Add some doc strings for LLB functions

This is just a first round of "would have been useful to me" sort of
documentation and examples for the LLB client API.
This is not in any way exhaustive.

Signed-off-by: Brian Goff <[email protected]>

Author: cpuguy83

client/llb/diff.go

@@ -90,6 +90,8 @@ func (m *DiffOp) Inputs() (out []Output) {
 	return out
 }
 
+// Diff returns a state that represents the diff of the lower and upper states.
+// The returned State is useful for use with [Merge] where you can merge the lower state with the diff.
 func Diff(lower, upper State, opts ...ConstraintsOpt) State {
 	if lower.Output() == nil {
 		if upper.Output() == nil {

client/llb/exec.go

@@ -649,6 +649,7 @@ type SSHInfo struct {
 	Optional bool
 }
 
+// AddSecret is a RunOption that adds a secret to the exec.
 func AddSecret(dest string, opts ...SecretOption) RunOption {
 	return runOptionFunc(func(ei *ExecInfo) {
 		s := &SecretInfo{ID: dest, Target: dest, Mode: 0400}
@@ -696,6 +697,7 @@ func SecretAsEnv(v bool) SecretOption {
 	})
 }
 
+// SecretFileOpt sets the secret's target file uid, gid and permissions.
 func SecretFileOpt(uid, gid, mode int) SecretOption {
 	return secretOptionFunc(func(si *SecretInfo) {
 		si.UID = uid
@@ -704,12 +706,15 @@ func SecretFileOpt(uid, gid, mode int) SecretOption {
 	})
 }
 
+// ReadonlyRootFS sets the execs's root filesystem to be read-only.
 func ReadonlyRootFS() RunOption {
 	return runOptionFunc(func(ei *ExecInfo) {
 		ei.ReadonlyRootFS = true
 	})
 }
 
+// WithProxy is a RunOption that sets the proxy environment variables in the resulting exec.
+// For example `HTTP_PROXY` is a standard environment variable for unix systems that programs may read.
 func WithProxy(ps ProxyEnv) RunOption {
 	return runOptionFunc(func(ei *ExecInfo) {
 		ei.ProxyEnv = &ps

client/llb/fileop.go

@@ -48,6 +48,7 @@ func NewFileOp(s State, action *FileAction, c Constraints) *FileOp {
 }
 
 // CopyInput is either llb.State or *FileActionWithState
+// It is used by [Copy] to to specify the source of the copy operation.
 type CopyInput interface {
 	isFileOpCopyInput()
 }
@@ -131,6 +132,10 @@ type fileActionWithState struct {
 
 func (fas *fileActionWithState) isFileOpCopyInput() {}
 
+// Mkdir creates a FileAction which creates a directory at the given path.
+// Example:
+//
+//	llb.Scratch().File(llb.Mkdir("/foo", 0755))
 func Mkdir(p string, m os.FileMode, opt ...MkdirOption) *FileAction {
 	var mi MkdirInfo
 	for _, o := range opt {
@@ -181,6 +186,7 @@ func (fn mkdirOptionFunc) SetMkdirOption(mi *MkdirInfo) {
 
 var _ MkdirOption = &MkdirInfo{}
 
+// WithParents is an option for Mkdir which creates parent directories if they do not exist.
 func WithParents(b bool) MkdirOption {
 	return mkdirOptionFunc(func(mi *MkdirInfo) {
 		mi.MakeParents = b
@@ -282,6 +288,10 @@ func (up *UserOpt) marshal(base pb.InputIndex) *pb.UserOpt {
 	return &pb.UserOpt{User: &pb.UserOpt_ByID{ByID: uint32(up.UID)}}
 }
 
+// Mkfile creates a FileAction which creates a file at the given path with the provided contents.
+// Example:
+//
+//	llb.Scratch().File(llb.Mkfile("/foo", 0644, []byte("hello world!")))
 func Mkfile(p string, m os.FileMode, dt []byte, opts ...MkfileOption) *FileAction {
 	var mi MkfileInfo
 	for _, o := range opts {
@@ -332,6 +342,10 @@ func (a *fileActionMkfile) toProtoAction(ctx context.Context, parent string, bas
 	}, nil
 }
 
+// Rm creates a FileAction which removes a file or directory at the given path.
+// Example:
+//
+//	llb.Scratch().File(Mkfile("/foo", 0644, []byte("not around for long..."))).File(llb.Rm("/foo"))
 func Rm(p string, opts ...RmOption) *FileAction {
 	var mi RmInfo
 	for _, o := range opts {
@@ -394,6 +408,25 @@ func (a *fileActionRm) toProtoAction(ctx context.Context, parent string, base pb
 	}, nil
 }
 
