Proposal Details

The proposal is to extend the loopclosure checker to additionally warn with a variables in a 3-clause for loop is written to after Go 1.22. A part of the new semantics is to copy the iterator variable at the end of the for loop. This is a READ that can race with writes from other goroutines.

Example:

for i := 0; i < N; i ++ {
  go func() {
    i *= 10  // warn "loop variable i written by func literal"
    println(i)
  }()
}

Criteria to use:

1. There is a 3-clause for loop declaring a variable v.
2. It is in a file with GoVersion >= 1.22.
3. The 'last' instruction in the loop body is a go statement.
4. The function executed is a function literal.
5. The function literal contains an assignment (or IncDecStmt).
6. The assignment updates the loop variable according to the following recursive definition in some left hand side:

• Updates(id) holds when id refers to the variable,
• Updates(e.X) holds when Updates(e),
• Updates(e[g]) holds when Updates(e),
• Updates(*e) holds when Updates(e),
• and otherwise Updates(e) does not hold.

Additionally:

• See loopclosure forEachLastStmt for the operational definition of 'last'.
• In addition to go statements this would be reported for errgroup.Go and t.Run() when t.Parrallel is present. It will not be reported for defer statements.

To discuss: The requirements give at the moment dereference pointers. This is not sufficient to ensure a race has occurred. Just that a race is overwhelmingly likely. This seems to be in the spirit of the existing loopclosure check. Curious whether others agree or think we should not report if there is indirection.