view tests/test-audit-subrepo.t @ 42044:bb271ec2fbfb

compression: introduce a `storage.revlog.zstd.level` configuration

This option control the zstd compression level used when compressing revlog
chunk. The usage of zstd for revlog compression has not graduated from
experimental yet, but we intend to fix that soon.

The option name for the compression level is more straight forward to pick, so
this changesets comes first.  Having a dedicated option for each compression
engine is useful because they don't support the same range of values.

I ran the same measurement as for the zlib compression level (in the parent
changesets). The variation in repository size is stay mostly in the same (small)
range. The "read/write" performance see smallish variation, but are overall much
better than zlib. Write performance show the same tend of having better write
performance for when reaching high-end compression.

Again, we don't intend to change the default zstd compression level (currently:
3) in this series. However this is worth investigating in the future.

The Performance comparison of zlib vs zstd is quite impressive. The repository
size stay in the same range, but the performance are much better in all
situations.

Comparison summary
==================

We are looking at:
- performance range for zlib
- performance range for zstd
- comparison of default zstd (level-3) to default zlib (level 6)
- comparison of the slowest zstd time to the fastest zlib time

Read performance:
-----------------
          |           zlib          |           zstd          | cmp | f2s
mercurial |   0.170159 -   0.189219 |   0.144127 -   0.149624 | 80% | 88%
pypy      |   2.679217 -   2.768691 |   1.532317 -   1.705044 | 60% | 63%
netbeans  | 122.477027 - 141.620281 |  72.996346 -  89.731560 | 58% | 73%
mozilla   | 147.867662 - 170.572118 |  91.700995 - 105.853099 | 56% | 71%

Write performance:
------------------
          |           zlib          |           zstd          | cmp | f2s
mercurial |  53.250304 - 56.2936129 |  40.877025 -  45.677286 | 75% | 86%
pypy      | 460.721984 - 476.589918 | 270.545409 - 301.002219 | 63% | 65%
netbeans  | 520.560316 - 715.930400 | 370.356311 - 428.329652 | 55% | 82%
mozilla   | 739.803002 - 987.056093 | 505.152906 - 591.930683 | 57% | 80%

Raw data
--------

repo      alg lvl  .hg/store size  00manifest.d read       write

mercurial zlib  1      49,402,813     5,963,475   0.170159  53.250304
mercurial zlib  6      47,197,397     5,875,730   0.182820  56.264320
mercurial zlib  9      47,121,596     5,849,781   0.189219  56.293612

mercurial zstd  1      49,737,084     5,966,355   0.144127  40.877025
mercurial zstd  3      48,961,867     5,895,208   0.146376  42.268142
mercurial zstd  5      48,200,592     5,938,676   0.149624  43.162875
mercurial zstd 10      47,833,520     5,913,353   0.145185  44.012489
mercurial zstd 15      47,314,604     5,728,679   0.147686  45.677286
mercurial zstd 20      47,330,502     5,830,539   0.145789  45.025407
mercurial zstd 22      47,330,076     5,830,539   0.143996  44.690460


pypy      zlib  1     370,830,572    28,462,425   2.679217 460.721984
pypy      zlib  6     340,112,317    27,648,747   2.768691 467.537158
pypy      zlib  9     338,360,736    27,639,003   2.763495 476.589918

pypy      zstd  1     362,377,479    27,916,214   1.532317 270.545409
pypy      zstd  3     354,137,693    27,905,988   1.686718 294.951509
pypy      zstd  5     342,640,043    27,655,774   1.705044 301.002219
pypy      zstd 10     334,224,327    27,164,493   1.567287 285.186239
pypy      zstd 15     329,000,363    26,645,965   1.637729 299.561332
pypy      zstd 20     324,534,039    26,199,547   1.526813 302.149827
pypy      zstd 22     324,530,595    26,198,932   1.525718 307.821218


netbeans  zlib  1   1,281,847,810   165,495,457 122.477027 520.560316
netbeans  zlib  6   1,205,284,353   159,161,207 139.876147 715.930400
netbeans  zlib  9   1,197,135,671   155,034,586 141.620281 678.297064

netbeans  zstd  1   1,259,581,737   160,840,613  72.996346 370.356311
netbeans  zstd  3   1,232,978,122   157,691,551  81.622317 396.733087
netbeans  zstd  5   1,208,034,075   160,246,880  83.080549 364.342626
netbeans  zstd 10   1,188,624,176   156,083,417  79.323935 403.594602
netbeans  zstd 15   1,176,973,589   153,859,477  89.731560 428.329652
netbeans  zstd 20   1,162,958,258   151,147,535  82.842667 392.335349
netbeans  zstd 22   1,162,707,029   151,150,220  82.565695 402.840655


