[flang][runtime] Resilient opening of anonymous unit (#93876)

When an I/O statement references a unit number that has not been
explicitly opened or predefined, the I/O runtime support library opens a
local "fort.N" file. If this fails, the program crashes, even when the
I/O statement has IOSTAT= or IOMSG= or ERR= control list items. Connect
the dots to enable resilience in these cases.

flang/runtime/external-unit.cpp

@@ -58,15 +58,13 @@ ExternalFileUnit *ExternalFileUnit::LookUpOrCreate(
 
 ExternalFileUnit *ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
     Direction dir, Fortran::common::optional<bool> isUnformatted,
-    const Terminator &terminator) {
-  // Make sure that the returned anonymous unit has been opened
+    IoErrorHandler &handler) {
+  // 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)};
+  ExternalFileUnit *result{GetUnitMap().LookUpOrCreate(unit, handler, exists)};
   if (result && !exists) {
-    IoErrorHandler handler{terminator};
     result->OpenAnonymousUnit(
         dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
         Action::ReadWrite, Position::Rewind, Convert::Unknown, handler);
@@ -143,6 +141,9 @@ bool ExternalFileUnit::OpenUnit(Fortran::common::optional<OpenStatus> status,
   }
   set_path(std::move(newPath), newPathLength);
   Open(status.value_or(OpenStatus::Unknown), action, position, handler);
+  if (handler.InError()) {
+    return impliedClose;
+  }
   auto totalBytes{knownSize()};
   if (access == Access::Direct) {
     if (!openRecl) {

flang/runtime/file.cpp

@@ -131,17 +131,17 @@ void OpenFile::Open(OpenStatus status, Fortran::common::optional<Action> action,
   }
   RUNTIME_CHECK(handler, action.has_value());
   pending_.reset();
-  if (position == Position::Append && !RawSeekToEnd()) {
+  if (fd_ >= 0 && position == Position::Append && !RawSeekToEnd()) {
     handler.SignalError(IostatOpenBadAppend);
   }
-  isTerminal_ = IsATerminal(fd_) == 1;
+  isTerminal_ = fd_ >= 0 && IsATerminal(fd_) == 1;
   mayRead_ = *action != Action::Write;
   mayWrite_ = *action != Action::Read;
   if (status == OpenStatus::Old || status == OpenStatus::Unknown) {
     knownSize_.reset();
 #ifndef _WIN32
     struct stat buf;
-    if (::fstat(fd_, &buf) == 0) {
+    if (fd_ >= 0 && ::fstat(fd_, &buf) == 0) {
       mayPosition_ = S_ISREG(buf.st_mode);
       knownSize_ = buf.st_size;
     }

flang/runtime/io-api-common.h

@@ -33,13 +33,17 @@ static inline RT_API_ATTRS Cookie NoopUnit(const Terminator &terminator,
 static inline RT_API_ATTRS ExternalFileUnit *GetOrCreateUnit(int unitNumber,
     Direction direction, Fortran::common::optional<bool> isUnformatted,
     const Terminator &terminator, Cookie &errorCookie) {
+  IoErrorHandler handler{terminator};
+  handler.HasIoStat();
   if (ExternalFileUnit *
       unit{ExternalFileUnit::LookUpOrCreateAnonymous(
-          unitNumber, direction, isUnformatted, terminator)}) {
+          unitNumber, direction, isUnformatted, handler)}) {
     errorCookie = nullptr;
     return unit;
   } else {
-    errorCookie = NoopUnit(terminator, unitNumber, IostatBadUnitNumber);
+    auto iostat{static_cast<enum Iostat>(handler.GetIoStat())};
+    errorCookie = NoopUnit(terminator, unitNumber,
+        iostat != IostatOk ? iostat : IostatBadUnitNumber);
     return nullptr;
   }
 }

flang/runtime/pseudo-unit.cpp

@@ -36,11 +36,11 @@ ExternalFileUnit *ExternalFileUnit::LookUpOrCreate(
 
 ExternalFileUnit *ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
     Direction direction, Fortran::common::optional<bool>,
-    const Terminator &terminator) {
+    IoErrorHandler &handler) {
   if (direction != Direction::Output) {
     terminator.Crash("ExternalFileUnit only supports output IO");
   }
-  return New<ExternalFileUnit>{terminator}(unit).release();
+  return New<ExternalFileUnit>{handler}(unit).release();
 }
 
 ExternalFileUnit *ExternalFileUnit::LookUp(const char *, std::size_t) {

flang/runtime/unit.h

@@ -120,7 +120,7 @@ public:
       int unit, const Terminator &, bool &wasExtant);
   static RT_API_ATTRS ExternalFileUnit *LookUpOrCreateAnonymous(int unit,
       Direction, Fortran::common::optional<bool> isUnformatted,
-      const Terminator &);
+      IoErrorHandler &);
   static RT_API_ATTRS ExternalFileUnit *LookUp(
       const char *path, std::size_t pathLen);
   static RT_API_ATTRS ExternalFileUnit &CreateNew(int unit, const Terminator &);