From: Ben Peart Date: Mon, 17 Jul 2017 20:09:17 GMT Subject: Re: [PATCH v2 1/1] sha1_file: Add support for downloading blobs on demand Message-ID: In-Reply-To: <20170717110602.6fac89ea@twelve2.svl.corp.google.com> On 7/17/2017 2:06 PM, Jonathan Tan wrote: > About the difference between this patch and my patch set [1], besides > the fact that this patch does not spawn separate processes for each > missing object, which does seem like an improvement to me, this patch > (i) does not use a list of promised objects (but instead communicates > with the hook for each missing object), and (ii) provides backwards > compatibility with other Git code (that does not know about promised > objects) in a different way. > > The costs and benefits of (i) are being discussed here [2]. As for (ii), > I still think that my approach is better - I have commented more about > this below. > > Maybe the best approach is a combination of both our approaches. Yes, in the context of the promised objects model patch series, the value of this patch series is primarily as a sample of how to use the sub-process mechanism to create a versioned interface for retrieving objects. > > [1] https://public-inbox.org/git/34efd9e9936fdab331655f5a33a098a72dc134f4.1499800530.git.jonathantanmy@google.com/ > > [2] https://public-inbox.org/git/20170713123951.5cab1adc@twelve2.svl.corp.google.com/ > > On Fri, 14 Jul 2017 09:26:51 -0400 > Ben Peart wrote: > >> +------------------------ >> +packet: git> command=get >> +packet: git> sha1=0a214a649e1b3d5011e14a3dc227753f2bd2be05 >> +packet: git> 0000 >> +------------------------ > > It would be useful to have this command support more than one SHA-1, so > that hooks that know how to batch can do so. > I agree. Since nothing was using that capability yet, I decided to keep it simple and not add support for a feature that wasn't being used. The reason the interface is versioned is so that if/when something does need that capability, it can be added. >> +static int subprocess_map_initialized; >> +static struct hashmap subprocess_map; > > The documentation of "tablesize" in "struct hashmap" states that it can > be used to check if the hashmap is initialized, so > subprocess_map_initialized is probably unnecessary. > Nice. That will make things a little simpler. >> static int check_and_freshen(const unsigned char *sha1, int freshen) >> { >> - return check_and_freshen_local(sha1, freshen) || >> - check_and_freshen_nonlocal(sha1, freshen); >> + int ret; >> + int already_retried = 0; >> + >> +retry: >> + ret = check_and_freshen_local(sha1, freshen) || >> + check_and_freshen_nonlocal(sha1, freshen); >> + if (!ret && core_virtualize_objects && !already_retried) { >> + already_retried = 1; >> + if (!read_object_process(sha1)) >> + goto retry; >> + } >> + >> + return ret; >> } > > Is this change meant to ensure that Git code that operates on loose > objects directly (bypassing storage-agnostic functions such as > sha1_object_info_extended() and has_sha1_file()) still work? If yes, > this patch appears incomplete (for example, read_loose_object() needs to > be changed too), and this seems like a difficult task - in my patch set > [1], I ended up deciding to create a separate type of storage and > instead looked at the code that operates on *packed* objects directly > (because there were fewer such methods) to ensure that they would work > correctly in the presence of a separate type of storage. > Yes, with this set of patches, we've been running successfully on completely sparse clones (no commits, trees, or blobs) for several months. read_loose_object() is only called by fsck when it is enumerating existing loose objects so does not need to be updated. We have a few thousand developers making ~100K commits per week so in our particular usage, I'm fairly confident it works correctly. That said, it is possible there is some code path I've missed. :) > [1] https://public-inbox.org/git/34efd9e9936fdab331655f5a33a098a72dc134f4.1499800530.git.jonathantanmy@google.com/ >