## Changes
Add mutex synchronization in cmdio logger Log() method.
## Why
Since we issue multiple calls to underlying writer, we should lock the
whole method, otherwise we can get broken messages. One that can be
easily reproduced today is
```
hyperfine -m 100 --show-output 'go test ./acceptance -run ^TestAccept$/^bundle$/^artifacts$/^whl_multiple$ -count=1'
...
-Uploading my_test_code-0.0.1-py3-none-any.whl...
-Uploading my_test_code_2-0.0.1-py3-none-any.whl...
+Uploading my_test_code-0.0.1-py3-none-any.whl...Uploading my_test_code_2-0.0.1-py3-none-any.whl...
Error: Command terminated with non-zero exit code 1 in benchmark iteration 54. Use the '-i'/'--ignore-failure' option if you want to ignore this. Alternatively, use the '--show-output' option to debug what went wrong.
```
An alternative could be to prepare a message fully in a local buffer and
write it in one call (I’m assuming underlying writer is still
synchronized). However, that’s more complicated and unclear if it’s
worth it, perf-wise.
## Tests
With this change I’m running the same hyperfine command with 1000
iterations with no failures.
## Changes
- Remove bundle.Parallel & bundle.ReadOnlyBundle.
- Add bundle.ApplyParallel, as a helper to migrate from bundle.Parallel.
- Keep ReadOnlyMutator as a separate type but it's now a subtype of
Mutator so it works on regular *Bundle. Having it as a separate type
prevents non-readonly mutators being passed to ApplyParallel
- validate.Validate becomes a function (was Mutator).
## Why
This a follow up to #2390 where we removed most of the tools to
construct chains of mutators. Same motivation applies here.
When it comes to read-only bundles, it's a leaky abstraction -- since
it's a shallow copy, it does not actually guarantee or enforce readonly
access to bundle. A better approach would be to run parallel operations
on independent narrowly-focused deep-copied structs, with just enough
information to carry out the task (this is not implemented here, but the
eventual goal). Now that we can just write regular code in phases and
not limited to mutator interface, we can switch to that approach.
## Tests
Existing tests.
---------
Co-authored-by: shreyas-goenka <88374338+shreyas-goenka@users.noreply.github.com>