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refactor(core): refactor CloudFormationLang.toJSON()
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Our previous implementation of `toJSON()` was quite hacky.

It replaced values inside the structure with objects that had a custom
`toJSON()` serializer, and then called `JSON.stringify()` on the result.

The resulting JSON would have special markers in it where the Token
values would be string-substituted back in.

It's actually easier and gives us more control to just implement
JSONification ourselves in a Token-aware recursive function.

This change has been split off from a larger, upcoming PR in order
to make the individual reviews smaller.
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@rix0rrr
rix0rrr committed Oct 30, 2020
1 parent a4a41b5 commit 53bf0b6
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339 changes: 259 additions & 80 deletions packages/@aws-cdk/core/lib/private/cloudformation-lang.ts
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Original file line number Diff line number Diff line change
@@ -1,10 +1,7 @@
import { Lazy } from '../lazy';
import { Reference } from '../reference';
import { DefaultTokenResolver, IFragmentConcatenator, IPostProcessor, IResolvable, IResolveContext } from '../resolvable';
import { TokenizedStringFragments } from '../string-fragments';
import { Token } from '../token';
import { Intrinsic } from './intrinsic';
import { resolve } from './resolve';
import { DefaultTokenResolver, IFragmentConcatenator, IResolveContext } from '../resolvable';
import { isResolvableObject, Token } from '../token';
import { TokenMap } from './token-map';

/**
* Routines that know how to do operations at the CloudFormation document language level
Expand All @@ -24,59 +21,12 @@ export class CloudFormationLang {
* @param space Indentation to use (default: no pretty-printing)
*/
public static toJSON(obj: any, space?: number): string {
// This works in two stages:
//
// First, resolve everything. This gets rid of the lazy evaluations, evaluation
// to the real types of things (for example, would a function return a string, an
// intrinsic, or a number? We have to resolve to know).
//
// We then to through the returned result, identify things that evaluated to
// CloudFormation intrinsics, and re-wrap those in Tokens that have a
// toJSON() method returning their string representation. If we then call
// JSON.stringify() on that result, that gives us essentially the same
// string that we started with, except with the non-token characters quoted.
//
// {"field": "${TOKEN}"} --> {\"field\": \"${TOKEN}\"}
//
// A final resolve() on that string (done by the framework) will yield the string
// we're after.
//
// Resolving and wrapping are done in go using the resolver framework.
class IntrinsincWrapper extends DefaultTokenResolver {
constructor() {
super(CLOUDFORMATION_CONCAT);
}

public resolveToken(t: IResolvable, context: IResolveContext, postProcess: IPostProcessor) {
// Return References directly, so their type is maintained and the references will
// continue to work. Only while preparing, because we do need the final value of the
// token while resolving.
if (Reference.isReference(t) && context.preparing) { return wrap(t); }

// Deep-resolve and wrap. This is necessary for Lazy tokens so we can see "inside" them.
return wrap(super.resolveToken(t, context, postProcess));
}
public resolveString(fragments: TokenizedStringFragments, context: IResolveContext) {
return wrap(super.resolveString(fragments, context));
}
public resolveList(l: string[], context: IResolveContext) {
return wrap(super.resolveList(l, context));
}
}

// We need a ResolveContext to get started so return a Token
return Lazy.stringValue({
produce: (ctx: IResolveContext) =>
JSON.stringify(resolve(obj, {
preparing: ctx.preparing,
scope: ctx.scope,
resolver: new IntrinsincWrapper(),
}), undefined, space),
// We used to do this by hooking into `JSON.stringify()` by adding in objects
// with custom `toJSON()` functions, but it's ultimately simpler just to
// reimplement the `stringify()` function from scratch.
produce: (ctx) => tokenAwareStringify(obj, space ?? 0, ctx),
});

function wrap(value: any): any {
return isIntrinsic(value) ? new JsonToken(deepQuoteStringsForJSON(value)) : value;
}
}

/**
Expand All @@ -97,44 +47,213 @@ export class CloudFormationLang {

// Otherwise return a Join intrinsic (already in the target document language to avoid taking
// circular dependencies on FnJoin & friends)
return { 'Fn::Join': ['', minimalCloudFormationJoin('', parts)] };
return fnJoinConcat(parts);
}
}

/**
* Token that also stringifies in the toJSON() operation.
* Return a CFN intrinsic mass concatting any number of CloudFormation expressions
*/
class JsonToken extends Intrinsic {
/**
* Special handler that gets called when JSON.stringify() is used.
*/
public toJSON() {
return this.toString();
}
function fnJoinConcat(parts: any[]) {
return { 'Fn::Join': ['', minimalCloudFormationJoin('', parts)] };
}

