[flang][runtime] Don't use endfile record number for EOF detection on… (#74640)

… input

The current EOF detection method (IsAtEOF()) depends on comparing the
current record number with the record number of the endfile record, if
it is known, which it is for units that have been written and then
rewound for input. For formatted input, this is wrong in the case of a
unit written with in-band newline characters. Rather than scan output
data to count newlines, it's best to just organically determine EOF by
detecting a failed or short read(), as we would have done anyway had the
endfile record number not been known. (I considered resetting the
endfile record number at the point of a REWIND, but not all rewinds are
followed by input; it seems wiser to defer the resetting until an actual
READ takes place.)

Fixes llvm-test-suite/Fortran/gfortran/regression/backslash_2.f90

Author: klausler

flang/runtime/unit.cpp

@@ -440,6 +440,14 @@ bool ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
   RUNTIME_CHECK(handler, direction_ == Direction::Input);
   if (!beganReadingRecord_) {
     beganReadingRecord_ = true;
+    // Don't use IsAtEOF() to check for an EOF condition here, just detect
+    // it from a failed or short read from the file.  IsAtEOF() could be
+    // wrong for formatted input if actual newline characters had been
+    // written in-band by previous WRITEs before a REWIND.  In fact,
+    // now that we know that the unit is being used for input (again),
+    // it's best to reset endfileRecordNumber and ensure IsAtEOF() will
+    // now be true on return only if it gets set by HitEndOnRead().
+    endfileRecordNumber.reset();
     if (access == Access::Direct) {
       CheckDirectAccess(handler);
       auto need{static_cast<std::size_t>(recordOffsetInFrame_ + *openRecl)};
@@ -452,17 +460,13 @@ bool ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
       }
     } else {
       recordLength.reset();
-      if (IsAtEOF()) {
-        handler.SignalEnd();
-      } else {
-        RUNTIME_CHECK(handler, isUnformatted.has_value());
-        if (*isUnformatted) {
-          if (access == Access::Sequential) {
-            BeginSequentialVariableUnformattedInputRecord(handler);
-          }
-        } else { // formatted sequential or stream
-          BeginVariableFormattedInputRecord(handler);
+      RUNTIME_CHECK(handler, isUnformatted.has_value());
+      if (*isUnformatted) {
+        if (access == Access::Sequential) {
+          BeginSequentialVariableUnformattedInputRecord(handler);
         }
+      } else { // formatted sequential or stream
+        BeginVariableFormattedInputRecord(handler);
       }
     }
   }
@@ -727,6 +731,7 @@ void ExternalFileUnit::EndIoStatement() {
 
 void ExternalFileUnit::BeginSequentialVariableUnformattedInputRecord(
     IoErrorHandler &handler) {
+  RUNTIME_CHECK(handler, access == Access::Sequential);
   std::int32_t header{0}, footer{0};
   std::size_t need{recordOffsetInFrame_ + sizeof header};
   std::size_t got{ReadFrame(frameOffsetInFile_, need, handler)};