[PATCH 1 of 4 tags-cache-split] tags: make tags cache compatible with future split of filenode cache

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1427258716 25200
#      Tue Mar 24 21:45:16 2015 -0700
# Node ID 31716032c6e7f8764a02b212dac872cc3436c052
# Parent  2cb6b78cf818a082ee9a2518968f88c1819efed0
tags: make tags cache compatible with future split of filenode cache

Upcoming patches will split the tags cache into separate, per
filter files as well as split the .hgtags filenode cache into a
standalone file. Doing this atomically in one commit would be a
massive, difficult-to-comprehend patch.

This patch adds future-proofing to tags cache reading so this work
can be split across multiple commits.

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.

The assumption with this commit is that it will land immediately
before per-filter tags cache files are introduced and per-filter tags
cache files will land before the .hgtags filenode split is performed.

The patches landing in this order effectively constitute a new
version of the tags cache since per-filter files introduce new files,
which can include new processing rules.

Assuming this patch lands first and the new external .hgtags
filenodes syntax doesn't land until per-filter files are introduced,
clients that know to read the per-filter tags files are guaranteed to
understand the external .hgtags filenodes syntax. So no backwards
compatibility concern should arise.

This patch could potentially be reordered and merged with the later
patch that introduces the external .hgtags filenodes cache.

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 single 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.
 #
@@ -271,8 +279,13 @@ 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
@@ -271,8 +271,33 @@ Dump cache:
   bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
   bbd179dfa0a71671c253b3ae0aa1513b60d199fa bar
   78391a272241d70354aa14c874552cad6b51bb42 bar
 
+External .hgtags filenode cache marker is handled
+
+  $ cat > .hg/cache/tags << EOF
+  > external 4 0c192d7d5e6b78a714de54a2e9627952a877e25a 2e21d3312350ce63785cda82526c951211e76bab
+  > 
+  > 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