[flang][runtime] Control stream truncation via runtime environment

flang-rt/include/flang-rt/runtime/environment.h

@@ -73,6 +73,7 @@ struct ExecutionEnvironment {
   bool noStopMessage{false}; // NO_STOP_MESSAGE=1 inhibits "Fortran STOP"
   bool defaultUTF8{false}; // DEFAULT_UTF8
   bool checkPointerDeallocation{true}; // FORT_CHECK_POINTER_DEALLOCATION
+  bool truncateStream{true}; // FORT_TRUNCATE_STREAM
 
   enum InternalDebugging { WorkQueue = 1 };
   int internalDebugging{0}; // FLANG_RT_DEBUG

flang-rt/lib/runtime/environment.cpp

@@ -141,6 +141,17 @@ void ExecutionEnvironment::Configure(int ac, const char *av[],
     }
   }
 
+  if (auto *x{std::getenv("FORT_TRUNCATE_STREAM")}) {
+    char *end;
+    auto n{std::strtol(x, &end, 10)};
+    if (n >= 0 && n <= 1 && *end == '\0') {
+      truncateStream = n != 0;
+    } else {
+      std::fprintf(stderr,
+          "Fortran runtime: FORT_TRUNCATE_STREAM=%s is invalid; ignored\n", x);
+    }
+  }
+
   if (auto *x{std::getenv("NO_STOP_MESSAGE")}) {
     char *end;
     auto n{std::strtol(x, &end, 10)};

flang-rt/lib/runtime/unit.cpp

@@ -783,8 +783,11 @@ void ExternalFileUnit::DoEndfile(IoErrorHandler &handler) {
   frameOffsetInFile_ += recordOffsetInFrame_ + furthestPositionInRecord;
   recordOffsetInFrame_ = 0;
   FlushOutput(handler);
-  Truncate(frameOffsetInFile_, handler);
-  TruncateFrame(frameOffsetInFile_, handler);
+  if (access != Access::Stream || executionEnvironment.truncateStream) {
+    // Stream output after positioning truncates with some compilers.
+    Truncate(frameOffsetInFile_, handler);
+    TruncateFrame(frameOffsetInFile_, handler);
+  }
   BeginRecord();
   impliedEndfile_ = false;
   anyWriteSinceLastPositioning_ = false;

flang/docs/Extensions.md

@@ -946,6 +946,17 @@ print *, [(j,j=1,10)]
   This design allows format-driven input with `DT` editing to retain
   control over advancement in child input, while otherwise allowing it.
 
+* When output takes place to a file under `ACCESS="STREAM"` after
+  repositioning it to an earlier position, some compilers will
+  truncate the file; this behavior is similar to the implicit
+  `ENDFILE` that takes place under sequential output after a
+  `BACKSPACE` or `REWIND` statement.
+  Truncation of streams is not specified in the standard, however,
+  and it does not take place with all compilers.
+  In this one, truncation is optional; it occurs by default,
+  but it can be disabled via `FORT_TRUNCATE_STREAM=0` in the
+  environment at execution time.
+
 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the

flang/docs/RuntimeEnvironment.md

@@ -55,3 +55,14 @@ The default is 72.
 Set `NO_STOP_MESSAGE=1` to disable the extra information about
 IEEE floating-point exception flags that the Fortran language
 standard requires for `STOP` and `ERROR STOP` statements.
+
+## `FORT_TRUNCATE_STREAM`
+
+Set `FORT_TRUNCATE_STREAM=1` to make output to a formatted unit
+with `ACCESS="STREAM"` truncate the file when the unit has been
+repositioned via `POS=` to an earlier point in the file.
+This behavior is analogous to the implicit writing of an ENDFILE record
+when output takes place to a sequential unit after
+executing a `BACKSPACE` or `REWIND` statement.
+Truncation of a stream-access unit is common to several other
+compilers, but it is not mentioned in the standard.