How I Found (and Fixed) a 14x Cost Bug in Airbyte's S3 Connector

We've all heard the "cloud horror stories." A $50,000 AWS bill over a weekend. A $100k charge in 12 hours from a runaway Lambda. These aren't just myths; they're bugs. And they're almost always in the application code.

I've become obsessed with this problem: FinOps tools tell you what you spent, but never why the code spent it.

So I went hunting. I decided to manually audit the codebase of Airbyte, a massive and high-quality open-source data platform, to see if I could find a "cloud cost bug" in the wild.

Then I Found It.

Inside the S3 source connector (source-s3/v4/zip_reader.py), I found a subtle but powerful N+1 bug. The code was trying to find a "signature" (a sequence of bytes) at the end of a ZIP file.

To do this, it was calling get_object inside a while loop, repeatedly asking for a slightly bigger chunk of the same file.

The "Before" Code (The Bug):

# ...
file_size = self.s3_client.head_object(...)
buffer_size = initial_buffer_size
while buffer_size <= max_buffer_size:  # This get_object call is inside a loop!
    chunk = self._fetch_data_from_s3(filename, file_size - buffer_size)
    index = chunk.rfind(signature)
    if index != -1:
        return chunk[index:]
    buffer_size *= 2  # Double the buffer and try again
return None

Why This Is a 14x Cost Multiplier

This while loop could execute up to 14 times to find the signature. That means 14 separate get_object API calls for every single file processed.

The Fix: One Call, Not Fourteen

The fix is to fetch the entire max-sized chunk once and search for the signature in memory.

The "After" Code (The Fix):

# ...
file_size = self.s3_client.head_object(...)
# Calculate the start byte for the single largest chunk
start_byte = max(0, file_size - max_buffer_size)

# Make ONE call for the whole chunk
chunk = self._fetch_data_from_s3(filename, start_byte)

# Search in memory
index = chunk.rfind(signature)
if index != -1:
    return chunk[index:]

return None

This simple refactor reduces 14 API calls to just 1 (plus the head_object), cutting the API cost for this operation by over 90%.

This is Why We Need "Cost-Aware" Linters

This bug is subtle. It's not "wrong," it's just expensive. It's the exact kind of thing that slips past code review and creates a "horror story" bill.

Manually finding this was a pain. That's why I'm building Wozz.

Wozz is a free GitHub Action that automatically scans your PRs for these "cost anti-patterns" and posts a comment before you merge. It's the "Snyk for Cloud Cost."