libdpkg: Use a varbuf for the field_state value

This should reduce the amount of m_realloc() calls done when parsing,
as the varbuf keeps track of the memory allocated and does not alternate
reducing and growing the buffer unnecessarily.

It might allow in the future to avoid copying around the field value
string just to be able to append a NUL character.

lib/dpkg/parse.c

@@ -86,7 +86,7 @@ const struct fieldinfo fieldinfos[]= {
 struct field_state {
   const char *fieldstart;
   const char *valuestart;
-  char *value;
+  struct varbuf value;
   int fieldlen;
   int valuelen;
   int fieldencountered[array_count(fieldinfos)];
@@ -120,11 +120,11 @@ pkg_parse_field(struct parsedb_state *ps, struct field_state *fs,
       parse_error(ps, pkg,
                   _("duplicate value for `%s' field"), fip->name);
 
-    fs->value = m_realloc(fs->value, fs->valuelen + 1);
-    memcpy(fs->value, fs->valuestart, fs->valuelen);
-    *(fs->value + fs->valuelen) = '\0';
+    varbuf_reset(&fs->value);
+    varbuf_add_buf(&fs->value, fs->valuestart, fs->valuelen);
+    varbuf_add_char(&fs->value, '\0');
 
-    fip->rcall(pkg, pkgbin, ps, fs->value, fip);
+    fip->rcall(pkg, pkgbin, ps, fs->value.buf, fip);
   } else {
     struct arbitraryfield *arp, **larpp;
 
@@ -448,7 +448,7 @@ int parsedb(const char *filename, enum parsedbflags flags,
     free(data);
 #endif
   }
-  free(fs.value);
+  varbuf_destroy(&fs.value);
   pop_cleanup(ehflag_normaltidy);
   if (close(fd)) ohshite(_("failed to close after read: `%.255s'"),filename);
   if (donep && !pdone) ohshit(_("no package information in `%.255s'"),filename);