memory leak

[gsathya]

Test case --

let out;

async function longLoop() {
  for (let i = 0; i < 1000000; i++) await undefined;
  out = 1;
}

longLoop().then(() => console.log(1 === out));

Steps to reproduce --

1. Run the above code through async-to-gen
2. Run the generated code using node and set an upper bound on memory

Actual output --

➜   node --max-old-space-size=10  async-leak-gen.js                                                                        
<--- Last few GCs --->

     194 ms: Mark-sweep 16.0 (44.0) -> 15.9 (44.0) MB, 46.2 / 0.0 ms [allocation failure] [GC in old space requested].
     242 ms: Mark-sweep 15.9 (44.0) -> 15.9 (44.0) MB, 48.6 / 0.0 ms [allocation failure] [GC in old space requested].
     291 ms: Mark-sweep 15.9 (44.0) -> 15.7 (28.0) MB, 48.7 / 0.0 ms [last resort gc].
     344 ms: Mark-sweep 15.7 (28.0) -> 15.7 (28.0) MB, 52.5 / 0.0 ms [last resort gc].


<--- JS stacktrace --->

==== JS stack trace =========================================

Security context: 0x170d0accfb51 <JS Object>
    2: NewPromiseCapability(aka NewPromiseCapability) [native promise.js:175] [pc=0x379a595496a3] (this=0x170d0ac04381 <undefined>,O=0x170d0acc3059 <JS Function Promise (SharedFunctionInfo 0x170d0ac74f51)>)
    3: then [native promise.js:~215] [pc=0x379a59554f01] (this=0x1c8d0c9c9839 <a Promise with map 0x3b1fbab1b9b9>,z=0x1c8d0c9ca109 <
JS Function s (SharedFunctionInfo 0x6e897d5d7c1)>,A=0x1c8d...

FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
 1: node::Abort() [node]
 2: 0x10a08dc [node]
 3: v8::Utils::ReportApiFailure(char const*, char const*) [node]
 4: v8::internal::V8::FatalProcessOutOfMemory(char const*, bool) [node]
 5: v8::internal::Factory::NewOneByteInternalizedString(v8::internal::Vector<unsigned char const>, unsigned int) [node]
 6: v8::internal::AstRawStringInternalizationKey::AsHandle(v8::internal::Isolate*) [node]
 7: v8::internal::StringTable::LookupKey(v8::internal::Isolate*, v8::internal::HashTableKey*) [node]
 8: v8::internal::AstValueFactory::GetString(unsigned int, bool, v8::internal::Vector<unsigned char const>) [node]
 9: v8::internal::AstValueFactory::GetOneByteStringInternal(v8::internal::Vector<unsigned char const>) [node]
10: v8::internal::Scanner::CurrentSymbol(v8::internal::AstValueFactory*) [node]
11: v8::internal::ParserBase<v8::internal::ParserTraits>::ParseAndClassifyIdentifier(v8::internal::ExpressionClassifier<v8::internal
::ParserTraits>*, bool*) [node]
12: v8::internal::ParserBase<v8::internal::ParserTraits>::ParsePrimaryExpression(v8::internal::ExpressionClassifier<v8::internal::Pa
rserTraits>*, bool*) [node]
13: v8::internal::ParserBase<v8::internal::ParserTraits>::ParseMemberExpression(v8::internal::ExpressionClassifier<v8::internal::Par
serTraits>*, bool*) [node]
14: v8::internal::ParserBase<v8::internal::ParserTraits>::ParseMemberWithNewPrefixesExpression(v8::internal::ExpressionClassifier<v8
::internal::ParserTraits>*, bool*) [node]
15: v8::internal::ParserBase<v8::internal::ParserTraits>::ParseLeftHandSideExpression(v8::internal::ExpressionClassifier<v8::interna
l::ParserTraits>*, bool*) [node]
16: v8::internal::ParserBase<v8::internal::ParserTraits>::ParsePostfixExpression(v8::internal::ExpressionClassifier<v8::internal::Pa
... <snip>

Expected output --
Running non async-to-gen version with ToT v8

➜   ./out.gn/x64.release/d8 ~/code/scratch/async-leak.js --max-old-space-size 10
true

[leebyron]

Thanks for the report!

Any sense of where the root of this is?

[gsathya]

You don't need the return here --
https://github.com/leebyron/async-to-gen/blob/master/index.js#L82

[leebyron]

Nice catch! Yes, that indeed was the root cause - it looks like it was previously maintaining all prior Promises in the loop within a chain where that was not necessary or desirable.

The fixing commit uses your a slight variation of your test. I confirmed the same output on my machine before the fix and with the fix the test passes.