revlog: move the details of revlog "v2" index inside revlog.utils.constants

revlog: move the details of revlog "v2" index inside revlog.utils.constants

the revlog module is quite large and this kind of format information would handy
for other module. So let us start to gather this information about the format in
a more appropriate place.

We update various reference to this information to use the new "source of truth"
in the process.

Differential Revision: https://phab.mercurial-scm.org/D10305

--- a/mercurial/pure/parsers.py	Mon Apr 05 12:21:01 2021 +0200
+++ b/mercurial/pure/parsers.py	Mon Apr 05 12:21:12 2021 +0200
@@ -243,21 +243,8 @@
 
 
 class Index2Mixin(object):
-    #  6 bytes: offset
-    #  2 bytes: flags
-    #  4 bytes: compressed length
-    #  4 bytes: uncompressed length
-    #  4 bytes: base rev
-    #  4 bytes: link rev
-    #  4 bytes: parent 1 rev
-    #  4 bytes: parent 2 rev
-    # 32 bytes: nodeid
-    #  8 bytes: sidedata offset
-    #  4 bytes: sidedata compressed length
-    #  20 bytes: Padding to align to 96 bytes (see RevlogV2Plan wiki page)
-    index_format = b">Qiiiiii20s12xQi20x"
-    index_size = struct.calcsize(index_format)
-    assert index_size == 96, index_size
+    index_format = revlog_constants.INDEX_ENTRY_V2.format
+    index_size = revlog_constants.INDEX_ENTRY_V2.size
     null_item = (0, 0, 0, -1, -1, -1, -1, nullid, 0, 0)
 
     def replace_sidedata_info(self, i, sidedata_offset, sidedata_length):

--- a/mercurial/revlog.py	Mon Apr 05 12:21:01 2021 +0200
+++ b/mercurial/revlog.py	Mon Apr 05 12:21:12 2021 +0200
@@ -43,6 +43,7 @@
     FLAG_INLINE_DATA,
     INDEX_ENTRY_V0,
     INDEX_ENTRY_V1,
+    INDEX_ENTRY_V2,
     REVLOGV0,
     REVLOGV1,
     REVLOGV1_FLAGS,
@@ -87,7 +88,6 @@
     storageutil,
     stringutil,
 )
-from .pure import parsers as pureparsers
 
 # blanked usage of all the name to prevent pyflakes constraints
 # We need these name available in the module for extensions.
@@ -352,20 +352,16 @@
         return p
 
 
-indexformatv2 = struct.Struct(pureparsers.Index2Mixin.index_format)
-indexformatv2_pack = indexformatv2.pack
-
-
 class revlogv2io(object):
     def __init__(self):
-        self.size = indexformatv2.size
+        self.size = INDEX_ENTRY_V2.size
 
     def parseindex(self, data, inline):
         index, cache = parsers.parse_index2(data, inline, revlogv2=True)
         return index, cache
 
     def packentry(self, entry, node, version, rev):
-        p = indexformatv2_pack(*entry)
+        p = INDEX_ENTRY_V2.pack(*entry)
         if rev == 0:
             p = versionformat_pack(version) + p[4:]
         return p

--- a/mercurial/revlogutils/constants.py	Mon Apr 05 12:21:01 2021 +0200
+++ b/mercurial/revlogutils/constants.py	Mon Apr 05 12:21:12 2021 +0200
@@ -57,6 +57,21 @@
 INDEX_ENTRY_V1 = struct.Struct(b">Qiiiiii20s12x")
 assert INDEX_ENTRY_V1.size == 32 * 2
 
+#  6 bytes: offset
+#  2 bytes: flags
+#  4 bytes: compressed length
+#  4 bytes: uncompressed length
+#  4 bytes: base rev
+#  4 bytes: link rev
+#  4 bytes: parent 1 rev
+#  4 bytes: parent 2 rev
+# 32 bytes: nodeid
+#  8 bytes: sidedata offset
+#  4 bytes: sidedata compressed length
+#  20 bytes: Padding to align to 96 bytes (see RevlogV2Plan wiki page)
+INDEX_ENTRY_V2 = struct.Struct(b">Qiiiiii20s12xQi20x")
+assert INDEX_ENTRY_V2.size == 32 * 3
+
 # revlog index flags
 
 # For historical reasons, revlog's internal flags were exposed via the