sql: test read-write permissions for given path and raise early

maybe fix #6485

electrum/util.py

@@ -1465,3 +1465,25 @@ def random_shuffled_copy(x: Iterable[T]) -> List[T]:
     x_copy = list(x)  # copy
     random.shuffle(x_copy)  # shuffle in-place
     return x_copy
+
+
+def test_read_write_permissions(path) -> None:
+    # note: There might already be a file at 'path'.
+    #       Make sure we do NOT overwrite/corrupt that!
+    temp_path = "%s.tmptest.%s" % (path, os.getpid())
+    echo = "fs r/w test"
+    try:
+        # test READ permissions for actual path
+        if os.path.exists(path):
+            with open(path, "rb") as f:
+                f.read(1)  # read 1 byte
+        # test R/W sanity for "similar" path
+        with open(temp_path, "w", encoding='utf-8') as f:
+            f.write(echo)
+        with open(temp_path, "r", encoding='utf-8') as f:
+            echo2 = f.read()
+        os.remove(temp_path)
+    except Exception as e:
+        raise IOError(e) from e
+    if echo != echo2:
+        raise IOError('echo sanity-check failed')