Problem

After upgrading to Rack 3, I found that Rack::Response generates an incorrect content-length header when:

• it is constructed with an empty array for the body, and
• write is called multiple times

Here is a simple reproduction script:

require "rack"

status = 200
headers = {}
response = Rack::Response.new([], status, headers)

# Write 11 total bytes
response.write "hello"
response.write "world"
response.write "!"

# Resulting content-length should be 11, but is 26
response.headers
# => {"content-length"=>"26"}

Underlying cause

When Rack::Response is constructed with an empty array, it sets this internal state:

This causes the value of @length to accumulate on every call to Rack::Response#write:

When chunked? is false, @length is incremented yet again, and then emitted as a content-length header:

The result is that the content-length value is too large, and the error gets larger on every call to write.