mozilla   zlib  1   2,775,497,186   298,527,987 147.867662 751.263721
mozilla   zlib  6   2,596,856,420   286,597,671 170.572118 987.056093
mozilla   zlib  9   2,587,542,494   287,018,264 163.622338 739.803002

mozilla   zstd  1   2,723,159,348   286,617,532  91.700995 570.042751
mozilla   zstd  3   2,665,055,001   286,152,013  95.240155 561.412805
mozilla   zstd  5   2,607,819,817   288,060,030 101.978048 505.152906
mozilla   zstd 10   2,558,761,085   283,967,648 104.113481 497.771202
mozilla   zstd 15   2,526,216,060   275,581,300 105.853099 591.930683
mozilla   zstd 20   2,485,114,806   266,478,859  95.268795 576.515389
mozilla   zstd 22   2,484,869,080   266,456,505  94.429282 572.785537

Test illegal name
-----------------

on commit:

  $ hg init hgname
  $ cd hgname
  $ mkdir sub
  $ hg init sub/.hg
  $ echo 'sub/.hg = sub/.hg' >> .hgsub
  $ hg ci -qAm 'add subrepo "sub/.hg"'
  abort: path 'sub/.hg' is inside nested repo 'sub'
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "sub/.hg"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +sub/.hg = sub/.hg
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 sub/.hg
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q hgname hgname2
  abort: path 'sub/.hg' is inside nested repo 'sub'
  [255]

Test absolute path
------------------

on commit:

  $ hg init absolutepath
  $ cd absolutepath
  $ hg init sub
  $ echo '/sub = sub' >> .hgsub
  $ hg ci -qAm 'add subrepo "/sub"'
  abort: path contains illegal component: /sub
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "/sub"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +/sub = sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 /sub
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q absolutepath absolutepath2
  abort: path contains illegal component: /sub
  [255]

Test root path
--------------

on commit:

  $ hg init rootpath
  $ cd rootpath
  $ hg init sub
  $ echo '/ = sub' >> .hgsub
  $ hg ci -qAm 'add subrepo "/"'
  abort: path ends in directory separator: /
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "/"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +/ = sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 /
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q rootpath rootpath2
  abort: path ends in directory separator: /
  [255]

Test empty path
---------------

on commit:

  $ hg init emptypath
  $ cd emptypath
  $ hg init sub
  $ echo '= sub' >> .hgsub
  $ hg ci -qAm 'add subrepo ""'
  hg: parse error at .hgsub:1: = sub
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo ""' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > += sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q emptypath emptypath2
  hg: parse error at .hgsub:1: = sub
  [255]

Test current path
-----------------

on commit:

  $ hg init currentpath
  $ cd currentpath
  $ hg init sub
  $ echo '. = sub' >> .hgsub
  $ hg ci -qAm 'add subrepo "."'
  abort: subrepo path contains illegal component: .
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "."' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +.= sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 .
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q currentpath currentpath2
  abort: subrepo path contains illegal component: .
  [255]

Test outer path
---------------

on commit:

  $ mkdir outerpath
  $ cd outerpath
  $ hg init main
  $ cd main
  $ hg init ../sub
  $ echo '../sub = ../sub' >> .hgsub
  $ hg ci -qAm 'add subrepo "../sub"'
  abort: path contains illegal component: ../sub
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "../sub"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +../sub = ../sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 ../sub
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q main main2
  abort: path contains illegal component: ../sub
  [255]
  $ cd ..

Test variable expansion
-----------------------

Subrepository paths shouldn't be expanded, but we fail to handle them
properly. Any local repository paths are expanded.

on commit:

  $ mkdir envvar
  $ cd envvar
  $ hg init main
  $ cd main
  $ hg init sub1
  $ cat <<'EOF' > sub1/hgrc
  > [hooks]
  > log = echo pwned
  > EOF
  $ hg -R sub1 ci -qAm 'add sub1 files'
  $ hg -R sub1 log -r. -T '{node}\n'
  39eb4b4d3e096527668784893a9280578a8f38b8
  $ echo '$SUB = sub1' >> .hgsub
  $ SUB=sub1 hg ci -qAm 'add subrepo "$SUB"'
  abort: subrepo path contains illegal component: $SUB
  [255]

prepare tampered repo (including the changes above as two commits):

  $ hg import --bypass -qm 'add subrepo "$SUB"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +$SUB = sub1
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 $SUB
  > EOF
  $ hg debugsetparents 0
  $ hg import --bypass -qm 'update subrepo "$SUB"' - <<'EOF'
  > diff --git a/.hgsubstate b/.hgsubstate
  > --- a/.hgsubstate
  > +++ b/.hgsubstate
  > @@ -1,1 +1,1 @@
  > -0000000000000000000000000000000000000000 $SUB
  > +39eb4b4d3e096527668784893a9280578a8f38b8 $SUB
  > EOF
  $ cd ..

