latch-unix: Decrease the stack usage in latch

1. Make latch behave as described and documented - clear all
outstanding latch writes when invoking latch_poll().
2. Decrease the size of the latch buffer. Less stack usage,
less cache thrashing.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/lib/latch-unix.c b/lib/latch-unix.c
index 2995076d6..f4d10c39a 100644
--- a/lib/latch-unix.c
+++ b/lib/latch-unix.c
@@ -43,9 +43,17 @@ latch_destroy(struct latch *latch)
 bool
 latch_poll(struct latch *latch)
 {
-    char buffer[_POSIX_PIPE_BUF];
+    char latch_buffer[16];
+    bool result = false;
+    int ret;
 
-    return read(latch->fds[0], buffer, sizeof buffer) > 0;
+    do {
+        ret = read(latch->fds[0], &latch_buffer, sizeof latch_buffer);
+        result |= ret > 0;
+    /* Repeat as long as read() reads a full buffer. */
+    } while (ret == sizeof latch_buffer);
+
+    return result;
 }
 
 /* Sets 'latch'.