run will return a relevant error.
 func (cfg Config) Run(ctx context.Context, fn func(context.Context) error) error {
 	retryDelay := func() time.Duration { return 2 * time.Second }
 	if cfg.RetryDelay != nil {

common/shell-local/doc.go

@@ -0,0 +1,14 @@
+/*
+Package shell-local is designed to make it easier to shell out locally on the
+machine running Packer. The top level tools in this package are probably not
+relevant to plugin maintainers, as they are implementation details shared
+between the HashiCorp-maintained shell-local provisioner and shell-local
+post-processor.
+
+The localexec sub-package can be used in any plugins that need local shell
+access, whether that is in a driver for a hypervisor, or a command to a third
+party cli tool. Please make sure that any third party tool dependencies are
+noted in your plugin's documentation.
+*/
+
+package shell_local

common/shell/shell.go

@@ -3,7 +3,17 @@ package shell
 
 import "github.com/hashicorp/packer/common"
 
-// Provisioner contains common fields to all shell provisioners
+// Provisioner contains common fields to all shell provisioners.
+// It is provided as a convenience to encourage plugin developers to
+// consider implementing these options, which we believe are valuable for all
+// shell-type provisioners. It also helps guarantee that option names for
+// similar options are the same across the various shell provisioners.
+// Validation and defaulting are left to the maintainer because appropriate
+// values and defaults will be different depending on which shell is used.
+// To use the Provisioner struct, embed it in your shell provisioner's config
+// using the `mapstructure:",squash"` struct tag. Examples can be found in the
+// HashiCorp-maintained "shell", "shell-local", "windows-shell" and "powershell"
+// provisioners.
 type Provisioner struct {
 	common.PackerConfig `mapstructure:",squash"`
 

common/shutdowncommand/config.go

@@ -8,6 +8,13 @@ import (
 	"github.com/hashicorp/packer/packer-plugin-sdk/template/interpolate"
 )
 
+// ShutdownConfig defines implementation details for shutting down a VM once it
+// is done provisioned.
+// It is provided as a convenience to encourage builder developers to
+// consider implementing these options, which we believe are valuable for all
+// builders. It also helps guarantee that option names for similar options
+// are the same across the various builders. Embed it in your builder config
+// using the `mapstructure:",squash"` struct tag.
 type ShutdownConfig struct {
 	// The command to use to gracefully shut down the machine once all
 	// provisioning is complete. By default this is an empty string, which

common/template/funcs.go

@@ -26,7 +26,8 @@ func DeprecatedTemplateFunc(funcName, useInstead string, deprecated func(string)
 	}
 }
 
-// Vault retrieves a secret from an HC vault KV store
+// Vault retrieves a secret from a HashiCorp Vault KV store.
+// It assumes the necessary environment variables are set.
 func Vault(path string, key string) (string, error) {
 
 	if token := os.Getenv("VAULT_TOKEN"); token == "" {
@@ -64,6 +65,8 @@ func Vault(path string, key string) (string, error) {
 	return "", errors.New("Vault path does not contain the requested key")
 }
 
+// Consul retrieves a value from a HashiCorp Consul KV store.
+// It assumes the necessary environment variables are set.
 func Consul(k string) (string, error) {
 	consulConfig := consulapi.DefaultConfig()
 	client, err := consulapi.NewClient(consulConfig)
@@ -88,6 +91,9 @@ func Consul(k string) (string, error) {
 	return value, nil
 }
 
+// GetAwsSecret retrieves a value from an AWS Secrets Manager.
+// It assumes that credentials are properly set in the AWS SDK's credential
+// chain.
 func GetAWSSecret(name, key string) (string, error) {
 	// Check if at least 1 parameter has been used
 	if len(name) == 0 {