on clone (and update) with various substitutions:

  $ hg clone -q main main2
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main2

  $ SUB=sub1 hg clone -q main main3
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main3

  $ SUB=sub2 hg clone -q main main4
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main4

on clone empty subrepo into .hg, then pull (and update), which at least fails:

  $ SUB=.hg hg clone -qr0 main main5
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main5
  $ test -d main5/.hg/.hg
  [1]
  $ SUB=.hg hg -R main5 pull -u
  pulling from $TESTTMP/envvar/main
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets 7a2f0e59146f
  .hgsubstate: untracked file differs
  abort: untracked files in working directory differ from files in requested revision
  [255]
  $ cat main5/.hg/hgrc | grep pwned
  [1]

on clone (and update) into .hg, which at least fails:

  $ SUB=.hg hg clone -q main main6
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main6
  $ cat main6/.hg/hgrc | grep pwned
  [1]

on clone (and update) into .hg/* subdir:

  $ SUB=.hg/foo hg clone -q main main7
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main7
  $ test -d main7/.hg/.hg
  [1]

on clone (and update) into outer tree:

  $ SUB=../out-of-tree-write hg clone -q main main8
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main8

on clone (and update) into e.g. $HOME, which doesn't work since subrepo paths
are concatenated prior to variable expansion:

  $ SUB="$TESTTMP/envvar/fakehome" hg clone -q main main9
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls main9 | wc -l
  \s*0 (re)

  $ ls
  main
  main2
  main3
  main4
  main5
  main6
  main7
  main8
  main9
  $ cd ..

Test tilde
----------

The leading tilde may be expanded to $HOME, but it can be a valid subrepo
path in theory. However, we want to prohibit it as there might be unsafe
handling of such paths.

on commit:

  $ hg init tilde
  $ cd tilde
  $ hg init './~'
  $ echo '~ = ~' >> .hgsub
  $ hg ci -qAm 'add subrepo "~"'
  abort: subrepo path contains illegal component: ~
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "~"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +~ = ~
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 ~
  > EOF
  $ cd ..

on clone (and update):

  $ hg clone -q tilde tilde2
  abort: subrepo path contains illegal component: ~
  [255]

Test direct symlink traversal
-----------------------------

#if symlink

on commit:

  $ mkdir hgsymdir
  $ hg init hgsymdir/root
  $ cd hgsymdir/root
  $ ln -s ../out
  $ hg ci -qAm 'add symlink "out"'
  $ hg init ../out
  $ echo 'out = out' >> .hgsub
  $ hg ci -qAm 'add subrepo "out"'
  abort: subrepo 'out' traverses symbolic link
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "out"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +out = out
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 out
  > EOF
  $ cd ../..

on clone (and update):

  $ mkdir hgsymdir2
  $ hg clone -q hgsymdir/root hgsymdir2/root
  abort: subrepo 'out' traverses symbolic link
  [255]
  $ ls hgsymdir2
  root

#endif

Test indirect symlink traversal
-------------------------------

#if symlink

on commit:

  $ mkdir hgsymin
  $ hg init hgsymin/root
  $ cd hgsymin/root
  $ ln -s ../out
  $ hg ci -qAm 'add symlink "out"'
  $ mkdir ../out
  $ hg init ../out/sub
  $ echo 'out/sub = out/sub' >> .hgsub
  $ hg ci -qAm 'add subrepo "out/sub"'
  abort: path 'out/sub' traverses symbolic link 'out'
  [255]

prepare tampered repo (including the commit above):

  $ hg import --bypass -qm 'add subrepo "out/sub"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +out/sub = out/sub
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 out/sub
  > EOF
  $ cd ../..

on clone (and update):

  $ mkdir hgsymin2
  $ hg clone -q hgsymin/root hgsymin2/root
  abort: path 'out/sub' traverses symbolic link 'out'
  [255]
  $ ls hgsymin2
  root

#endif

Test symlink traversal by variable expansion
--------------------------------------------

#if symlink

  $ FAKEHOME="$TESTTMP/envvarsym/fakehome"

on commit:

  $ mkdir envvarsym
  $ cd envvarsym
  $ hg init main
  $ cd main
  $ ln -s "`echo "$FAKEHOME" | sed 's|\(.\)/.*|\1|'`"
  $ hg ci -qAm 'add symlink to top-level system directory'

  $ hg init sub1
  $ echo pwned > sub1/pwned
  $ hg -R sub1 ci -qAm 'add sub1 files'
  $ hg -R sub1 log -r. -T '{node}\n'
  f40c9134ba1b6961e12f250868823f0092fb68a8
  $ echo '$SUB = sub1' >> .hgsub
  $ SUB="$FAKEHOME" hg ci -qAm 'add subrepo "$SUB"'
  abort: subrepo path contains illegal component: $SUB
  [255]

prepare tampered repo (including the changes above as two commits):

