Not really fixed, but I have a verbose error message for file creation...

Not really fixed, but I have a verbose error message for file creation failures which happen under windows.

mc_work/dorun.go

@@ -4,8 +4,10 @@ package mcwork
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"math"
 	"math/rand"
 	"os"
@@ -62,6 +64,43 @@ func isNotSane(mcPrm *McPrm) error {
 	return nil
 }
 
+// fExists returns true if the file exists.
+// It is part of the exercise, working around file permissions on windows
+func fExists (name string) bool {
+	var err error
+	if _, err := os.Stat (name); err == nil {
+		return true
+	}
+	if errors.Is (err, fs.ErrNotExist) {
+		return false
+	}
+	return true
+}
+
+
+// createFix is a wrapper around os.Create() which I seem to need for windows.
+// On windows, I cannot overwrite an existing file. To be very exact, the
+// O_CREATE fails if a file exists. O_TRUNC also fails.
+func createFix (name string) (*os.File, error) {
+	var rErr = `Remove the file %s and try again. This only happens on windows: %w`
+	var f *os.File
+	var err error
+	if f, err = os.Create(name); err == nil {
+		return f, nil // Most common path (no problem)
+	}
+	
+	if !fExists(name) { // does not exist, but we have a real error.
+		return nil, err // Return original error
+	} // Next step try to remove original file
+    if e := os.Remove (name); e != nil {
+		return nil, fmt.Errorf (rErr, name, e)
+	}
+	if f, err = os.Create(name); err == nil { // second attempt, after removing name
+		return f, nil
+	}
+	return nil, err
+}
+
 // setupRun does things like get the cooling rate, seed the random numbers.
 func setupRun(mcPrm *McPrm, cprm *cprm) error {
 	var err error
@@ -83,7 +122,7 @@ func setupRun(mcPrm *McPrm, cprm *cprm) error {
 		if mcPrm.fOutName, err = setSuffix(mcPrm.fOutName, ".csv"); err != nil {
 			return err
 		}
-		if cprm.fOutRaw, err = os.Create(mcPrm.fOutName); err != nil {
+		if cprm.fOutRaw, err = createFix(mcPrm.fOutName); err != nil {
 			return err
 		}
 		cprm.fOut = bufio.NewWriter(cprm.fOutRaw)