From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37091) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Wi3hx-0006EA-IZ for qemu-devel@nongnu.org; Wed, 07 May 2014 11:24:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Wi3hr-00065y-DZ for qemu-devel@nongnu.org; Wed, 07 May 2014 11:24:33 -0400 Date: Wed, 7 May 2014 11:24:24 -0400 From: Luiz Capitulino Message-ID: <20140507112424.4727b05d@redhat.com> In-Reply-To: <1399427861-26440-1-git-send-email-famz@redhat.com> References: <1399427861-26440-1-git-send-email-famz@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v3] qapi: Document optional arguments' backwards compatibility List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng Cc: Kevin Wolf , qemu-trivial@nongnu.org, qemu-devel@nongnu.org, Markus Armbruster On Wed, 7 May 2014 09:57:41 +0800 Fam Zheng wrote: > From: Eric Blake > > Signed-off-by: Eric Blake > Signed-off-by: Fam Zheng Applied to the qmp branch, thanks. > > --- > v3: More text from Eric. > > Signed-off-by: Fam Zheng > --- > docs/qapi-code-gen.txt | 32 ++++++++++++++++++++++++++++---- > 1 file changed, 28 insertions(+), 4 deletions(-) > > diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt > index d78921f..a6cba0a 100644 > --- a/docs/qapi-code-gen.txt > +++ b/docs/qapi-code-gen.txt > @@ -49,10 +49,34 @@ example of a complex type is: > { 'type': 'MyType', > 'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } } > > -The use of '*' as a prefix to the name means the member is optional. Optional > -members should always be added to the end of the dictionary to preserve > -backwards compatibility. > - > +The use of '*' as a prefix to the name means the member is optional. > + > +The default initialization value of an optional argument should not be changed > +between versions of QEMU unless the new default maintains backward > +compatibility to the user-visible behavior of the old default. > + > +With proper documentation, this policy still allows some flexibility; for > +example, documenting that a default of 0 picks an optimal buffer size allows > +one release to declare the optimal size at 512 while another release declares > +the optimal size at 4096 - the user-visible behavior is not the bytes used by > +the buffer, but the fact that the buffer was optimal size. > + > +On input structures (only mentioned in the 'data' side of a command), changing > +from mandatory to optional is safe (older clients will supply the option, and > +newer clients can benefit from the default); changing from optional to > +mandatory is backwards incompatible (older clients may be omitting the option, > +and must continue to work). > + > +On output structures (only mentioned in the 'returns' side of a command), > +changing from mandatory to optional is in general unsafe (older clients may be > +expecting the field, and could crash if it is missing), although it can be done > +if the only way that the optional argument will be omitted is when it is > +triggered by the presence of a new input flag to the command that older clients > +don't know to send. Changing from optional to mandatory is safe. > + > +A structure that is used in both input and output of various commands > +must consider the backwards compatibility constraints of both directions > +of use. > > A complex type definition can specify another complex type as its base. > In this case, the fields of the base type are included as top-level fields