[flang][runtime] Add a critical section for LookUpOrCreateAnonymous.

[sihuan]

In parallel regions LookUpOrCreateAnonymous may return an anonymous unit that has not yet been opened, which may cause a runtime error.
This commit ensures that the returned anonymous unit has been opened.
For details see: #68856 (comment)

Fixes #68856

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: SiHuaN (sihuan)

Changes

In parallel regions LookUpOrCreateAnonymous may return an anonymous unit that has not yet been opened, which may cause a runtime error.
This commit ensures that the returned anonymous unit has been opened.
For details see: #68856 (comment)

Fixes #68856

---

Full diff: https://github.com/llvm/llvm-project/pull/74468.diff

1 Files Affected:

• (modified) flang/runtime/unit.cpp (+4)

diff --git a/flang/runtime/unit.cpp b/flang/runtime/unit.cpp
index 995656b9480c4..fa9efee78127e 100644
--- a/flang/runtime/unit.cpp
+++ b/flang/runtime/unit.cpp
@@ -20,6 +20,7 @@ namespace Fortran::runtime::io {
 // The per-unit data structures are created on demand so that Fortran I/O
 // should work without a Fortran main program.
 static Lock unitMapLock;
+static Lock createOpenLock;
 static UnitMap *unitMap{nullptr};
 static ExternalFileUnit *defaultInput{nullptr}; // unit 5
 static ExternalFileUnit *defaultOutput{nullptr}; // unit 6
@@ -52,6 +53,9 @@ ExternalFileUnit *ExternalFileUnit::LookUpOrCreate(
 ExternalFileUnit *ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
     Direction dir, std::optional<bool> isUnformatted,
     const Terminator &terminator) {
+  // Make sure that the returned anonymous unit has been opened
+  // not just created in the unitmap.
+  CriticalSection critical{createOpenLock};
   bool exists{false};
   ExternalFileUnit *result{
       GetUnitMap().LookUpOrCreate(unit, terminator, exists)};

[klausler]

This lock can be declared static in the only function that uses it.

[sihuan]

If I move this declaration into *ExternalFileUnit::LookUpOrCreateAnonymous, like the following:

ExternalFileUnit *ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
    Direction dir, std::optional<bool> isUnformatted,
    const Terminator &terminator) {
  // Make sure that the returned anonymous unit has been opened
  // not just created in the unitMap.
  static Lock createOpenLock;
  CriticalSection critical{createOpenLock};
  bool exists{false};
  ExternalFileUnit *result{
      GetUnitMap().LookUpOrCreate(unit, terminator, exists)};
  if (result && !exists) {
    IoErrorHandler handler{terminator};
    result->OpenAnonymousUnit(
        dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
        Action::ReadWrite, Position::Rewind, Convert::Unknown, handler);
    result->isUnformatted = isUnformatted;
  }
  return result;
}

I will have to add the -lstdc++ flag when compiling with flang, or I will get a linker error:

$ cat test.f90
! test.f90
!$omp parallel
write(1,*) 'OK'
!$omp end parallel
end
$ ../../llvm-project/build/bin/flang-new test.f90 -fopenmp
/usr/bin/ld: /path/to/llvm-project/build/lib/libFortranRuntime.a(unit.cpp.o): in function `Fortran::runtime::io::ExternalFileUnit::LookUpOrCreateAnonymous(int, Fortran::runtime::io::Direction, std::optional<bool>, Fortran::runtime::Terminator const&)':
/path/to/llvm-project/flang/runtime/unit.cpp:57:(.text+0x32e): undefined reference to `__cxa_guard_acquire'
/usr/bin/ld: /path/to/llvm-project/flang/runtime/unit.cpp:57:(.text+0x369): undefined reference to `__cxa_guard_release'
flang-new: error: linker command failed with exit code 1 (use -v to see invocation)
$ ../../llvm-project/build/bin/flang-new test.f90 -fopenmp -lstdc++
$ OMP_NUM_THREADS=4 ./a.out  
$ cat fort.1
 OK
 OK
 OK    

[NimishMishra]

Thanks for the patch. LGTM