`Error: read(): Is a directory` when locking a project containing (dune) pins

[gridbugs]

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To reproduce, clone https://github.com/gridbugs/ocaml.org and checkout the dune-pkg branch. The branch makes a couple of changes to allow dune package management to solve this project (except for the issue described here).

Run dune pkg lock.

The output will be:

Error: read(): Is a directory
Error: read(): Is a directory

(and no lockdir is generated).

The number of times the error prints out appears to be the number of pins in the project - removing one of the pins causes it to just print once and removing both allows dune pkg lock to succeed.

I've reproduce this on both macos and linux.

[rgrinberg]

The --debug-backtraces option might help you determine the cause of this exception.

[gridbugs]

Ah useful. The output is:

$ dune pkg lock --debug-backtraces
Error: read(): Is a directory
Raised by primitive operation at Dune_digest.Direct_impl.file in file
  "src/dune_digest/dune_digest.ml", line 30, characters 23-29
Called from Stdune__Exn.protectx in file "otherlibs/stdune/src/exn.ml", line
  10, characters 8-11
Re-raised at Stdune__Exn.protectx in file "otherlibs/stdune/src/exn.ml", line
  16, characters 4-11
Called from Dune_pkg__Source.fetch_and_hash_archive_cached.(fun) in file
  "src/dune_pkg/source.ml", line 55, characters 25-48
Called from Fiber__Core.O.(>>|).(fun) in file "vendor/fiber/src/core.ml",
  line 253, characters 36-41
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
  line 76, characters 8-11

[rgrinberg]

I guess since we're trying to compute the checksum for a directory. Not sure why we're doing that, but if you're using a git pin, no checksums are needed since we rely on the git sha.