Function xsltEmitMessage 

pub fn xsltEmitMessage(ctxt: *mut _xsltTransformContext, msg: &[u8])

Emit an xsl:message payload through the transform error handler (or the global xsltGenericError channel), WITHOUT moving the transform context out of the OK state.

Upstream xsltMessage (transform.c 1.1.45) routes the message to ctxt->error when a per-context handler is installed (lxml’s xsltSetTransformErrorFunc / _receiveXSLTError), otherwise to the global generic channel, and does NOT emit a context line — only the raw message bytes, via handler(errctx, "%s", message). The context only leaves OK here when the terminate attribute is "yes" (handled by the caller, process_message).

Safety

• ctxt must be NULL or a valid _xsltTransformContext; its error / errctx fields are read and a registered handler must be a valid extern “C” function pointer callable with errctx and the message.