storage: read/write sanity checks

related: #4110
supersedes: #4528

electrum/storage.py

@@ -50,6 +50,9 @@ class StorageEncryptionVersion(IntEnum):
     XPUB_PASSWORD = 2
 
 
+class StorageReadWriteError(Exception): pass
+
+
 class WalletStorage(Logger):
 
     def __init__(self, path, *, manual_upgrades=False):
@@ -61,7 +64,7 @@ class WalletStorage(Logger):
         DB_Class = JsonDB
         self.logger.info(f"wallet path {self.path}")
         self.pubkey = None
-        # TODO we should test r/w permissions here (whether file exists or not)
+        self._test_read_write_permissions(self.path)
         if self.file_exists():
             with open(self.path, "r", encoding='utf-8') as f:
                 self.raw = f.read()
@@ -74,6 +77,27 @@ class WalletStorage(Logger):
             # avoid new wallets getting 'upgraded'
             self.db = DB_Class('', manual_upgrades=False)
 
+    @classmethod
+    def _test_read_write_permissions(cls, path):
+        # 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, "r", encoding='utf-8') 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 StorageReadWriteError(e) from e
+        if echo != echo2:
+            raise StorageReadWriteError('echo sanity-check failed')
 
     def load_plugins(self):
         wallet_type = self.db.get('wallet_type')