[compat] Allow overriding by only COLUMNS or LINES in compat_get_terminal_size

Now the semantic of this function is identical to
shutil.get_terminal_size() in Python 3.3+. The new behavior also
corresponds to the old get_term_width(), which is removed in
003c69a84b

youtube_dl/compat.py

@@ -416,7 +416,7 @@ if hasattr(shutil, 'get_terminal_size'):  # Python >= 3.3
 else:
     _terminal_size = collections.namedtuple('terminal_size', ['columns', 'lines'])
 
-    def compat_get_terminal_size():
+    def compat_get_terminal_size(fallback=(80, 24)):
         columns = compat_getenv('COLUMNS', None)
         if columns:
             columns = int(columns)
@@ -428,14 +428,20 @@ else:
         else:
             lines = None
 
-        try:
-            sp = subprocess.Popen(
-                ['stty', 'size'],
-                stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-            out, err = sp.communicate()
-            lines, columns = map(int, out.split())
-        except Exception:
-            pass
+        if columns <= 0 or lines <= 0:
+            try:
+                sp = subprocess.Popen(
+                    ['stty', 'size'],
+                    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+                out, err = sp.communicate()
+                _columns, _lines = map(int, out.split())
+            except Exception:
+                _columns, _lines = _terminal_size(*fallback)
+
+            if columns <= 0:
+                columns = _columns
+            if lines <= 0:
+                lines = _lines
         return _terminal_size(columns, lines)
 
 try: