moby/builder/parser/parser.go

// This package implements a parser and parse tree dumper for Dockerfiles.
package parser

import (
	"bufio"
	"io"
	"regexp"
	"strings"
	"unicode"
)

// Node is a structure used to represent a parse tree.
//
// In the node there are three fields, Value, Next, and Children. Value is the
// current token's string value. Next is always the next non-child token, and
// children contains all the children. Here's an example:
//
// (value next (child child-next child-next-next) next-next)
//
// This data structure is frankly pretty lousy for handling complex languages,
// but lucky for us the Dockerfile isn't very complicated. This structure
// works a little more effectively than a "proper" parse tree for our needs.
//
type Node struct {
	Value      string          // actual content
	Next       *Node           // the next item in the current sexp
	Children   []*Node         // the children of this sexp
	Attributes map[string]bool // special attributes for this node
	Original   string          // original line used before parsing
}

var (
	dispatch                map[string]func(string) (*Node, map[string]bool, error)
	TOKEN_WHITESPACE        = regexp.MustCompile(`[\t\v\f\r ]+`)
	TOKEN_LINE_CONTINUATION = regexp.MustCompile(`\\$`)
	TOKEN_COMMENT           = regexp.MustCompile(`^#.*$`)
)

func init() {
	// Dispatch Table. see line_parsers.go for the parse functions.
	// The command is parsed and mapped to the line parser. The line parser
	// recieves the arguments but not the command, and returns an AST after
	// reformulating the arguments according to the rules in the parser
	// functions. Errors are propogated up by Parse() and the resulting AST can
	// be incorporated directly into the existing AST as a next.
	dispatch = map[string]func(string) (*Node, map[string]bool, error){
		"user":       parseString,
		"onbuild":    parseSubCommand,
		"workdir":    parseString,
		"env":        parseEnv,
		"maintainer": parseString,
		"from":       parseString,
		"add":        parseStringsWhitespaceDelimited,
		"copy":       parseStringsWhitespaceDelimited,
		"run":        parseMaybeJSON,
		"cmd":        parseMaybeJSON,
		"entrypoint": parseMaybeJSON,
		"expose":     parseStringsWhitespaceDelimited,
		"volume":     parseMaybeJSONToList,
		"insert":     parseIgnore,
	}
}

// parse a line and return the remainder.
func parseLine(line string) (string, *Node, error) {
	if line = stripComments(line); line == "" {
		return "", nil, nil
	}

	if TOKEN_LINE_CONTINUATION.MatchString(line) {
		line = TOKEN_LINE_CONTINUATION.ReplaceAllString(line, "")
		return line, nil, nil
	}

	cmd, args := splitCommand(line)

	node := &Node{}
	node.Value = cmd

	sexp, attrs, err := fullDispatch(cmd, args)
	if err != nil {
		return "", nil, err
	}

	if sexp.Value != "" || sexp.Next != nil || sexp.Children != nil {
		node.Next = sexp
		node.Attributes = attrs
		node.Original = line
	}

	return "", node, nil
}

// The main parse routine. Handles an io.ReadWriteCloser and returns the root
// of the AST.
func Parse(rwc io.Reader) (*Node, error) {
	root := &Node{}
	scanner := bufio.NewScanner(rwc)

	for scanner.Scan() {
		line, child, err := parseLine(strings.TrimLeftFunc(scanner.Text(), unicode.IsSpace))
		if err != nil {
			return nil, err
		}

		if line != "" && child == nil {
			for scanner.Scan() {
				newline := scanner.Text()

				if newline == "" {
					continue
				}

				line, child, err = parseLine(line + newline)
				if err != nil {
					return nil, err
				}

				if child != nil {
					break
				}
			}
		}

		if child != nil {
			root.Children = append(root.Children, child)
		}
	}

	return root, nil
}