Fix Permission Denied on Win32 install operation

[jonahbeckford]

Problem

The Windows preconditions are:

• an Opam package already exists in the Opam switch
• one of its files has read-only permissions, and
• that file is individually "install"-ed by Opam

Then when the package is recompiled it will fail with Permission denied.

For example:

02:23.245  FILE(.install)         Wrote Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\.opam-switch\install\dune.install in 0.000s
02:23.245  TRACK                  before install: 34 elements scanned in 0.000s
02:23.245  ACTION                 Installing dune.2.9.0.

02:23.245  SYSTEM                 install Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\.opam-switch\build\dune.2.9.0\_build\install\default\bin\dune.exe -> Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe (755)
02:23.245  PARALLEL               Exception while computing job 318106069: ✶ dune.2.9.0

#=== ERROR while installing dune.2.9.0 ========================================#
Sys_error("Z:\\source\\diskuv-ocaml-starter\\build\\dev\\Debug\\_opam\\bin\\dune.exe: Permission denied")
Backtrace:
  Raised at OpamParallel.Make.aux_map.loop.fail in file "src/core/opamParallel.ml", line 225, characters 8-76
  Called from OpamParallel.Make.aux_map.loop in file "src/core/opamParallel.ml", line 255, characters 48-56
  Called from OpamParallel.Make.aux_map in file "src/core/opamParallel.ml", line 293, characters 12-44
  Called from OpamParallel.Make.map in file "src/core/opamParallel.ml", line 301, characters 15-57
  Called from OpamSolution.parallel_apply in file "src/client/opamSolution.ml", line 664, characters 8-209

The file was initially installed read-only.

Specifically, from the Cygwin and Windows' perspectives the file and its ACL looks like:

$ ls -l $(cygpath 'Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe')
-r-x------+ 1 beckf beckf 6927360 Sep  4 18:00 /cygdrive/z/source/diskuv-ocaml-starter/build/dev/Debug/_opam/bin/dune.exe

$ getfacl -e $(cygpath 'Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe')
# file: /cygdrive/z/source/diskuv-ocaml-starter/build/dev/Debug/_opam/bin/dune.exe
# owner: beckf
# group: beckf
user::rwx
group::---              #effective:---
group:SYSTEM:rwx        #effective:---
group:Administrators:rwx        #effective:---
mask::---
other::---

and from the MSYS2 perspective the file looks like:

$ ls -l $(cygpath 'Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe')
-r-xr-xr-x 1 beckf beckf 6927360 Sep  4 18:00 /z/source/diskuv-ocaml-starter/build/dev/Debug/_opam/bin/dune.exe

Solution

Do a chmod 640 and delete the file before trying to do a truncate+write into it.

[dra27]

Apologies for the delay looking at this! GitHub still doesn't let one do code suggestions anywhere in the file, so I've pushed a commit with my suggestions, but that commit may still be removed. My comments:

• Unix.chmod is an emulation on Windows - it only controls the readonly bit, so I'd suggest 0o666 as that more accurately reflects what will happen (Dune does the same, although that might be down to shared authorship!)
• In setup_copy, we should preserve the cp-like semantics, which are inode preserving (i.e. it truncates if possible). To this end, instead of always deleting the target on Windows, I instead moved the chmod change around the opening of the file. open_out_gen doesn't give very rich information, so instead of open_out_gen followed later by Unix.descr_of_out_channel, I switched to Unix.openfile and use Unix.out_channel_of_descr. This is safe for Windows as binary mode is wanted and no attempt will be made to change that.
• The remove_file function could do with the same logic, so I added it there too (propagates to copy_file as well)

[rjbou]

Thanks!