fix(http): stale skip_bytes in cache-write consumer after 100 Continue with transform

[JakeChampion]

When an origin sends 100 Continue before a compressible response, setup_100_continue_transfer() sets client_response_hdr_bytes to the intermediate header size. setup_server_transfer_to_transform() then creates the tunnel buffer with hdr_size=0 - body only, but does not reset client_response_hdr_bytes for non-chunked responses. perform_cache_write_action() passes the stale value as skip_bytes to the cache-write consumer, causing ink_release_assert in producer_run when the response body is smaller than the 100 Continue headers.

Fix: Set cache_write_skip to 0 when a transform is present, since setup_server_transfer_to_transform() never writes headers into the tunnel buffer.

---

Standard flow with no 100 Continue, no compress plugin transform:

ATS creates a tunnel buffer that contains [response headers][response body]
When writing to cache, it needs to skip past the headers so only the body gets stored
It uses a variable called skip_bytes for this
skip_bytes = size of response headers

What goes wrong with 100 Continue + compress transform:

1 - Origin sends 100 Continue:

Origin -> ATS: HTTP/1.1 100 Continue\r\n\r\n (25 bytes)
ATS forwards this to the client and records client_response_hdr_bytes = 25 (the size of those headers)
This is used later to know how much header data was written to the tunnel buffer

2 - Origin sends the real 200 OK response:
Because compress plugin is active, ATS sets up a server-to-transform tunnel
The buffer for this tunnel contains only the body, no headers go into this buffer at all, since the compress transform handles headers separately
client_response_hdr_bytes is not reset here for non-chunked responses, it is still 25 bytes from Step 1

3 - Cache write:
perform_cache_write_action() says wants to skip past the headers in the tunnel buffer before writing to cache and uses client_response_hdr_bytes as it's skip_bytes
In this scenario that means it tries to skip 25 bytes

But the tunnel buffer only has the body bytes in it, making the assertion fire and crash ATS

[JakeChampion]

I think AuTest 1of4 was a flakey fail

[ezelkow1]

[approve ci autest 1]

[Copilot]

Pull request overview

This PR fixes a crash in HttpTunnel::producer_run that occurs when an origin server sends a 100 Continue response before a compressible, non-chunked final response that triggers a transform (like the compress plugin with cache=true for untransformed caching).

The root cause is that setup_100_continue_transfer() sets client_response_hdr_bytes to the size of the 100 Continue headers. Later, when setup_server_transfer_to_transform() creates the tunnel buffer for the final response, it calls server_transfer_init(buf, 0) with hdr_size=0, meaning no headers are written to the buffer. However, for non-chunked responses, client_response_hdr_bytes is not reset. When perform_cache_write_action() passes this stale value as skip_bytes to the cache-write consumer, and the response body is smaller than the 100 Continue headers, the assertion skip_bytes <= read_avail() fails.

Changes:

• Fixed the cache write skip_bytes calculation to account for transforms
• Added comprehensive regression test with custom origin and client scripts

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/proxy/http/HttpSM.cc	Fixed perform_cache_write_action() to set cache_write_skip=0 when transform is present, since setup_server_transfer_to_transform() creates buffers without headers
tests/gold_tests/pluginTest/compress/compress-cache-untransformed.test.py	Regression test that reproduces the crash scenario with 100 Continue + compress transform + untransformed caching
tests/gold_tests/pluginTest/compress/compress_100_continue_origin.py	Custom origin server that sends 100 Continue followed by a small compressible response
tests/gold_tests/pluginTest/compress/compress_100_continue_client.py	Client that sends POST with Expect: 100-continue and Accept-Encoding: gzip
tests/gold_tests/pluginTest/compress/etc/cache-true-untransformed.config	Compress plugin config with cache=false to enable untransformed caching only

[masaori335]

[approve ci autest]

[masaori335]

Thank you for adding this test case. But I guess Proxy-Verifier supports 100 Continue support. ( we can merge this first and change it later )

@bneradt can we cover this case with proxy-verifier?

[masaori335]

Makes sense. Thank you!

[bryancall]

[approve ci]