From: Siraaj Khandkar <siraaj@khandkar.net>
Date: Tue, 24 Mar 2020 21:56:32 +0000 (-0400)
Subject: Re-gain inter-message pushback
X-Git-Url: https://git.xandkar.net/?a=commitdiff_plain;h=de463e20353d8ae1b03127ebf93c4fa4d2433eee;p=khatus.git

Re-gain inter-message pushback

by closing FDs after EOM, while accepting possibility of message loss,
since we close before EOF.
---

diff --git a/x5/khatus.c b/x5/khatus.c
index d1574a5..98eea2d 100644
--- a/x5/khatus.c
+++ b/x5/khatus.c
@@ -418,13 +418,26 @@ fifo_read_all(Config *cfg, char *buf)
 			reading++;
 			debug("reading: %s\n", f->name);
 			switch (fifo_read_one(f, buf)) {
+			/*
+			 * ### MESSAGE LOSS ###
+			 * is introduced by closing at EOM in addition to EOF,
+			 * since there may be unread messages remaining in the
+			 * pipe. However,
+			 *
+			 * ### INTER-MESSAGE PUSHBACK ###
+			 * is also gained, since pipes block at the "open" call.
+			 *
+			 * This is an acceptable trade-off because we are a
+			 * stateless reporter of a _most-recent_ status, not a
+			 * stateful accumulator.
+			 */
+			case END_OF_MESSAGE:
 			case END_OF_FILE:
 			case FAILURE:
 				close(f->fd);
 				f->fd = -1;
 				reading--;
 				break;
-			case END_OF_MESSAGE:
 			case RETRY:
 				break;
 			default:
@@ -533,6 +546,10 @@ main(int argc, char *argv[])
 		clock_gettime(CLOCK_MONOTONIC, &t1); // FIXME: check errors
 		timespecsub(&t1, &t0, &td);
 		debug("td {tv_sec = %ld, tv_nsec = %ld}\n", td.tv_sec, td.tv_nsec);
+		/*
+		 * Skip sleep when we aren't done reading,
+		 * to avoid filling-up the pipe during sleep.
+		 */
 		if (!reading && timespeccmp(&td, &ti, <)) {
 			/* Pushback on data producers by refusing to read the
 			 * pipe more frequently than the interval.