[Scons-users] TempFileMunge conflicts with ccache

[Andrew C. Morrow]

In this case, I'm sort of just relaying word from someone who was trying to
build MongoDB this way. I don't use ccache, I use the SCons cache. But I
thought the assumption baked into Action was worth flagging.

On Wed, Dec 19, 2018 at 10:12 AM Mats Wichmann <mats at wichmann.us> wrote:

> On 12/19/18 8:03 AM, Andrew C. Morrow wrote:
> > While investigating this issue, I think I discovered a potentially more
> > serious issue. Evaluation of a build where CC is set to a multi token
> value
> > like ccache gcc will not rebuild files if the compiler is upgraded,
> because
> > CommandAction.get_implicit_deps only considers the first token of the
> > command line as an implicit dependency which must be checked for
> > up-to-date-ness. In the case of CC="ccache gcc" build, that would result
> in
> > a rebuild when the ccache binary changed, but not when the gcc binary
> > changed.
> >
> > I'm not sure if there is anything really that can be done about that, but
> > maybe some guard rails around setting tool variables to multi token
> values
> > would be a good idea?
> >
> >
> >
> > On Thu, Dec 6, 2018 at 12:14 PM Andrew C. Morrow <
> andrew.c.morrow at gmail.com>
> > wrote:
> >
> >>
> >> In the MongoDB build, we enable TempFileMunge for non-windows platforms,
> >> since sometimes we have truly astronomically long link lines which break
> >> even the linux command line length limitations.
> >>
> >> https://github.com/mongodb/mongo/blob/master/SConstruct#L1478-L1496
> >>
> >> A user reported that their builds failed when trying to use ccache:
> >> https://jira.mongodb.org/browse/SERVER-38389
> >>
> >> It appears that TempFileMunge just assumes that only the first token of
> >> the command is actually the "compiler", and ends up feeding the response
> >> file directly to ccache, rather than invoking, say `ccache
> >> g++ @/path/to/temp/file`.
> >>
> >> On the other hand, could this even work in principle? Is ccache smart
> >> enough to know to "see into" the response file in order to interact with
> >> its cache correctly? This CMake bug suggests not:
> >> https://github.com/kripken/emscripten/issues/4750
> >>
> >> I did steer the user towards using the SCons cache instead of ccache,
> and
> >> provided a workaround of setting MAXLINELENGTH=<huge-number>
> >>
> >> At the very least, if ccache and response files can't be mixed, perhaps
> >> TempFileMunge could include some guard rails here to bail out if it sees
> >> the first token to be ccache.
> >>
> >> Thoughts?
>
>
> Isn't the more normal usage model to let ccache masquerade as gcc, in
> which case there's only one compiler token value on the line?
>
> That is, from the manpage:
>
> SYNOPSIS
>        ...
>        ccache compiler [compiler options]
>        compiler [compiler options]                   (via symbolic link)
>
> use the second form?  that doesn't necessarily excuse scons not
> understanding about the other scenario, but could provide a way forward.
>
>
>
>
>
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