Fix infinite loop in recv_inject() when peer closes connection

[brndnsvr]

The refill block in recv_inject() used if (l > -1) to check recv() return value, which incorrectly treated EOF (recv returning 0) as valid data. This caused tight CPU loops (40-60% per process) and CLOSE_WAIT connections when peers closed their side of the connection.

Changes:

• Replace if (l > -1) with proper three-way check:
  • l > 0: Success, set inject_len
  • l == 0: EOF, set io_status_close and return
  • l < 0: Error, handle EAGAIN/EWOULDBLOCK with io_status_retry, others with io_status_error
• Move inject_off reset before recv() for code hygiene
• Add io_status_ok reporting when returning buffered data

[Copilot]

Pull request overview

This PR fixes an infinite loop bug in recv_inject() that occurred when a peer closed its side of the connection, causing high CPU usage (40-60% per process) and connections stuck in CLOSE_WAIT state. The fix replaces an incorrect two-way check with a proper three-way check to distinguish between successful data reception, EOF, and errors.

Key Changes:

• Replace if (l > -1) with proper handling for recv() return values: success (l > 0), EOF (l == 0), and errors (l < 0)
• Move inject_off reset inside the success block to prevent resetting on EOF/error
• Add explicit status reporting for all code paths including buffered data returns

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