revlog: avoid possible collision between directory and temporary index

Since 6.4, we create a temporary index file to write the split data without
overwriting the inline version too early. However, the store encoding does not
prevent these new `.i.s` file to collide with a directory with the same name.
While the odds for such a collision to happens are fairly low, the collision
would prevent Mercurial from working.

The store encoding have a mitigation solution in place to prevent such
collisions from happening for `.i` and `.d` files, but not for other extensions.
We cannot update this encoding scheme to solve the issue since it would diverge
from older version of Mercurial.

Instead, we create an alternative directory tree dedicated to such files.
The use of the `.i` extension combined with store encoding will prevent
collisions there.

mercurial/revlog.py

@@ -2026,7 +2026,15 b' class revlog:'
 
         The file will only exist if a splitting operation is in progress, but
         it is always expected at the same location."""
-        return self.radix + b'.i.s'
+        parts = os.path.split(self.radix)
+        if len(parts) > 1:
+            # adds a '-s' prefix to the ``data/` or `meta/` base
+            head = parts[0] + b'-s'
+            return os.path.join(head, *parts[1:])
+        else:
+            # the revlog is stored at the root of the store (changelog or
+            # manifest), no risk of collision.
+            return self.radix + b'.i.s'
 
     def _enforceinlinesize(self, tr, side_write=True):
         """Check if the revlog is too big for inline and convert if so.

tests/test-transaction-rollback-on-revlog-split.t

@@ -84,6 +84,8 b' setup a repository for tests'
   > Directory_With,Special%Char/Complex_File.babar
   > foo/bar/babar_celeste/foo
   > 1234567890/1234567890/1234567890/1234567890/1234567890/1234567890/1234567890/1234567890/1234567890/1234567890/f
+  > some_dir/sub_dir/foo_bar
+  > some_dir/sub_dir/foo_bar.i.s/tutu
   > "
   $ for f in $files; do
   >     mkdir -p `dirname $f`
@@ -112,7 +114,7 b' setup a repository for tests'
 Reference size:
   $ f -s file
   file: size=135168
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.d: size=267307
   .hg/store/data/file.i: size=320
 
@@ -140,7 +142,7 b' but truncate the index and the data to r'
 Reference size:
   $ f -s file
   file: size=1024
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
 
   $ cat > .hg/hgrc <<EOF
@@ -162,11 +164,12 b' Reference size:'
 The inline revlog still exist, but a split version exist next to it
 
   $ cat .hg/store/journal | tr '\0' ' ' | grep '\.s'
-  [1]
-  $ f -s .hg/store/data/file*
+  data/some_dir/sub_dir/foo_bar.i.s/tutu.i 1174
+  data/some_dir/sub_dir/foo_bar.i.s/tutu.d 0
+  $ f -s .hg/store/data*/file*
+  .hg/store/data-s/file: size=320
   .hg/store/data/file.d: size=267307
   .hg/store/data/file.i: size=132395
-  .hg/store/data/file.i.s: size=320
 
 
 The first file.i entry should match the "Reference size" above.
@@ -177,19 +180,19 b' A "temporary file" entry exist for the s'
   $ cat .hg/store/journal | tr -s '\000' ' ' | grep data/file
   data/file.i 1174
   data/file.d 0
-  $ cat .hg/store/journal.backupfiles | tr -s '\000' ' ' | tr -s '\00' ' '| grep data/file
+  $ cat .hg/store/journal.backupfiles | tr -s '\000' ' ' | tr -s '\00' ' '| grep 'data.*/file'
    data/file.i data/journal.backup.file.i 0
-   data/file.i.s 0
+   data-s/file 0
 
 recover is rolling the split back, the fncache is still valid
 
   $ hg recover
   rolling back interrupted transaction
   (verify step skipped, run `hg verify` to check your repository content)
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
   $ hg tip
-  changeset:   1:cc8dfb126534
+  changeset:   1:64b04c8dc267
   tag:         tip
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
@@ -214,7 +217,7 b' where the data file is left as garbage.'
 Reference size:
   $ f -s file
   file: size=1024
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
 
   $ cat > .hg/hgrc <<EOF
@@ -242,12 +245,12 b' Reference size:'
 
 The inline revlog still exist, but a split version exist next to it
 
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
+  .hg/store/data-s/file: size=320
   .hg/store/data/file.d: size=267307
   .hg/store/data/file.i: size=132395
-  .hg/store/data/file.i.s: size=320
 
-  $ cat .hg/store/journal | tr -s '\000' ' ' | grep data/file
+  $ cat .hg/store/journal | tr -s '\000' ' ' | grep 'data.*/file'
   data/file.i 1174
   data/file.d 0
 
@@ -256,10 +259,10 b' recover is rolling the split back, the f'
   $ hg recover
   rolling back interrupted transaction
   (verify step skipped, run `hg verify` to check your repository content)
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
   $ hg tip
-  changeset:   1:cc8dfb126534
+  changeset:   1:64b04c8dc267
   tag:         tip
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
@@ -279,7 +282,7 b' Now retry the procedure but intercept th'
 Reference size:
   $ f -s file
   file: size=1024
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
 
   $ cat > .hg/hgrc <<EOF
@@ -307,11 +310,11 b' Reference size:'
 
 The inline revlog was over written on disk
 
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.d: size=267307
   .hg/store/data/file.i: size=320
 
-  $ cat .hg/store/journal | tr -s '\000' ' ' | grep data/file
+  $ cat .hg/store/journal | tr -s '\000' ' ' | grep 'data.*/file'
   data/file.i 1174
   data/file.d 0
 
@@ -320,10 +323,10 b' recover is rolling the split back, the f'
   $ hg recover
   rolling back interrupted transaction
   (verify step skipped, run `hg verify` to check your repository content)
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.i: size=1174
   $ hg tip
-  changeset:   1:cc8dfb126534
+  changeset:   1:64b04c8dc267
   tag:         tip
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
@@ -357,13 +360,13 b' Repeat the original test but let hg roll'
 
 The split was rollback
 
-  $ f -s .hg/store/data/file*
+  $ f -s .hg/store/data*/file*
   .hg/store/data/file.d: size=0
   .hg/store/data/file.i: size=1174
 
 
   $ hg tip
-  changeset:   1:cc8dfb126534
+  changeset:   1:64b04c8dc267
   tag:         tip
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000

tests/test-upgrade-repo.t

@@ -839,6 +839,7 b' store directory has files we expect'
   00changelog.i
   00manifest.i
   data
+  data-s
   fncache
   phaseroots
   requires
@@ -862,6 +863,7 b' old store should be backed up'
   00changelog.i
   00manifest.i
   data
+  data-s
   fncache
   phaseroots
   requires