Logging

Because the daemon protocol only has one sender stream and one receiver stream logging messages need to be carefully interleaved with requests and responses. Usually this means that after the operation and all of its inputs (the request) has been read logging hijacks the sender stream (in the server case) and uses it to send typed logging messages while the request is being processed. When the response has been generated it will send STDERR_LAST to mark that what follows is the response data to the request. If the request failed a STDERR_ERROR message is sent with the error and no response is sent.

While not in this state between request reading and response sending all messages and activities are buffered until next time the logger can send data.

The logging messages supported are:

• STDERR_LAST
• STDERR_ERROR
• STDERR_NEXT
• STDERR_READ
• STDERR_WRITE
• STDERR_START_ACTIVITY
• STDERR_STOP_ACTIVITY
• STDERR_RESULT

STDERR_LAST

Marks the end of the logs, normal processing can resume.

• 0x616c7473 :: UInt64 (hardcoded, 'alts' in ASCII)

STDERR_ERROR

This also marks the end of this log "session" and so it has the same effect as STDERR_LAST. On the client the error is thrown as an exception and no response is read.

If protocol version is 1.26 or newer

• 0x63787470 :: UInt64 (hardcoded, 'cxtp' in ASCII)
• error :: Error

If protocol version is older than 1.26

• 0x63787470 :: UInt64 (hardcoded, 'cxtp' in ASCII)
• msg :: String (If logger is JSON, invalid UTF-8 is replaced with U+FFFD)
• exitStatus :: Int

STDERR_NEXT

Normal string log message.

• 0x6f6c6d67 :: UInt64 (hardcoded, 'olmg' in ASCII)
• msg :: String (If logger is JSON, invalid UTF-8 is replaced with U+FFFD)

STDERR_READ

Reader interface used by ImportsPaths and AddToStoreNar (between 1.21 and 1.23). It works by sending a desired buffer length and then on the receiver stream it reads the actual buffer length followed by a bytes buffer of that length padded to 8 bytes. If it receives 0 bytes it sees this as an unexpected EOF.

• 0x64617461 :: UInt64 (hardcoded, 'data' in ASCII)
• desiredLen :: Size

STDERR_WRITE

Writer interface used by ExportPath. Simply writes a buffer.

• 0x64617416 :: UInt64 (hardcoded)
• buffer :: Bytes

STDERR_START_ACTIVITY

Begins an activity. In other tracing frameworks this would be called a span.

Implemented in protocol 1.20. To achieve backwards compatible with older versions of the protocol instead of sending an STDERR_START_ACTIVITY the level is checked against enabled logging level and the text field is sent as a simple log message with STDERR_NEXT.

• 0x53545254 :: UInt64 (hardcoded, 'STRT' in ASCII)
• act :: UInt64
• level :: Verbosity
• type :: ActivityType
• text :: String (If logger is JSON, invalid UTF-8 is replaced with U+FFFD)
• fields :: List of Field
• parent :: UInt64

act is atomic (nextId++ + (getPid() << 32))

STDERR_STOP_ACTIVITY

Stops the given activity. The activity id should not send any more results. Just sends ActivityId.

Implemented in protocol 1.20. When backwards compatible with older versions of the protocol and this message would have been sent it is instead ignored.

• 0x53544f50 :: UInt64 (hardcoded, 'STOP' in ASCII)
• act :: UInt64

STDERR_RESULT

Sends results for a given activity.

Implemented in protocol 1.20. When backwards compatible with older versions of the protocol and this message would have been sent it is instead ignored.

• 0x52534c54 :: UInt64 (hardcoded, 'RSLT' in ASCII)
• act :: UInt64
• type :: ResultType
• fields :: List of Field