sidedata: introduce a new requirement to protect the feature

The feature will be usable only within repository supported by version of
Mercurial with the support.

Since the support is not fully implemented yet, the requirement has a temporary,
experimental name.

Differential Revision: https://phab.mercurial-scm.org/D6886

mercurial/configitems.py

@@ -733,6 +733,10 b" coreconfigitem('format', 'usegeneraldelt"
 coreconfigitem('format', 'usestore',
     default=True,
 )
+coreconfigitem('format', 'use-side-data',
+    default=False,
+    experimental=True,
+)
 coreconfigitem('format', 'internal-phase',
     default=False,
     experimental=True,

mercurial/localrepo.py

@@ -394,6 +394,10 b" REVLOGV2_REQUIREMENT = 'exp-revlogv2.1'"
 # This is why once a repository has enabled sparse-read, it becomes required.
 SPARSEREVLOG_REQUIREMENT = 'sparserevlog'
 
+# A repository with the sidedataflag requirement will allow to store extra
+# information for revision without altering their original hashes.
+SIDEDATA_REQUIREMENT = 'exp-sidedata-flag'
+
 # Functions receiving (ui, features) that extensions can register to impact
 # the ability to load repositories with custom requirements. Only
 # functions defined in loaded extensions are called.
@@ -814,6 +818,9 b' def resolverevlogstorevfsoptions(ui, req'
     if sparserevlog:
         options[b'generaldelta'] = True
 
+    sidedata = SIDEDATA_REQUIREMENT in requirements
+    options[b'side-data'] = sidedata
+
     maxchainlen = None
     if sparserevlog:
         maxchainlen = revlogconst.SPARSE_REVLOG_MAX_CHAIN_LENGTH
@@ -917,6 +924,7 b' class localrepository(object):'
         'generaldelta',
         'treemanifest',
         REVLOGV2_REQUIREMENT,
+        SIDEDATA_REQUIREMENT,
         SPARSEREVLOG_REQUIREMENT,
         bookmarks.BOOKMARKS_IN_STORE_REQUIREMENT,
     }
@@ -3153,6 +3161,10 b' def newreporequirements(ui, createopts):'
         requirements.add('generaldelta')
         if ui.configbool('format', 'sparse-revlog'):
             requirements.add(SPARSEREVLOG_REQUIREMENT)
+
+    # experimental config: format.use-side-data
+    if ui.configbool('format', 'use-side-data'):
+        requirements.add(SIDEDATA_REQUIREMENT)
     if ui.configbool('experimental', 'treemanifest'):
         requirements.add('treemanifest')
 

mercurial/revlog.py

@@ -388,6 +388,7 b' class revlog(object):'
             self._maxdeltachainspan = opts['maxdeltachainspan']
         if self._mmaplargeindex and 'mmapindexthreshold' in opts:
             mmapindexthreshold = opts['mmapindexthreshold']
+        self.hassidedata = bool(opts.get('side-data', False))
         self._sparserevlog = bool(opts.get('sparse-revlog', False))
         withsparseread = bool(opts.get('with-sparse-read', False))
         # sparse-revlog forces sparse-read
@@ -1849,6 +1850,10 b' class revlog(object):'
 
         if sidedata is None:
             sidedata = {}
+        elif not self.hassidedata:
+            raise error.ProgrammingError(
+                _("trying to add sidedata to a revlog who don't support them")
+                )
 
         if flags:
             node = node or self.hash(text, p1, p2)