[ccache] Caching failed compilations

[Joel Rosdahl]

(Resend since my original mail didn't reach the mailing list properly.)

(Sorry for the delayed reply, I have been on vacation.)

On cache-hit, there's currently no reason to actually look inside the file,
> right? It just does the copy blind (I forget exactly how). Reading the
> initial data from every binary on every cache-hit (the case we want to be
> most optimal) sounds like a Bad Thing.


No no, doing an extra read of initial data is not needed. If something I
wrote implied that I must have been unclear.

A cache hit in the current implementation (simplified, of course):

1. Stat object file in cache. If it exists, we have a hit.
2. Open file.
3. Read a chunk of the file into a buffer.
4. Write buffer content to the destination object file.
5. Repeat 3 and 4 until EOF.
6. Close file.

A cache hit in the suggested solution where special data (e.g. encoding the
exit code) is written to the cached "object file":

1. Stat object file in cache. If it exists, we have a hit.
2. Open file.
3. Read a chunk of the file into a buffer.
4. If the buffer contains special data (e.g. starts with a ccache-specific
header), exit with the encoded exit code (and write stderr, etc.). Else:
5. Write buffer content to the destination object file.
6. Repeat 3 and 5 until EOF.
7. Close file.

So, exactly the same system calls in both cases for a normal cache hit.
That's what I tried to summarize with "On a cache hit, we need to open and
read the file regardless of whether it's a real object file or special data
encoding an exit code".

The most common case must always be the "quick path" [...]


Heh, I certainly know that it's important not to slow down common cases
with e.g. more system calls. I didn't mean to ask you to explain what you
meant by the term "slow path" but why you thought that there *would be* a
slow path in the special-data-in-object-file solution. Again I must have
been unclear, sorry about that. (I'm a bit surprised that you felt the need
to explain basic stuff about what's important, to be honest.)

And as described above, I see no reason for why special-data-in-object-file
would be slower. Yes, the non-zero-exit-code case could perhaps be
optimized with a stat size hack, but that doesn't feel important.

I hope this is more clear. I'm beginning to lose faith in my English
communication skills. :-)

-- Joel