Fix race condition during parallel coverage testing using Template compiled_path option/method

This race condition can only happen during parallel testing. Assume
you have 2 processes, both trying to compile the same file at the
same time.  When using the compiled_path option/method, this
involves writing a file and loading it.  That's done with
File.binwrite.  However, the way File.binwrite works is it opens
the file for writing, truncating it if it exists, then writes the
data.  So in the following situation, you lose the race, and you get
an exception like `undefined method '__tilt_2720' for module
'Tilt::CompiledTemplates'` when trying to load the template:

1. Process A runs File.binwrite
2. Process A truncates file
3. Process A writes file
4. Process B runs File.binwrite
5. Process B truncates file
6. Process A loads (empty) file
7. Process B writes file
8. Process B loads (nonempty) file

In this situation, process A loaded an empty file. Loading an
empty file isn't an error, so processing continues. You don't have
problems until you try to access the method that should have been
defined and it isn't there.

Fix this by writing to a temporary file name that includes the
pid, and then renaming the file to the expected name after
the file has been written successfully. Writing to separate files
avoids the race condition, and the rename is atomic, so this
should avoid the race condition.

Author: jeremyevans

CHANGELOG.md

@@ -1,3 +1,7 @@
+## master
+
+* Fix race condition during parallel coverage testing using Template compiled_path option/method (jeremyevans)
+
 ## 2.6.0 (2025-01-13)
 
 * Support :compiled_path option, needed for compiled paths when using :scope_class and fixed locals (jeremyevans)

lib/tilt/template.rb

@@ -473,7 +473,12 @@ def eval_compiled_method(method_source, offset, scope_class)
     end
 
     def load_compiled_method(path, method_source)
-      File.binwrite(path, method_source)
+      # Write to a temporary path specific to the current process, and
+      # rename after writing. This prevents issues during parallel
+      # coverage testing.
+      tmp_path = "#{path}-#{$$}"
+      File.binwrite(tmp_path, method_source)
+      File.rename(tmp_path, path)
 
       # Use load and not require, so unbind_compiled_method does not
       # break if the same path is used more than once.