fix internal payment check (#1604)

lnbits/core/crud.py

@@ -597,6 +597,7 @@ async def check_internal(
 async def check_internal_pending(
     payment_hash: str, conn: Optional[Connection] = None
 ) -> bool:
+    """Returns False if the internal payment is not pending anymore (and thus paid), otherwise True"""
     row = await (conn or db).fetchone(
         """
         SELECT pending FROM apipayments
@@ -605,7 +606,7 @@ async def check_internal_pending(
         (payment_hash,),
     )
     if not row:
-        return False
+        return True
     else:
         return row["pending"]
 

lnbits/core/services.py

@@ -154,10 +154,11 @@ async def pay_invoice(
             extra=extra,
         )
 
+        # we check if an internal invoice exists that has already been paid (not pending anymore)
         if not await check_internal_pending(invoice.payment_hash, conn=conn):
             raise PaymentFailure("Internal invoice already paid.")
 
-        # check_internal() returns the checking_id of the invoice we're waiting for
+        # check_internal() returns the checking_id of the invoice we're waiting for (pending only)
         internal_checking_id = await check_internal(invoice.payment_hash, conn=conn)
         if internal_checking_id:
             logger.debug(f"creating temporary internal payment with id {internal_id}")