libdpkg: Refactor common code into parse_error_msg()

This moves all the buffer size knowledge to the same function, and
easily allows to fix possible buffer overflows in case fmt is longer
than the destination buffer. It also nicely reduces slightly the
duplicated code.

lib/dpkg/parsehelp.c

@@ -32,16 +32,23 @@
 #include <dpkg/string.h>
 #include <dpkg/parsedump.h>
 
-static void
+static const char *
 parse_error_msg(struct parsedb_state *ps, const struct pkginfo *pigp,
-                char *buf)
+                const char *fmt)
 {
+  static char msg[1024];
+  char filename[256];
+
+  str_escape_fmt(filename, ps->filename, sizeof(filename));
+
   if (pigp && pigp->name)
-    sprintf(buf, _("parsing file '%.255s' near line %d package '%.255s':\n "),
-            ps->filename, ps->lno, pigp->name);
+    sprintf(msg, _("parsing file '%.255s' near line %d package '%.255s':\n"
+                   " %.255s"), filename, ps->lno, pigp->name, fmt);
   else
-    sprintf(buf, _("parsing file '%.255s' near line %d:\n "),
-            ps->filename, ps->lno);
+    sprintf(msg, _("parsing file '%.255s' near line %d:\n"
+                   " %.255s"), filename, ps->lno, fmt);
+
+  return msg;
 }
 
 void
@@ -49,14 +56,9 @@ parse_error(struct parsedb_state *ps,
             const struct pkginfo *pigp, const char *fmt, ...)
 {
   va_list args;
-  char buf1[768], buf2[1000], *q;
-
-  parse_error_msg(ps, pigp, buf1);
-  q = str_escape_fmt(buf2, buf1, sizeof(buf2));
-  strcat(q,fmt);
 
   va_start(args, fmt);
-  ohshitv(buf2, args);
+  ohshitv(parse_error_msg(ps, pigp, fmt), args);
 }
 
 void
@@ -64,15 +66,10 @@ parse_warn(struct parsedb_state *ps,
            const struct pkginfo *pigp, const char *fmt, ...)
 {
   va_list args;
-  char buf1[768], buf2[1000], *q;
-
-  parse_error_msg(ps, pigp, buf1);
-  q = str_escape_fmt(buf2, buf1, sizeof(buf2));
-  strcat(q, fmt);
 
   va_start(args, fmt);
   ps->warncount++;
-  warningv(buf2, args);
+  warningv(parse_error_msg(ps, pigp, fmt), args);
   va_end(args);
 }