Refactor consumer callbacks to (n)ack *after* execution.

The existing behaviour is basically the same as always having
no_ack=TRUE, in that messages are unequivocally acknowledged. However,
it is sometimes desirable to distinguish processing which fails from
that which succeeds, so callbacks which throw an error will now nack
messages instead -- without requeing them.

Since nack without requeue is only *semantically* (not operationally)
different than acknowledgment, existing workflows are unlikely to be
affected.

Documentation has been updated to reflect this behaviour.

Part of #2.

R/consume.R

@@ -46,7 +46,8 @@
 #' @details
 #'
 #' Unless \code{no_ack} is \code{TRUE}, messages are acknowledged automatically
-#' after the callback executes.
+#' after the callback executes. If it fails, messages are nacked instead before
+#' surfacing the underlying error to the caller.
 #'
 #' @examples
 #' \dontrun{
@@ -79,8 +80,27 @@ amqp_consume <- function(conn, queue, fun, tag = "", no_ack = FALSE,
   }
   stopifnot(is.function(fun))
   args <- amqp_table(...)
+  if (!no_ack) {
+    # Wrap fun to control error conditions and ensure messages are acknowledged.
+    wrapped <- function(msg, chan) {
+      should_ack <- TRUE
+      tryCatch({
+        fun(msg)
+      }, error = function(cond) {
+        should_ack <<- FALSE
+        amqp_nack_on_channel(conn, chan, msg$delivery_tag, requeue = FALSE)
+        stop(cond)
+      }, finally = {
+        if (should_ack) {
+          amqp_nack_on_channel(conn, chan, msg$delivery_tag)
+        }
+      })
+    }
+  } else {
+    wrapped <- fun
+  }
   .Call(
-    R_amqp_create_consumer, conn$ptr, queue, tag, fun, new.env(), no_ack,
+    R_amqp_create_consumer, conn$ptr, queue, tag, wrapped, new.env(), no_ack,
     exclusive, args$ptr
   )
 }
@@ -145,6 +165,8 @@ amqp_listen <- function(conn, timeout = 10L) {
 #' \code{\link{amqp_cancel_consumer}} or by garbage collection when the original
 #' connection object expires.
 #'
+#' Messages to background consumers are always acknowledged.
+#'
 #' @seealso \code{\link{amqp_consume}} to consume messages in the main thread.
 #' @export
 #' @import later

src/consume.c

@@ -139,9 +139,7 @@ SEXP R_amqp_listen(SEXP ptr, SEXP timeout)
   tv.tv_usec = 0;
   time_t start = time(NULL);
 
-  SEXP message, body;
-  SEXP R_fcall = PROTECT(allocList(2));
-  SET_TYPEOF(R_fcall, LANGSXP);
+  SEXP message, body, R_fcall;
 
   int ack;
   amqp_rpc_reply_t reply;
@@ -235,46 +233,25 @@ SEXP R_amqp_listen(SEXP ptr, SEXP timeout)
                                          &env.message.properties));
       amqp_destroy_envelope(&env);
 
-      /* Acknowledge the message before we run the callback, just in case it
-       * fails. */
-
-      if (!elt->no_ack) {
-        ack = amqp_basic_ack(conn->conn, elt->chan.chan, env.delivery_tag, 0);
-
-        /* We want to handle errors here before running the callback, since it's
-         * possible the message will be redelivered and the callback may not be
-         * idempotent. */
-        switch (ack) {
-        case AMQP_STATUS_OK:
-          break;
-        case AMQP_STATUS_CONNECTION_CLOSED:
-          /* fallthrough */
-        case AMQP_STATUS_SOCKET_CLOSED:
-          /* fallthrough */
-        case AMQP_STATUS_SOCKET_ERROR:
-          /* fallthrough */
-          conn->is_connected = 0;
-          Rf_error("Disconnected from server.");
-          break;
-        default:
-          Rf_error("Failed to acknowledge message: %s.\n",
-                   amqp_error_string2(ack));
-          break;
-        }
-      }
-
+      R_fcall = PROTECT(allocList(elt->no_ack ? 2 : 3));
+      SET_TYPEOF(R_fcall, LANGSXP);
       SETCAR(R_fcall, elt->fun);
       SETCADR(R_fcall, message);
+      if (!elt->no_ack) {
+        /* The channel is passed to the callback wrapper so that it can be used
+         * for acknowledgements. */
+        SEXP chan_ptr = PROTECT(R_MakeExternalPtr(&elt->chan, R_NilValue, R_NilValue));
+        SETCADDR(R_fcall, chan_ptr);
+      }
       Rf_eval(R_fcall, elt->rho);
 
-      UNPROTECT(2);
+      UNPROTECT(elt->no_ack ? 3 : 4);
     }
 
     current_wait = time(NULL) - start;
     R_CheckUserInterrupt(); // Escape hatch.
   }
 
-  UNPROTECT(1);
   return R_NilValue;
 }