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From: anuragg-saxenaa (@anuragg-saxenaa) <[email protected]>
To: pgjdbc/pgjdbc <[email protected]>
Subject: [pgjdbc/pgjdbc] PR #4027: fix: drain CopyOut messages after cancel to restore connection usability
Date: Mon, 13 Apr 2026 14:39:00 +0000
Message-ID: <[email protected]> (raw)

## Problem

When `PGCopyInputStream` is closed before EOF, `close()` calls `cancelCopy()` on the underlying `CopyOut` operation. The `CopyIn` branch in `QueryExecutorImpl.cancelCopy()` correctly sends `CopyFail` and then **drains remaining server messages** (`CopyData`, `CommandComplete`, `ReadyForQuery`) in a loop until the connection lock is released.

The `CopyOut` branch only called `sendQueryCancel()` — an out-of-band cancel signal sent on a **separate socket** — but never drained the remaining `CopyData` and `ReadyForQuery` messages from the **main connection buffer**. This left stale server messages on the wire that corrupted subsequent queries, making the connection unusable.

Reproducer (from the issue):
```java
try (PGCopyInputStream in = new PGCopyInputStream(conn, COPY_SQL)) {
    byte[] partial = new byte[5];
    in.read(partial); // read less than full output
    // close() → cancelCopy() → stale messages left on wire
}
// Next query or COPY fails because connection is in broken state
```

## Fix

After `sendQueryCancel()`, add the same `processCopyResults` drain loop used by the `CopyIn` path. This ensures all remaining `CopyData` messages and the final `ReadyForQuery` are consumed, fully resetting the connection before the lock is released.

## Test

Added `connectionIsUsableAfterPartialRead()` in `PGCopyInputStreamTest`: performs a partial read, closes the stream, then opens a second `PGCopyInputStream` on the same connection and verifies all rows are returned correctly.

Fixes #1290

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  To: github://pgjdbc/pgjdbc
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