+// Copy produces a FileAction which copies a file or directory from the source to the destination.
+// The "input" parameter is the contents to copy from.
+// "src" is the path to copy from within the "input".
+// "dest" is the path to copy to within the destination (the state being operated on).
+// See [CopyInput] for the valid types of input.
+//
+// Example:
+//
+//	st := llb.Local(".")
+//	llb.Scratch().File(llb.Copy(st, "/foo", "/bar"))
+//
+// The example copies the local (client) directory "./foo" to a new empty directory at /bar.
+//
+// Note: Copying directories can have different behavior based on if the destination exists or not.
+// When the destination already exists, the contents of the source directory is copied underneath the destination, including the directory itself.
+// You may need to supply a copy option to copy the dir contents only.
+// You may also need to pass in a [CopyOption] which creates parent directories if they do not exist.
+//
+// See [CopyOption] for more details on what options are available.
 func Copy(input CopyInput, src, dest string, opts ...CopyOption) *FileAction {
 	var state *State
 	var fas *fileActionWithState

client/llb/merge.go

@@ -70,6 +70,31 @@ func (m *MergeOp) Inputs() []Output {
 	return m.inputs
 }
 
+// Merge merges multiple states into a single state. This is useful in
+// conjunction with [Diff] to create set of patches which are independent of
+// each other to a base state without affecting the cache of other merged
+// states.
+// As an example, lets say you have a rootfs with the following directories:
+//
+//	/ /bin /etc /opt /tmp
+//
+// Now lets say you want to copy a directory /etc/foo from one state and a
+// binary /bin/bar from another state.
+// [Copy] makes a duplicate of file on top of another directory.
+// Merge creates a directory whose contents is an overlay of 2 states on top of each other.
+//
+// With "Merge" you can do this:
+//
+//	fooState := Diff(rootfs, fooState)
+//	barState := Diff(rootfs, barState)
+//
+// Then merge the results with:
+//
+//	Merge(rootfs, fooDiff, barDiff)
+//
+// The resulting state will have both /etc/foo and /bin/bar, but because Merge
+// was used, changing the contents of "fooDiff" does not require copying
+// "barDiff" again.
 func Merge(inputs []State, opts ...ConstraintsOpt) State {
 	// filter out any scratch inputs, which have no effect when merged
 	var filteredInputs []State

client/llb/source.go

@@ -91,6 +91,10 @@ func (s *SourceOp) Inputs() []Output {
 	return nil
 }
 
+// Image returns a state that represents a docker image in a registry.
+// Example:
+//
+//	st := llb.Image("busybox:latest")
 func Image(ref string, opts ...ImageOption) State {
 	r, err := reference.ParseNormalizedNamed(ref)
 	if err == nil {
@@ -215,6 +219,17 @@ type ImageInfo struct {
 	RecordType    string
 }
 
+// Git returns a state that represents a git repository.
+// Example:
+//
+//	st := llb.Git("https://github.com/moby/buildkit.git#v0.11.6")
+//
+// The example fetches the v0.11.6 tag of the buildkit repository.
+// You can also use a commit hash or a branch name.
+//
+// Other URL formats are supported such as "[email protected]:moby/buildkit.git", "git://...", "ssh://..."
+// Formats that utilize SSH may need to supply credentials as a [GitOption].
+// You may need to check the source code for a full list of supported formats.
 func Git(remote, ref string, opts ...GitOption) State {
 	url := strings.Split(remote, "#")[0]
 
@@ -346,10 +361,12 @@ func MountSSHSock(sshID string) GitOption {
 	})
 }
 
+// Scratch returns a state that represents an empty filesystem.
 func Scratch() State {
 	return NewState(nil)
 }
 
+// Local returns a state that represents a directory local to the client.
 func Local(name string, opts ...LocalOption) State {
 	gi := &LocalInfo{}
 

client/llb/sourcemap.go

@@ -7,6 +7,16 @@ import (
 	digest "github.com/opencontainers/go-digest"
 )
 
+// SourceMap maps a source file/location to an LLB state/definition.
+// SourceMaps are used to provide information for debugging and helpful error messages to the user.
+// As an example, lets say you have a Dockerfile with the following content:
+//
+//	FROM alpine
+//	RUN exit 1
+//
+// When the "RUN" statement exits with a non-zero exit code buildkit will treat
+// it as an error and is able to provide the user with a helpful error message
+// pointing to exactly the line in the Dockerfile that caused the error.
 type SourceMap struct {
 	State      *State
 	Definition *Definition

client/llb/state.go

@@ -49,6 +49,10 @@ func NewState(o Output) State {
 	return s
 }
 
+// State represents all operations that must be done to produce a given output.
+// States are immutable, and all operations return a new state linked to the previous one.
+// State is the core type of the LLB API and is used to build a graph of operations.
+// The graph is then marshaled into a definition that can be executed by a backend (such as buildkitd).
 type State struct {
 	out   Output
 	prev  *State