Guard new leaf generation with a mutex

[phbnf]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 26.74%. Comparing base (07aa77d) to head (537c854).
Report is 80 commits behind head on main.

Files	Patch %	Lines
hammer/hammer.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #166       +/-   ##
===========================================
- Coverage   55.99%   26.74%   -29.26%     
===========================================
  Files           8       16        +8     
  Lines         759     1537      +778     
===========================================
- Hits          425      411       -14     
- Misses        265     1047      +782     
- Partials       69       79       +10     

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[mhutchinson]

@phbnf reminder to rebase and merge this when you get a mo

[phbnf]

Oops thanks.