sprockets should raise a warning if it finds multiple manifest files

[michaelglass]

We recently ran into an issue where some caching gone wrong left us with multiple .sprockets-manifest-1234567.json files.

It'd be lovely to have a warning in this situation.

Relevant method to change is here:

sprockets/lib/sprockets/manifest_utils.rb

Lines 36 to 41 in 0cb3314

	def find_directory_manifest(dirname)
	entries = File.directory?(dirname) ? Dir.entries(dirname) : []
	entry = entries.find { |e| e =~ MANIFEST_RE } ||
	generate_manifest_path
	File.join(dirname, entry)
	end

If this seems like a reasonable direction, I'd be happy to bring a PR.

Expected behavior

• Sprockets loads the most recently modified manifest file
• Sprockets logs a warning that it expected only one manifest file

Actual behavior

• Sprockets loads (I believe) the alphabetically earliest manifest file, and silently ignores the second file

System configuration

• Sprockets version
  3.7.2 (but 4* is also affected)
• Ruby version
  2.3.4

[schneems]

I’ve seen this in the wild before. I have no idea what would cause it though.

I would be interested in a PR to start. Likely an error would be a good idea. But let’s start with the basics of the implementation of how we can detect it.

[michaelglass]

For us, we produced multiple manifest by doing some funny business with caching between environments.

We had three different workspaces from which we deployed
We would occasionally clear out those workspaces and have no cache. In this case, sprockets would generate a new manifest which would end up in the cache.
Occasionally, that cache would be unpacked on top of a workspace that had another manifest. Now we have two manifest files.

We were at fault. Our caching was broken. But it was hard to diagnose because of the silence of sprockets.