EXC_BAD_ACCESS in Lexer.Cursor.Position.advance()

[jaredmbourgeois]

Issue Kind

Bug

Source Code

await withDiscardingTaskGroup { taskGroup in 
    for content in fileContents {
        taskGroup.addTask { 
            // EXC_BAD_ACCESS here, requires `await MainActor.run`
            // Maybe this isn't meant to be used concurrently, but having to run on `MainActor` is a huge bottleneck.
            Parser.parse(source: content)
        }
    }
}

Description

Running into this EXC_BAD_ACCESS when running concurrent Parser.parse.

Crash is in SwiftParser/Lexer/Cursor.swift, line 648 (602.0.0) when attempting to input.popFirst() inside of Lexer.Cursor.Position.advance

[ahoppen]

Two quick questions:

• Does this happen with any source file (eg. also with one only containing a single line func foo() {}) or only large source files
• Does this happen when building in debug and release configuration or only in a debug configuration?

If this happens only in debug and/or with deeply-ish nested source files, this may be #390, which is what I suspect because SwiftParser doesn’t otherwise care which thread it’s running on.

If you could share the contents of the source file that you’re parsing, that would be helpful so we can investigate how to reduce stack usage or decrease the maximum nesting level here

swift-syntax/Sources/SwiftParser/Parser.swift

Lines 133 to 137 in d7cde2f

	#if DEBUG
	static let defaultMaximumNestingLevel = 20
	#else
	static let defaultMaximumNestingLevel = 256
	#endif

[jaredmbourgeois]

Two quick questions:

• Does this happen with any source file (eg. also with one only containing a single line func foo() {}) or only large source files
• Does this happen when building in debug and release configuration or only in a debug configuration?

If this happens only in debug and/or with deeply-ish nested source files, this may be #390, which is what I suspect because SwiftParser doesn’t otherwise care which thread it’s running on.

If you could share the contents of the source file that you’re parsing, that would be helpful so we can investigate how to reduce stack usage or decrease the maximum nesting level here

swift-syntax/Sources/SwiftParser/Parser.swift

Lines 133 to 137 in d7cde2f

#if DEBUG
static let defaultMaximumNestingLevel = 20
#else
static let defaultMaximumNestingLevel = 256
#endif

hey @ahoppen, appreciate the reply!
I encountered this at work and on vacation until next week - will see if it's OK to share the file(s) with the crash - honestly haven't looked into which file, but none of these files should be particularly large with max nesting level of I'd guess ~5.

We've been parsing our project in CI for close to 2 years and I just encountered this when running locally, haven't tried in CI with these changes, so I'll try a release build to see if the defaultMaximumNestingLevel fixes the issue.

I was suspecting something changed with the swift version shipped with Xcode 26.0.0, would be nice if it's defaultMaximumNestingLevel. I'll reply back here next week. Thank you!

[ahoppen]

Absolutely, take your time. If you have reproduction steps, we’d really appreciate that. It could be that something in swift-syntax has changed and increased our stack usage in debug builds. If so, it would be good to track that down and try to fix it.

[jaredmbourgeois]

hey @ahoppen, apologies for never getting back to this, but I just tested with 603.0.0 and this issue has been fixed! Appreciate your help here!