Subject: Re: [PATCH 3/4] reftable/stack: register lockfiles during compaction

From: Patrick Steinhardt <[email-masked]>
Date: Wed, 6 Mar 2024 12:59:52 +0100

On Tue, Mar 05, 2024 at 05:30:48PM -0600, Justin Tobler wrote:
> On 24/03/04 12:10PM, Patrick Steinhardt wrote:
> > We do not register any of the locks we acquire when compacting the
> > reftable stack via our lockfiles interfaces. These locks will thus not
> > be released when Git gets killed.
> > 
> > Refactor the code to register locks as lockfiles.
> > 
> > Signed-off-by: Patrick Steinhardt <[email-masked]>
> > ---
> > ...
> > +	/*
> > +	 * Write the new "tables.list" contents with the compacted table we
> > +	 * have just written. In case the compacted table became empty we
> > +	 * simply skip writing it.
> > +	 */
> > +	for (i = 0; i < first; i++)
> > +		strbuf_addf(&tables_list_buf, "%s\n", st->readers[i]->name);
> > +	if (!is_empty_table)
> > +		strbuf_addf(&tables_list_buf, "%s\n", new_table_name.buf);
> 
> Something not really related to this patch, but I noticed and had a
> question about.
> 
> If I'm understanding this correctly, when a newly compacted table is
> empty, it becomes possible for a range of indexes to no longer exist 
> within the stack. If this occurs in the middle of the stack, future
> compaction will likely combine the tables on either side and restore the
> missing index range. If the empty table was at the end of the stack,
> would this effectly reset the max index to something lower for future
> tables written to the stack? If so, could this lead to issues with
> separate concurrent table writes?

Very good question indeed, but I think we should be fine here. This is
mostly because concurrent writers will notice when "tables.list" has
changed, and, if so, abort the transaction with an out-of-date error.

A few scenarios with concurrent processes, one process which compacts
the stack (C) and one which modifies it (M):

  - M acquires the lock before C compacts: M sees the whole stack and
    uses the latest update index to update it, resulting in a newly
    written table. When C then locks afterwards, it may decide to
    compact and drop some tables in the middle of the stack. This may
    lead to a gap in update indices, but this is fine.

  - M acquires the lock while C compacts: M sees the whole stack and
    uses the latest update index to update the stack. C then acquires
    the lock to write the merged tables, notices that its compacted
    tables still exist and are in the same order, and thus removes them.
    We now have a gap in update indices, but this is totally fine.

  - M acquires the lock after C compacts: M will refresh "tables.list"
    after it has acquired the lock itself. Thus, it won't ever see the
    now-dropped empty table.

M cannot write its table when C has the "tables.list" lock, so this
scenario cannot happen. In the same spirit, two Ms cannot race with each
other either as only one can have the "tables.list" lock, and the other
one would abort with an out-of-date error when it has subsequently
acquired the lock and found the "tables.list" contents to have been
updated concurrently.

> > ...  
> > diff --git a/reftable/system.h b/reftable/system.h
> > index 6b74a81514..5d8b6dede5 100644
> > --- a/reftable/system.h
> > +++ b/reftable/system.h
> > @@ -12,7 +12,9 @@ license that can be found in the LICENSE file or at
> >  /* This header glues the reftable library to the rest of Git */
> >  
> >  #include "git-compat-util.h"
> > +#include "lockfile.h"
> >  #include "strbuf.h"
> > +#include "tempfile.h"
> >  #include "hash-ll.h" /* hash ID, sizes.*/
> >  #include "dir.h" /* remove_dir_recursively, for tests.*/
> 
> Naive question, why do we include the headers in `system.h`? I assume
> this is because they are common? Are there other benefits to this
> indirection?

Well, "system.h" is supposedly the glue between the common Git codebase
and the reftable library, so all Git-specific headers should be added
here instead of being added individually to the respective files in the
library. Whether that is ultimately a sensible thing and whether it
really helps us all that much is a different question though.

Patrick