/**
* Deep escape strings for use in a JSON context
* Perform a JSON.stringify()-like operation, except aware of Tokens and CloudFormation intrincics
*
* Tokens will be resolved and if they resolve to CloudFormation intrinsics, the intrinsics
* will be lifted to the top of a giant `{ Fn::Join}` expression.
*
* We are looking to do the following transforms:
*
* (a) Token in a string context
*
* { "field": "a${TOKEN}b" } -> "{ \"field\": \"a" ++ resolve(TOKEN) ++ "b\" }"
* { "a${TOKEN}b": "value" } -> "{ \"a" ++ resolve(TOKEN) ++ "b\": \"value\" }"
*
* (b) Standalone token
*
* { "field": TOKEN } ->
*
* if TOKEN resolves to a string (or is a non-encoded or string-encoded intrinsic) ->
* "{ \"field\": \"" ++ resolve(TOKEN) ++ "\" }"
* if TOKEN resolves to a non-string (or is a non-string-encoded intrinsic) ->
* "{ \"field\": " ++ resolve(TOKEN) ++ " }"
*
* (Where ++ is the CloudFormation string-concat operation (`{ Fn::Join }`).
*
* -------------------
*
* Here come complex type interpretation rules, which we are unable to simplify because
* some clients our there are already taking dependencies on the unintended side effects
* of the old implementation.
*
* 1. If TOKEN is embedded in a string with a prefix or postfix, we'll render the token
* as a string regardless of whether it returns an intrinsic or a literal.
*
* 2. If TOKEN resolves to an intrinsic:
* - We'll treat it as a string if the TOKEN itself was not encoded or string-encoded
* (this covers the 99% case of what CloudFormation intrinsics resolve to).
* - We'll treat it as a non-string otherwise; the intrinsic MUST resolve to a number;
* * if resolves to a list { Fn::Join } will fail
* * if it resolves to a string after all the JSON will be malformed at API call time.
*
* 3. Otherwise, the type of the value it resolves to (string, number, complex object, ...)
* determines how the value is rendered.
*/
function deepQuoteStringsForJSON(x: any): any {
if (typeof x === 'string') {
// Whenever we escape a string we strip off the outermost quotes
// since we're already in a quoted context.
const stringified = JSON.stringify(x);
return stringified.substring(1, stringified.length - 1);
function tokenAwareStringify(root: any, space: number, ctx: IResolveContext) {
let indent = 0;

const ret = new Array<Segment>();
recurse(root);
switch (ret.length) {
case 0: return '';
case 1: return renderSegment(ret[0]);
default:
return fnJoinConcat(ret.map(renderSegment));
}

if (Array.isArray(x)) {
return x.map(deepQuoteStringsForJSON);
/**
* Stringify a JSON element
*/
function recurse(obj: any): void {
if (Token.isUnresolved(obj)) {
return handleToken(obj);
}
if (Array.isArray(obj)) {
return renderCollection('[', ']', obj, recurse);
}
if (typeof obj === 'object' && obj != null && !(obj instanceof Date)) {
return renderCollection('{', '}', definedEntries(obj), ([key, value]) => {
recurse(key);
pushLiteral(prettyPunctuation(':'));
recurse(value);
});
}
// Otherwise we have a scalar, defer to JSON.stringify()s serialization
pushLiteral(JSON.stringify(obj));
}

if (typeof x === 'object') {
for (const key of Object.keys(x)) {
x[key] = deepQuoteStringsForJSON(x[key]);
/**
* Render an object or list
*/
function renderCollection<A>(pre: string, post: string, xs: Iterable<A>, each: (x: A) => void) {
pushLiteral(pre);
indent += space;
let atLeastOne = false;
for (const [comma, item] of sepIter(xs)) {
if (comma) { pushLiteral(','); }
pushLineBreak();
each(item);
atLeastOne = true;
}
indent -= space;
if (atLeastOne) { pushLineBreak(); }
pushLiteral(post);
}

return x;
/**
* Handle a Token.
*
* Can be any of:
*
* - Straight up IResolvable
* - Encoded string, number or list
*/
function handleToken(token: any) {
if (typeof token === 'string') {
// Encoded string, treat like a string if it has a token and at least one other
// component, otherwise treat like a regular token and base the output quoting on the
// type of the result.
const fragments = TokenMap.instance().splitString(token);
if (fragments.length > 1) {
pushLiteral('"');
fragments.visit({
visitLiteral: pushLiteral,
visitToken: (tok) => {
const resolved = ctx.resolve(tok);
if (isIntrinsic(resolved)) {
pushIntrinsic(quoteInsideIntrinsic(resolved));
} else {
// We're already in a string context, so stringify and escape
pushLiteral(quoteString(`${resolved}`));
}
},
// This potential case is the result of poor modeling in the tokenized string, it should not happen
visitIntrinsic: () => { throw new Error('Intrinsic not expected in a freshly-split string'); },
});
pushLiteral('"');
return;
}
}

