[Libguestfs] [libnbd PATCH 1/4] generator: Use static buffer for discaring reads
No need to malloc()/free() a throw-away buffer, when we can just use a
static one. We normally don't even have to worry about thread-safety
issues between separate connections (as long as we never read the
buffer, we don't care if separate nbd handles clobber the same buffer
at the same time); but for DUMP_PACKETS, we have to use stack
allocations instead of static allocation (this makes stack utilization
much larger, but this is okay for an expensive debugging mode that
isn't even going to be compiled in normal practice).
---
generator/states.c | 37 ++++++++++++++++++++-----------------
1 file changed, 20 insertions(+), 17 deletions(-)
diff --git a/generator/states.c b/generator/states.c
index 654281c..d2aa51d 100644
--- a/generator/states.c
+++ b/generator/states.c
@@ -1,5 +1,5 @@
/* nbd client library in userspace: state machine
- * Copyright (C) 2013-2020 Red Hat Inc.
+ * Copyright (C) 2013-2022 Red Hat Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -44,33 +44,39 @@ static int
recv_into_rbuf (struct nbd_handle *h)
{
ssize_t r;
- char *buf = NULL;
void *rbuf;
size_t rlen;
- if (h->rlen == 0)
- return 0; /* move to next state */
-
/* As a special case h->rbuf is allowed to be NULL, meaning
* throw away the data.
+ *
+ * When building with DUMP_PACKETS, it's worth debugging even
+ * discarded packets; this makes our stack frame larger, but
+ * DUMP_PACKETS is already for developers. Otherwise, we share a
+ * single static sink buffer across all nbd handles; we don't care
+ * about thread-safety issues with two clients discarding data at
+ * the same time, because we never read the sink.
*/
+#ifdef DUMP_PACKETS
+ char sink[1024];
+#else
+ static char sink[BUFSIZ];
+#endif
+
+ if (h->rlen == 0)
+ return 0; /* move to next state */
+
if (h->rbuf) {
rbuf = h->rbuf;
rlen = h->rlen;
}
else {
- buf = malloc (BUFSIZ);
- if (buf == NULL) {
- set_error (errno, "malloc");
- return -1;
- }
- rbuf = buf;
- rlen = MIN (h->rlen, BUFSIZ);
+ rbuf = sink;
+ rlen = MIN (h->rlen, sizeof sink);
}
r = h->sock->ops->recv (h, h->sock, rbuf, rlen);
if (r == -1) {
- free (buf);
if (errno == EAGAIN || errno == EWOULDBLOCK)
return 1; /* more data */
/* sock->ops->recv called set_error already. */
@@ -78,17 +84,14 @@ recv_into_rbuf (struct nbd_handle *h)
}
if (r == 0) {
set_error (0, "recv: server disconnected unexpectedly");
- free (buf);
return -1;
}
#ifdef DUMP_PACKETS
- if (h->rbuf != NULL)
- nbd_internal_hexdump (h->rbuf, r, stderr);
+ nbd_internal_hexdump (rbuf, r, stderr);
#endif
if (h->rbuf)
h->rbuf += r;
h->rlen -= r;
- free (buf);
if (h->rlen == 0)
return 0; /* move to next state */
else
--
2.37.1