  $ hg import --bypass -qm 'add subrepo "$SUB"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +$SUB = sub1
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 $SUB
  > EOF
  $ hg debugsetparents 1
  $ hg import --bypass -qm 'update subrepo "$SUB"' - <<'EOF'
  > diff --git a/.hgsubstate b/.hgsubstate
  > --- a/.hgsubstate
  > +++ b/.hgsubstate
  > @@ -1,1 +1,1 @@
  > -0000000000000000000000000000000000000000 $SUB
  > +f40c9134ba1b6961e12f250868823f0092fb68a8 $SUB
  > EOF
  $ cd ..

on clone (and update) without fakehome directory:

  $ rm -fR "$FAKEHOME"
  $ SUB="$FAKEHOME" hg clone -q main main2
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ test -d "$FAKEHOME"
  [1]

on clone (and update) with empty fakehome directory:

  $ rm -fR "$FAKEHOME"
  $ mkdir "$FAKEHOME"
  $ SUB="$FAKEHOME" hg clone -q main main3
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls "$FAKEHOME"

on clone (and update) with non-empty fakehome directory:

  $ rm -fR "$FAKEHOME"
  $ mkdir "$FAKEHOME"
  $ touch "$FAKEHOME/a"
  $ SUB="$FAKEHOME" hg clone -q main main4
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls "$FAKEHOME"
  a

on clone empty subrepo with non-empty fakehome directory,
then pull (and update):

  $ rm -fR "$FAKEHOME"
  $ mkdir "$FAKEHOME"
  $ touch "$FAKEHOME/a"
  $ SUB="$FAKEHOME" hg clone -qr1 main main5
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls "$FAKEHOME"
  a
  $ test -d "$FAKEHOME/.hg"
  [1]
  $ SUB="$FAKEHOME" hg -R main5 pull -u
  pulling from $TESTTMP/envvarsym/main
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets * (glob)
  .hgsubstate: untracked file differs
  abort: untracked files in working directory differ from files in requested revision
  [255]
  $ ls "$FAKEHOME"
  a
  $ test -d "$FAKEHOME/.hg"
  [1]

on clone empty subrepo with hg-managed fakehome directory,
then pull (and update):

  $ rm -fR "$FAKEHOME"
  $ hg init "$FAKEHOME"
  $ touch "$FAKEHOME/a"
  $ hg -R "$FAKEHOME" ci -qAm 'add fakehome file'
  $ SUB="$FAKEHOME" hg clone -qr1 main main6
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls "$FAKEHOME"
  a
  $ SUB="$FAKEHOME" hg -R main6 pull -u
  pulling from $TESTTMP/envvarsym/main
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets * (glob)
  .hgsubstate: untracked file differs
  abort: untracked files in working directory differ from files in requested revision
  [255]
  $ ls "$FAKEHOME"
  a

on clone only symlink with hg-managed fakehome directory,
then pull (and update):

  $ rm -fR "$FAKEHOME"
  $ hg init "$FAKEHOME"
  $ touch "$FAKEHOME/a"
  $ hg -R "$FAKEHOME" ci -qAm 'add fakehome file'
  $ SUB="$FAKEHOME" hg clone -qr0 main main7
  $ ls "$FAKEHOME"
  a
  $ SUB="$FAKEHOME" hg -R main7 pull -uf
  pulling from $TESTTMP/envvarsym/main
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 3 changes to 2 files
  new changesets * (glob)
  abort: subrepo path contains illegal component: $SUB
  [255]
  $ ls "$FAKEHOME"
  a

  $ cd ..

#endif

Test drive letter
-----------------

Windows has a weird relative path that can change the drive letter, which
should also be prohibited on Windows.

prepare tampered repo:

  $ hg init driveletter
  $ cd driveletter
  $ hg import --bypass -qm 'add subrepo "X:"' - <<'EOF'
  > diff --git a/.hgsub b/.hgsub
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsub
  > @@ -0,0 +1,1 @@
  > +X: = foo
  > diff --git a/.hgsubstate b/.hgsubstate
  > new file mode 100644
  > --- /dev/null
  > +++ b/.hgsubstate
  > @@ -0,0 +1,1 @@
  > +0000000000000000000000000000000000000000 X:
  > EOF
  $ cd ..

on clone (and update):

#if windows

  $ hg clone -q driveletter driveletter2
  abort: path contains illegal component: X:
  [255]

#else

  $ hg clone -q driveletter driveletter2
  $ ls driveletter2
  X:

#endif