const resolved = ctx.resolve(token);
if (isIntrinsic(resolved)) {
if (isResolvableObject(token) || typeof token === 'string') {
// If the input was an unencoded IResolvable or a string-encoded value,
// treat it like it was a string (for the 99% case)
pushLiteral('"');
pushIntrinsic(quoteInsideIntrinsic(resolved));
pushLiteral('"');
} else {
pushIntrinsic(resolved);
}
return;
}

// Otherwise we got an arbitrary JSON structure from the token, recurse
recurse(resolved);
}

/**
* Push a literal onto the current segment if it's also a literal, otherwise open a new Segment
*/
function pushLiteral(lit: string) {
let last = ret[ret.length - 1];
if (last?.type !== 'literal') {
last = { type: 'literal', parts: [] };
ret.push(last);
}
last.parts.push(lit);
}

/**
* Add a new intrinsic segment
*/
function pushIntrinsic(intrinsic: any) {
ret.push({ type: 'intrinsic', intrinsic });
}

/**
* Push a line break if we are pretty-printing, otherwise don't
*/
function pushLineBreak() {
if (space > 0) {
pushLiteral(`\n${' '.repeat(indent)}`);
}
}

/**
* Add a space after the punctuation if we are pretty-printing, no space if not
*/
function prettyPunctuation(punc: string) {
return space > 0 ? `${punc} ` : punc;
}
}

/**
* A Segment is either a literal string or a CloudFormation intrinsic
*/
type Segment = { type: 'literal'; parts: string[] } | { type: 'intrinsic'; intrinsic: any };

/**
* Render a segment
*/
function renderSegment(s: Segment): NonNullable<any> {
switch (s.type) {
case 'literal': return s.parts.join('');
case 'intrinsic': return s.intrinsic;
}
}

const CLOUDFORMATION_CONCAT: IFragmentConcatenator = {
Expand Down Expand Up @@ -194,7 +313,7 @@ function isIntrinsic(x: any) {
const keys = Object.keys(x);
if (keys.length !== 1) { return false; }

return keys[0] === 'Ref' || isNameOfCloudFormationIntrinsic(keys[0]);
return keys[0] === 'Ref' || isNameOfCloudFormationIntrinsic(keys[0]) || isNameOfCdkIntrinsic(keys[0]);
}

export function isNameOfCloudFormationIntrinsic(name: string): boolean {
Expand All @@ -204,3 +323,63 @@ export function isNameOfCloudFormationIntrinsic(name: string): boolean {
// these are 'fake' intrinsics, only usable inside the parameter overrides of a CFN CodePipeline Action
return name !== 'Fn::GetArtifactAtt' && name !== 'Fn::GetParam';
}

export function isNameOfCdkIntrinsic(name: string): boolean {
return name.startsWith('$Cdk::');
}

/**
* Separated iterator
*/
function* sepIter<A>(xs: Iterable<A>): IterableIterator<[boolean, A]> {
let comma = false;
for (const item of xs) {
yield [comma, item];
comma = true;
}
}

/**
* Object.entries() but skipping undefined values
*/
function* definedEntries<A extends object>(xs: A): IterableIterator<[string, any]> {
for (const [key, value] of Object.entries(xs)) {
if (value !== undefined) {
yield [key, value];
}
}
}

/**
* Quote string literals inside an intrinsic
*/
function quoteInsideIntrinsic(x: any): any {
if (typeof x === 'object' && x != null && Object.keys(x).length === 1) {
const key = Object.keys(x)[0];
const params = x[key];
switch (key) {
case 'Fn::If':
return { 'Fn::If': [params[0], quoteInsideIntrinsic(params[1]), quoteInsideIntrinsic(params[2])] };
case 'Fn::Join':
return { 'Fn::Join': [quoteInsideIntrinsic(params[0]), params[1].map(quoteInsideIntrinsic)] };
case 'Fn::Sub':
if (Array.isArray(params)) {
return { 'Fn::Sub': [quoteInsideIntrinsic(params[0]), params[1]] };
} else {
return { 'Fn::Sub': quoteInsideIntrinsic(params[0]) };
}
}
}
if (typeof x === 'string') {
return quoteString(x);
}
return x;
}

/**
* Quote the characters inside a string, for use inside toJSON
*/
function quoteString(s: string) {
s = JSON.stringify(s);
return s.substring(1, s.length - 1);
}
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