[OE-core] [PATCH] ccache: Disable CCACHE_HASHDIR by default

[Mike Crowe]

On Monday 13 March 2017 at 13:51:46 +0000, Mike Crowe wrote:
> On Monday 13 March 2017 at 13:33:27 +0000, Burton, Ross wrote:
> > On 11 March 2017 at 16:54, Mike Crowe <mac at mcrowe.com> wrote:
> > 
> > > ccache apparently does this so that paths in the debug information will
> > > always be correct. In an OE world these paths may already be missing or
> > > incorrect due to rm_work or the use of a shared sstate cache, so it doesn't
> > > seem as if we're losing much by disabling this feature.
> > >
> > 
> > In an OE world we tell GCC to rewrite them to be target paths anyway, so
> > this isn't a problem.  Maybe worth rewriting the commit message?
> > 
> > (see bitbake.conf, -fdebug-prefix-map)
> 
> I wasn't aware of that.
> 
> ccache does have some technology to detect this situation:
> 
>   Exception: The CWD will not be included in the hash if *base_dir* is set
>   (and matches the CWD) and the compiler option *-fdebug-prefix-map* is
>   used.
> 
> I think this means that if CCACHE_BASEDIR is set appropriately then it
> wouldn't be necessary to set CCACHE_NOHASHDIR. (Looking at the ccache code,
> I think that "matches the CWD" means "CWD is under *base_dir*" rather than
> the two needing to be identical.)
> 
> I shall investigate why things weren't working correctly for us. In the
> meantime I don't think my patch is yet proven to be doing the right thing.

Unfortunately ccache only supports this special behaviour for the last
-fdebug-prefix-map argument. All the arguments make it through to the
compiler, but the last one isn't enough to stop the current directory being
hashed by default.

I've raised this as https://github.com/ccache/ccache/issues/163 .

It seems that for the time being setting CCACHE_NOHASHDIR is required to
make ccache effective for us. I'll reword the commit message based on
Ross's input and this information.

Thanks.

Mike.