fix: only enforce O_EXCL on MemMapFs OpenFile when O_CREATE is set

Per POSIX, O_EXCL is meaningful only together with O_CREATE. MemMapFs
OpenFile returned ErrFileExists whenever O_EXCL was set on an existing
file, so reopening a file with O_RDWR|O_EXCL failed even though no
creation was requested. Guard the existence check with O_CREATE so that
O_EXCL without O_CREATE opens the existing file, matching os.OpenFile.

Fixes #415

memmap_test.go

@@ -135,6 +135,35 @@ func TestOpenFileExcl(t *testing.T) {
 	checkPathError(t, err, "Open")
 }
 
+// Ensure os.O_EXCL only rejects an existing file when os.O_CREATE is also
+// requested. Per POSIX, O_EXCL is meaningful only in combination with
+// O_CREATE, so opening an existing file with O_RDWR|O_EXCL must succeed.
+func TestOpenFileExclWithoutCreate(t *testing.T) {
+	const fileName = "/myExclNoCreate"
+	const fileMode = os.FileMode(0o644)
+
+	fs := NewMemMapFs()
+
+	// Create the file up front.
+	f, err := fs.OpenFile(fileName, os.O_CREATE|os.O_RDWR|os.O_EXCL, fileMode)
+	if err != nil {
+		t.Fatalf("initial create failed: %s", err)
+	}
+	f.Close()
+
+	// Reopening with O_EXCL but without O_CREATE must succeed.
+	f, err = fs.OpenFile(fileName, os.O_RDWR|os.O_EXCL, fileMode)
+	if err != nil {
+		t.Fatalf("OpenFile(O_RDWR|O_EXCL) on existing file should succeed, got: %s", err)
+	}
+	f.Close()
+
+	// O_CREATE|O_EXCL on an existing file must still fail.
+	if _, err := fs.OpenFile(fileName, os.O_CREATE|os.O_RDWR|os.O_EXCL, fileMode); err == nil {
+		t.Fatalf("OpenFile(O_CREATE|O_EXCL) on existing file should fail, but it didn't")
+	}
+}
+
 // Ensure Permissions are set on OpenFile/Mkdir/MkdirAll
 func TestPermSet(t *testing.T) {
 	const fileName = "/myFileTest"