qapi: introduce QAPISchemaIfCond.cgen()

Instead of building prepocessor conditions from a list of string, use
the result generated from QAPISchemaIfCond.cgen() and hide the
implementation details.

Note: this patch introduces a minor regression, generating a redundant
pair of parenthesis. This is mostly fixed in a later patch in this
series ("qapi: replace if condition list with dict [..]")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210804083105.97531-5-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/common.py

@@ -12,7 +12,12 @@
 # See the COPYING file in the top-level directory.
 
 import re
-from typing import Match, Optional, Sequence
+from typing import (
+    List,
+    Match,
+    Optional,
+    Union,
+)
 
 
 #: Magic string that gets removed along with all space to its right.
@@ -194,22 +199,26 @@ def guardend(name: str) -> str:
                  name=c_fname(name).upper())
 
 
-def gen_if(ifcond: Sequence[str]) -> str:
-    ret = ''
-    for ifc in ifcond:
-        ret += mcgen('''
+def cgen_ifcond(ifcond: Union[str, List[str]]) -> str:
+    if not ifcond:
+        return ''
+    return '(' + ') && ('.join(ifcond) + ')'
+
+
+def gen_if(cond: str) -> str:
+    if not cond:
+        return ''
+    return mcgen('''
 #if %(cond)s
-''', cond=ifc)
-    return ret
+''', cond=cond)
 
 
-def gen_endif(ifcond: Sequence[str]) -> str:
-    ret = ''
-    for ifc in reversed(ifcond):
-        ret += mcgen('''
+def gen_endif(cond: str) -> str:
+    if not cond:
+        return ''
+    return mcgen('''
 #endif /* %(cond)s */
-''', cond=ifc)
-    return ret
+''', cond=cond)
 
 
 def must_match(pattern: str, string: str) -> Match[str]: