[PATCH 2 of 2] tags: make tags cache compatible with future split of filenode cache

[Gregory Szorc]

On Tue, Mar 24, 2015 at 9:46 PM, Gregory Szorc <gregory.szorc at gmail.com>
wrote:

> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1427258716 25200
> #      Tue Mar 24 21:45:16 2015 -0700
> # Node ID 314d22b3f6418885c3c395f57ce2b2a95e69b3c5
> # Parent  30988005a918c28154c77e98160bd6ea3a3b16a1
> tags: make tags cache compatible with future split of filenode cache
>
> Pierre-Yves has plans to establish separate, per-filter tags cache
> files.
>
> I have plans to establish a separate, shared cache file for .hgtags
> filenode data so we don't have redundant lookups of potentially
> expensive filenode data.
>
> This patch adds future-proofing to tags cache reading so per-filter
> tags cache can land in a way that doesn't make the transition to a
> separate .hgtags filenode cache painful.
>
> With this patch applied, clients are able to recognize the planned
> future format of the tags cache with external .hgtags filenode data.
> When support for that cache lands, clients can read existing .hgtags
> filenode data from the prior tags cache files and then write out the
> new file format without having to recompute .hgtags filenodes. For
> users of large repositories, this will potentially save minutes of
> wall time.
>
> Without this patch, we would have to create a new version of the tags
> cache files when rolling out the separate .hgtags filenode cache. This
> is because the file formats would need to be different. This is
> because we want the tags cache to record the tip rev and node for
> quick cache freshness checks. We can't simply add this data to an
> existing cache file because if an old client came along, it would
> choke when reading this data (the format would have to be different
> so it wouldn't be confused for a .hgtags filenode mapping of a single
> head).
>
> The assumption here is that per-filter tags caches will land shortly
> after this patch. If an old client talks to a future repository, that
> client is either going to:
>
> a) read the old .hg/cache/tags file
> b) read the per-filter tags cache
>
> The old/existing tags cache will *never* have external .hgtags
> filenodes, so it will always be able to read that file. (It will likely
> have to process a bunch of new heads, but that's the price for using an
> old client.)
>
> The new, per-filter tags cache *may* have external .hgtags filenodes.
> But, since the code for recognizing the declaration of external .hgtags
> (this patch) exists for all clients that support per-filter tags cache
> reading (which will land after this patch), all clients are guaranteed
> to parse per-filter tags cache files with or without external .hgtags
> filenodes. Thus, moving data to an external .hgtags filenodes cache
> can occur without backwards compatibility concerns and without
> potentially expensive recomputation of .hgtags filenodes values.
>
> diff --git a/mercurial/tags.py b/mercurial/tags.py
> --- a/mercurial/tags.py
> +++ b/mercurial/tags.py
> @@ -27,8 +27,16 @@ import time
>  # The first part consists of lines of the form:
>  #
>  #   <headrev> <headnode> [<hgtagsnode>]
>  #
> +# *OR* a line of the form:
> +#
> +#   "external" <tiprev> <tipnode>
> +#
> +# The first form is the historical method of storing the .hgtags filenode
> +# mapping inline. The second form (which is reserved for future use) uses
> +# a separate file for this data.
> +#
>  # <headrev> is an integer revision and <headnode> is a 40 character hex
>  # node for that changeset. These redundantly identify a repository
>  # head from the time the cache was written.
>  #
> @@ -262,8 +270,13 @@ def _readtagcache(ui, repo):
>          try:
>              for line in cachelines:
>                  if line == "\n":
>                      break
> +
> +                # Future version of cache encountered. Do nothing yet.
> +                if line.startswith("external "):
> +                    continue
> +
>                  line = line.split()
>                  cacherevs.append(int(line[0]))
>                  headnode = bin(line[1])
>                  cacheheads.append(headnode)
> diff --git a/tests/test-tags.t b/tests/test-tags.t
> --- a/tests/test-tags.t
> +++ b/tests/test-tags.t
> @@ -224,8 +224,33 @@ Dump cache:
>    bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
>    bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
>    78391a272241d70354aa14c874552cad6b51bb42 bar
>
> +External .hgtags filenode cache marker is handled
> +
> +  $ cat > .hg/cache/tags << EOF
> +  > external 4 0c192d7d5e6b78a714de54a2e9627952a877e25a
> +  >
> +  > bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
> +  > bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
> +  > 78391a272241d70354aa14c874552cad6b51bb42 bar
> +  > EOF
> +
> +  $ hg tags
> +  tip                                4:0c192d7d5e6b
> +  bar                                1:78391a272241
> +
> +We should get an old style cache again
> +
> +  $ cat .hg/cache/tags
> +  4 0c192d7d5e6b78a714de54a2e9627952a877e25a
> 0c04f2a8af31de17fab7422878ee5a2dadbc943d
> +  3 6fa450212aeb2a21ed616a54aea39a4a27894cd7
> 7d3b718c964ef37b89e550ebdafd5789e76ce1b0
> +  2 7a94127795a33c10a370c93f731fd9fea0b79af6
> 0c04f2a8af31de17fab7422878ee5a2dadbc943d
> +
> +  bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
> +  bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
> +  78391a272241d70354aa14c874552cad6b51bb42 bar
> +
>  Test tag removal:
>
>    $ hg tag --remove bar     # rev 5
>    $ hg tip -vp
>

Please disregard this patch: I've got a new series consolidating
Pierre-Yves's and my work. This patch will be rolled into that one.
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