sparse: reliably avoid writing to store without a lock

With the code as written before this patch we can still end up writing to
store in `debugsparse`. Obviously we'll write to it if by accident a store
requirement is modified, but more importantly we write to it if another
concurrent transaction modifies the requirements file on disk.

We can't rule this out since we're not holding the store lock,
so it's better to explicitly pass a permission to write instead
of inferring it based on file contents.

mercurial/scmutil.py

@@ -1668,7 +1668,7 def istreemanifest(repo) -> bool:
     return requirementsmod.TREEMANIFEST_REQUIREMENT in repo.requirements
 
 
-def writereporequirements(repo, requirements=None) -> None:
+def writereporequirements(repo, requirements=None, maywritestore=True) -> None:
     """writes requirements for the repo
 
     Requirements are written to .hg/requires and .hg/store/requires based
@@ -1681,10 +1681,11 def writereporequirements(repo, requirem
     if wcreq is not None:
         writerequires(repo.vfs, wcreq)
     if storereq is not None:
-        writerequires(repo.svfs, storereq)
+        writerequires(repo.svfs, storereq, maywrite=maywritestore)
     elif repo.ui.configbool(b'format', b'usestore'):
         # only remove store requires if we are using store
-        repo.svfs.tryunlink(b'requires')
+        if maywritestore:
+            repo.svfs.tryunlink(b'requires')
 
 
 def readrequires(vfs, allowmissing):
@@ -1701,9 +1702,11 def readrequires(vfs, allowmissing):
     return set(read(b'requires').splitlines())
 
 
-def writerequires(opener, requirements) -> None:
+def writerequires(opener, requirements, maywrite=True) -> None:
     on_disk = readrequires(opener, True)
     if not (on_disk == set(requirements)):
+        if not maywrite:
+            raise error.Abort(_(b"store requirements are not as expected"))
         with opener(b'requires', b'w', atomictemp=True) as fp:
             for r in sorted(requirements):
                 fp.write(b"%s\n" % r)

mercurial/sparse.py

@@ -632,10 +632,10 def _updateconfigandrefreshwdir(
 
         if requirements.SPARSE_REQUIREMENT in oldrequires and removing:
             repo.requirements.discard(requirements.SPARSE_REQUIREMENT)
-            scmutil.writereporequirements(repo)
+            scmutil.writereporequirements(repo, maywritestore=False)
         elif requirements.SPARSE_REQUIREMENT not in oldrequires:
             repo.requirements.add(requirements.SPARSE_REQUIREMENT)
-            scmutil.writereporequirements(repo)
+            scmutil.writereporequirements(repo, maywritestore=False)
 
         try:
             writeconfig(repo, includes, excludes, profiles)
@@ -644,7 +644,7 def _updateconfigandrefreshwdir(
             if repo.requirements != oldrequires:
                 repo.requirements.clear()
                 repo.requirements |= oldrequires
-                scmutil.writereporequirements(repo)
+                scmutil.writereporequirements(repo, maywritestore=False)
             writeconfig(repo, oldincludes, oldexcludes, oldprofiles)
             raise