pylibfdt/libfdt.i: fix backwards compatibility of return values

When our Python functions wrap `fdt_getprop()` they return a list
containing `[*data, length]`.

In SWIG v4.2 and earlier SWIG would discard `*data` if it is NULL/None.
Causing the return value to just be `length`.

But starting in SWIG v4.3 it no longer discards `*data`. So the return
value is now `[None, length]`.

Handle this compatibility issue in libfdt.i by checking if the return
value looks like the older 4.2 return value, and casting it to the newer
style.

See swig/swig#2907

Signed-off-by: Brandon Maier <[email protected]>
Signed-off-by: David Gibson <[email protected]>

pylibfdt/libfdt.i

@@ -114,11 +114,14 @@ def check_err_null(val, quiet=()):
         FdtException if val indicates an error was reported and the error
         is not in @quiet.
     """
-    # Normally a list is returned which contains the data and its length.
-    # If we get just an integer error code, it means the function failed.
+    # Compatibility for SWIG v4.2 and earlier. SWIG 4.2 would drop the first
+    # item from the list if it was None, returning only the second item.
     if not isinstance(val, list):
-        if -val not in quiet:
-            raise FdtException(val)
+        val = [None, val]
+
+    if val[0] is None:
+        if -val[1] not in quiet:
+            raise FdtException(val[1])
     return val
 
 class FdtRo(object):
@@ -395,8 +398,8 @@ class FdtRo(object):
         """
         pdata = check_err_null(
                 fdt_get_property_by_offset(self._fdt, prop_offset), quiet)
-        if isinstance(pdata, (int)):
-            return pdata
+        if pdata[0] is None:
+            return pdata[1]
         return Property(pdata[0], pdata[1])
 
     def getprop(self, nodeoffset, prop_name, quiet=()):
@@ -417,8 +420,8 @@ class FdtRo(object):
         """
         pdata = check_err_null(fdt_getprop(self._fdt, nodeoffset, prop_name),
                                quiet)
-        if isinstance(pdata, (int)):
-            return pdata
+        if pdata[0] is None:
+            return pdata[1]
         return Property(prop_name, bytearray(pdata[0]))
 
     def hasprop(self, nodeoffset, prop_name, quiet=()):
@@ -444,10 +447,10 @@ class FdtRo(object):
         """
         pdata = check_err_null(fdt_getprop(self._fdt, nodeoffset, prop_name),
                                quiet + (NOTFOUND,))
-        if isinstance(pdata, (int)):
-            if pdata == -NOTFOUND:
+        if pdata[0] is None:
+            if pdata[1] == -NOTFOUND:
                 return False
-            return pdata
+            return pdata[1]
         return True
 
     def get_phandle(self, nodeoffset):