qapi/common.py: Add indent manager

Code style tools really dislike the use of global keywords, because it
generally involves re-binding the name at runtime which can have strange
effects depending on when and how that global name is referenced in
other modules.

Make a little indent level manager instead.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-12-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 36 insertions, 20 deletions

diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
index cee63eb..b35318b 100644
--- a/scripts/qapi/common.py
+++ b/scripts/qapi/common.py
@@ -93,33 +93,50 @@ eatspace = '\033EATSPACE.'
 pointer_suffix = ' *' + eatspace
 
 
-def genindent(count):
-    ret = ''
-    for _ in range(count):
-        ret += ' '
-    return ret
+class Indentation:
+    """
+    Indentation level management.
+
+    :param initial: Initial number of spaces, default 0.
+    """
+    def __init__(self, initial: int = 0) -> None:
+        self._level = initial
+
+    def __int__(self) -> int:
+        return self._level
+
+    def __repr__(self) -> str:
+        return "{}({:d})".format(type(self).__name__, self._level)
 
+    def __str__(self) -> str:
+        """Return the current indentation as a string of spaces."""
+        return ' ' * self._level
 
-indent_level = 0
+    def __bool__(self) -> bool:
+        """True when there is a non-zero indentation."""
+        return bool(self._level)
 
+    def increase(self, amount: int = 4) -> None:
+        """Increase the indentation level by ``amount``, default 4."""
+        self._level += amount
 
-def push_indent(indent_amount=4):
-    global indent_level
-    indent_level += indent_amount
+    def decrease(self, amount: int = 4) -> None:
+        """Decrease the indentation level by ``amount``, default 4."""
+        if self._level < amount:
+            raise ArithmeticError(
+                f"Can't remove {amount:d} spaces from {self!r}")
+        self._level -= amount
 
 
-def pop_indent(indent_amount=4):
-    global indent_level
-    indent_level -= indent_amount
+indent = Indentation()
 
 
 # Generate @code with @kwds interpolated.
-# Obey indent_level, and strip eatspace.
+# Obey indent, and strip eatspace.
 def cgen(code, **kwds):
     raw = code % kwds
-    if indent_level:
-        indent = genindent(indent_level)
-        raw = re.sub(r'^(?!(#|$))', indent, raw, flags=re.MULTILINE)
+    if indent:
+        raw = re.sub(r'^(?!(#|$))', str(indent), raw, flags=re.MULTILINE)
     return re.sub(re.escape(eatspace) + r' *', '', raw)
 
 
diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
index 9fdbe5b..708f72c 100644
--- a/scripts/qapi/visit.py
+++ b/scripts/qapi/visit.py
@@ -18,9 +18,8 @@ from .common import (
     c_name,
     gen_endif,
     gen_if,
+    indent,
     mcgen,
-    pop_indent,
-    push_indent,
 )
 from .gen import QAPISchemaModularCVisitor, ifcontext
 from .schema import QAPISchemaObjectType
@@ -69,7 +68,7 @@ bool visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
     if (visit_optional(v, "%(name)s", &obj->has_%(c_name)s)) {
 ''',
                          name=memb.name, c_name=c_name(memb.name))
-            push_indent()
+            indent.increase()
         ret += mcgen('''
     if (!visit_type_%(c_type)s(v, "%(name)s", &obj->%(c_name)s, errp)) {
         return false;
@@ -78,7 +77,7 @@ bool visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
                      c_type=memb.type.c_name(), name=memb.name,
                      c_name=c_name(memb.name))
         if memb.optional:
-            pop_indent()
+            indent.decrease()
             ret += mcgen('''
     }
 ''')