[flang] Defer "next input record" processing until handlers established

External input was detecting "end of file" conditions in
BeginExternal...Input() and BeginUnformattedInput() routines
before EnableHandlers() could have been called.  Defer the
"start next record" processing to the input data item
handlers (and EndIoStatement() for when there are no data
items).

Differential Revision: https://reviews.llvm.org/D85161

flang/runtime/io-api.cpp

@@ -149,9 +149,6 @@ Cookie BeginExternalListIO(
   unit.SetDirection(DIR, handler);
   IoStatementState &io{unit.BeginIoStatement<ExternalListIoStatementState<DIR>>(
       unit, sourceFile, sourceLine)};
-  if constexpr (DIR == Direction::Input) {
-    unit.BeginReadingRecord(handler);
-  }
   return &io;
 }
 
@@ -185,9 +182,6 @@ Cookie BeginExternalFormattedIO(const char *format, std::size_t formatLength,
   IoStatementState &io{
       unit.BeginIoStatement<ExternalFormattedIoStatementState<DIR>>(
           unit, format, formatLength, sourceFile, sourceLine)};
-  if constexpr (DIR == Direction::Input) {
-    unit.BeginReadingRecord(handler);
-  }
   return &io;
 }
 
@@ -218,9 +212,7 @@ Cookie BeginUnformattedIO(
       unit, sourceFile, sourceLine)};
   IoErrorHandler handler{terminator};
   unit.SetDirection(DIR, handler);
-  if constexpr (DIR == Direction::Input) {
-    unit.BeginReadingRecord(handler);
-  } else {
+  if constexpr (DIR == Direction::Output) {
     if (unit.access == Access::Sequential && !unit.isFixedRecordLength) {
       // Create space for (sub)record header to be completed by
       // UnformattedIoStatementState<Direction::Output>::EndIoStatement()
@@ -838,6 +830,7 @@ bool IONAME(OutputDescriptor)(Cookie cookie, const Descriptor &) {
 
 bool IONAME(InputDescriptor)(Cookie cookie, const Descriptor &) {
   IoStatementState &io{*cookie};
+  io.BeginReadingRecord();
   io.GetIoErrorHandler().Crash("InputDescriptor: not yet implemented"); // TODO
 }
 
@@ -855,6 +848,7 @@ bool IONAME(OutputUnformattedBlock)(Cookie cookie, const char *x,
 bool IONAME(InputUnformattedBlock)(
     Cookie cookie, char *x, std::size_t length, std::size_t elementBytes) {
   IoStatementState &io{*cookie};
+  io.BeginReadingRecord();
   if (auto *unf{io.get_if<UnformattedIoStatementState<Direction::Input>>()}) {
     return unf->Receive(x, length, elementBytes);
   }
@@ -883,6 +877,7 @@ bool IONAME(InputInteger)(Cookie cookie, std::int64_t &n, int kind) {
         "InputInteger64() called for a non-input I/O statement");
     return false;
   }
+  io.BeginReadingRecord();
   if (auto edit{io.GetNextDataEdit()}) {
     if (edit->descriptor == DataEdit::ListDirectedNullValue) {
       return true;
@@ -922,6 +917,7 @@ static bool InputReal(Cookie cookie, REAL &x) {
         "InputReal() called for a non-input I/O statement");
     return false;
   }
+  io.BeginReadingRecord();
   if (auto edit{io.GetNextDataEdit()}) {
     if (edit->descriptor == DataEdit::ListDirectedNullValue) {
       return true;
@@ -968,6 +964,7 @@ static bool InputComplex(Cookie cookie, REAL x[2]) {
         "InputComplex() called for a non-input I/O statement");
     return false;
   }
+  io.BeginReadingRecord();
   for (int j{0}; j < 2; ++j) {
     if (auto edit{io.GetNextDataEdit()}) {
       if (edit->descriptor == DataEdit::ListDirectedNullValue) {
@@ -1012,6 +1009,7 @@ bool IONAME(InputAscii)(Cookie cookie, char *x, std::size_t length) {
         "InputAscii() called for a non-input I/O statement");
     return false;
   }
+  io.BeginReadingRecord();
   if (auto edit{io.GetNextDataEdit()}) {
     if (edit->descriptor == DataEdit::ListDirectedNullValue) {
       return true;
@@ -1044,6 +1042,7 @@ bool IONAME(InputLogical)(Cookie cookie, bool &truth) {
         "InputLogical() called for a non-input I/O statement");
     return false;
   }
+  io.BeginReadingRecord();
   if (auto edit{io.GetNextDataEdit()}) {
     if (edit->descriptor == DataEdit::ListDirectedNullValue) {
       return true;

flang/runtime/io-stmt.cpp

@@ -185,7 +185,10 @@ int NoopCloseStatementState::EndIoStatement() {
 }
 
 template <Direction DIR> int ExternalIoStatementState<DIR>::EndIoStatement() {
-  if (!unit().nonAdvancing) {
+  if constexpr (DIR == Direction::Input) {
+    BeginReadingRecord(); // in case of READ with no data items
+  }
+  if (!unit().nonAdvancing && GetIoStat() != IostatEnd) {
     unit().AdvanceRecord(*this);
   }
   if constexpr (DIR == Direction::Output) {
@@ -260,6 +263,16 @@ void ExternalIoStatementState<DIR>::HandleRelativePosition(std::int64_t n) {
   return unit().HandleRelativePosition(n);
 }
 
+template <Direction DIR>
+void ExternalIoStatementState<DIR>::BeginReadingRecord() {
+  if constexpr (DIR == Direction::Input) {
+    if (!beganReading_) {
+      beganReading_ = true;
+      unit().BeginReadingRecord(*this);
+    }
+  }
+}
+
 template <Direction DIR, typename CHAR>
 ExternalFormattedIoStatementState<DIR, CHAR>::ExternalFormattedIoStatementState(
     ExternalFileUnit &unit, const CHAR *format, std::size_t formatLength,
@@ -315,6 +328,10 @@ MutableModes &IoStatementState::mutableModes() {
       [](auto &x) -> MutableModes & { return x.get().mutableModes(); }, u_);
 }
 
+void IoStatementState::BeginReadingRecord() {
+  std::visit([](auto &x) { return x.get().BeginReadingRecord(); }, u_);
+}
+
 IoErrorHandler &IoStatementState::GetIoErrorHandler() const {
   return std::visit(
       [](auto &x) -> IoErrorHandler & {

flang/runtime/io-stmt.h

@@ -59,6 +59,7 @@ public:
   IoErrorHandler &GetIoErrorHandler() const;
   ExternalFileUnit *GetExternalFileUnit() const; // null if internal unit
   MutableModes &mutableModes();
+  void BeginReadingRecord();
 
   // N.B.: this also works with base classes
   template <typename A> A *get_if() const {
@@ -108,6 +109,7 @@ struct IoStatementBase : public DefaultFormatControlCallbacks {
   int EndIoStatement();
   std::optional<DataEdit> GetNextDataEdit(IoStatementState &, int = 1);
   ExternalFileUnit *GetExternalFileUnit() const { return nullptr; }
+  void BeginReadingRecord() {}
 };
 
 struct InputStatementState {};
@@ -247,6 +249,10 @@ public:
   void BackspaceRecord();
   void HandleRelativePosition(std::int64_t);
   void HandleAbsolutePosition(std::int64_t);
+  void BeginReadingRecord();
+
+private:
+  bool beganReading_{false};
 };
 
 template <Direction DIR, typename CHAR>