[PATCH] Verify source files for a module only once during the build

[Dmitri Gribenko]

On Fri, Feb 7, 2014 at 9:54 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> On Feb 7, 2014, at 1:46 PM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
>
>> On Fri, Feb 7, 2014 at 9:39 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
>>> On Feb 7, 2014, at 12:47 PM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
>>> I wanted to avoid the need to do the atomic-rename dance.
>>>
>>> What is you concern with it ?
>>
>> No real concerns, just a bit more code to write.
>>
>>> What is the
>>> potential for mtime confusion that you see?  We could provide a
>>> function in libclang to get the current timestamp so that clients
>>> don't have to invent their own, potentially incorrect way to get it.
>>>
>>>
>>> I really want to reduce complexity here and potential for out-of-sync,
>>> because now you have
>>>
>>>
>>> 1) The builder needs to provide an increasing timestamp by getting clock
>>> time (or libclang call ?)
>>> 2) we will compare that clock time with the file system modification time
>>> which can come from any kind of underlying file system
>>>
>>>
>>> vs
>>>
>>> 1) The builder needs to provide an increasing timestamp
>>>
>>>
>>> I much prefer the latter simpler approach.
>>
>> I can see how clients can break any of these while implementing (1) --
>> for example, by using the local time instead of UTC time, and having
>> the build happen when the DST adjustment is made.  But (2) is just an
>> OS-level thing, it can not go wrong.
>>
>> Also, imagine that we have a good client build system and a bad client
>> build system.  A good client uses correct timestamps, a bad client
>> uses timestamps + 1 billion.  Then after the bad client creates a
>> module, the good client will never rebuild it, because its timestamps
>> will always be "in the past”.
>
> A bad client will always be a problem but this is the responsibility of the builder, if the builder timestamps are self-consistent we don’t need to worry about any time changes or adjustments or what have you, it will not even need to be time based, we just don’t care.

A bad client will only create a problem for itself if clang uses
filesystem mtime on the timestamp file.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/