revlog: move the details of revlog "v0" index inside revlog.utils.constants

revlog: move the details of revlog "v0" 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.

Differential Revision: https://phab.mercurial-scm.org/D10303

--- a/mercurial/revlog.py	Mon Apr 05 12:20:40 2021 +0200
+++ b/mercurial/revlog.py	Mon Apr 05 12:20:52 2021 +0200
@@ -41,6 +41,7 @@
 from .revlogutils.constants import (
     FLAG_GENERALDELTA,
     FLAG_INLINE_DATA,
+    INDEX_ENTRY_V0,
     REVLOGV0,
     REVLOGV1,
     REVLOGV1_FLAGS,
@@ -218,19 +219,6 @@
     node = attr.ib(default=None)
 
 
-# index v0:
-#  4 bytes: offset
-#  4 bytes: compressed length
-#  4 bytes: base rev
-#  4 bytes: link rev
-# 20 bytes: parent 1 nodeid
-# 20 bytes: parent 2 nodeid
-# 20 bytes: nodeid
-indexformatv0 = struct.Struct(b">4l20s20s20s")
-indexformatv0_pack = indexformatv0.pack
-indexformatv0_unpack = indexformatv0.unpack
-
-
 class revlogoldindex(list):
     @property
     def nodemap(self):
@@ -284,7 +272,7 @@
 
 class revlogoldio(object):
     def __init__(self):
-        self.size = indexformatv0.size
+        self.size = INDEX_ENTRY_V0.size
 
     def parseindex(self, data, inline):
         s = self.size
@@ -295,7 +283,7 @@
         while off + s <= l:
             cur = data[off : off + s]
             off += s
-            e = indexformatv0_unpack(cur)
+            e = INDEX_ENTRY_V0.unpack(cur)
             # transform to revlogv1 format
             e2 = (
                 offset_type(e[0], 0),
@@ -315,6 +303,13 @@
         return index, None
 
     def packentry(self, entry, node, version, rev):
+        """return the binary representation of an entry
+
+        entry:   a tuple containing all the values (see index.__getitem__)
+        node:    a callback to convert a revision to nodeid
+        version: the changelog version
+        rev:     the revision number
+        """
         if gettype(entry[0]):
             raise error.RevlogError(
                 _(b'index entry flags need revlog version 1')
@@ -328,7 +323,7 @@
             node(entry[6]),
             entry[7],
         )
-        return indexformatv0_pack(*e2)
+        return INDEX_ENTRY_V0.pack(*e2)
 
 
 # index ng:

--- a/mercurial/revlogutils/constants.py	Mon Apr 05 12:20:40 2021 +0200
+++ b/mercurial/revlogutils/constants.py	Mon Apr 05 12:20:52 2021 +0200
@@ -9,6 +9,8 @@
 
 from __future__ import absolute_import
 
+import struct
+
 from ..interfaces import repository
 
 ### main revlog header
@@ -32,6 +34,16 @@
 
 ### individual entry
 
+## index v0:
+#  4 bytes: offset
+#  4 bytes: compressed length
+#  4 bytes: base rev
+#  4 bytes: link rev
+# 20 bytes: parent 1 nodeid
+# 20 bytes: parent 2 nodeid
+# 20 bytes: nodeid
+INDEX_ENTRY_V0 = struct.Struct(b">4l20s20s20s")
+
 # revlog index flags
 
 # For historical reasons, revlog's internal flags were exposed via the