[pacmano1]

Closes #511.

The File Writer fills in ${...} variables in S3 custom headers for every message, but the pooled S3Connection kept the headers of the message that created the pool, so every later write sent the first message's values.

Change

• FileDispatcher.send: after borrowing a connection, an S3 connection gets this message's headers.
• S3Connection: keeps them in its own field (setCustomHeaders), which getCustomHeaders() uses when set. Not in schemeProps, because every connection in a pool shares the first message's S3SchemeProperties.
• Nothing else changes: other schemes, the pool key, and the File Reader, which keeps its own headers.

Thread safety: Donkey clones the connector properties for each message, including a copy of the header map, and the pool hands a connection to one sender until it's returned, so the headers set on a borrowed connection belong to that sender's message.

Tests

• New FileDispatcherTest: two sends through one pooled S3Connection with a mocked S3 client, checking x-test-id on each upload request. Fails without the fix (expected:<[1, 2]> but was:<[1, 1]>), passes with it.
• Full build: donkey 23, server 676, client 18, command 6, no failures. Same on top of Upgrade the AWS SDK to 2.55.12, realign Jackson on 2.18.11 and upgrade Swagger to 2.2.41 #506.

How to verify

Needs an S3 bucket and credentials the engine can use, for example AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_SESSION_TOKEN and AWS_REGION in the engine's environment, with Use Default Credential Provider Chain on.

1. New channel: Channel Reader source; source transformer JavaScript step $c('clinic', String(msg));; File Writer in S3 mode, S3 Bucket <bucket>/s3-headers, file name ${message.messageId}.txt, template ${message.rawData}; in Amazon S3 Advanced Settings, Custom HTTP Headers x-amz-meta-clinic = ${clinic} and x-test-id = ${message.messageId}.
2. Deploy, then send clinic A and clinic B.
3. aws s3api head-object --bucket <bucket> --key s3-headers/2.txt --query Metadata
   • Before: {"clinic": "clinic A", "x-test-id": "1"}
   • After: {"clinic": "clinic B", "x-test-id": "2"}

Live-tested against real S3 on Java 21, on top of #506 (AWS SDK 2.55.12), the SDK this will ship with. Before: 2.txt carried message 1's values, with Keep Connection Open on and off (also reproduced on main). After: every object carried its own message's values, with Keep Connection Open on and off, and with the destination queue on and 2 threads (6 messages).

Co-authored with Claude Code.

[mgaffigan]

Can we test this in CI? s3mock or similar?

[github-actions]

Test Results

130 files  +1  130 suites  +1   3m 50s ⏱️ + 1m 26s
738 tests +1  738 ✅ +1  0 💤 ±0  0 ❌ ±0 
828 runs  +1  822 ✅ +1  6 💤 ±0  0 ❌ ±0 

Results for commit a86b5c3. ± Comparison against base commit 6d348a0.