[PATCH v2 15/17] virfdstream: Emulate skip for block devices
This is similar to one of previous patches.
When receiving stream (on virStorageVolUpload() and subsequent
virStreamSparseSendAll()) we may receive a hole. If the volume we
are saving the incoming data into is a regular file we just
lseek() and ftruncate() to create the hole. But this won't work
if the file is a block device. If that is the case we must write
zeroes so that any subsequent reader reads nothing just zeroes
(just like they would from a hole in a regular file).
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1852528
Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com>
---
src/util/virfdstream.c | 59 +++++++++++++++++++++++++++++++-----------
1 file changed, 44 insertions(+), 15 deletions(-)
diff --git a/src/util/virfdstream.c b/src/util/virfdstream.c
index 44b4855549..40825a8811 100644
--- a/src/util/virfdstream.c
+++ b/src/util/virfdstream.c
@@ -505,6 +505,7 @@ virFDStreamThreadDoRead(virFDStreamDataPtr fdst,
static ssize_t
virFDStreamThreadDoWrite(virFDStreamDataPtr fdst,
bool sparse,
+ bool isBlock,
const int fdin,
const int fdout,
const char *fdinname,
@@ -512,7 +513,6 @@ virFDStreamThreadDoWrite(virFDStreamDataPtr fdst,
{
ssize_t got = 0;
virFDStreamMsgPtr msg = fdst->msg;
- off_t off;
bool pop = false;
switch (msg->type) {
@@ -540,19 +540,48 @@ virFDStreamThreadDoWrite(virFDStreamDataPtr fdst,
}
got = msg->stream.hole.len;
- off = lseek(fdout, got, SEEK_CUR);
- if (off == (off_t) -1) {
- virReportSystemError(errno,
- _("unable to seek in %s"),
- fdoutname);
- return -1;
- }
-
- if (ftruncate(fdout, off) < 0) {
- virReportSystemError(errno,
- _("unable to truncate %s"),
- fdoutname);
- return -1;
+ if (isBlock) {
+ g_autofree char * buf = NULL;
+ const size_t buflen = 1 * 1024 * 1024; /* 1MiB */
+ size_t toWrite = got;
+
+ /* While for files it's enough to lseek() and ftruncate() to create
+ * a hole which would emulate zeroes on read(), for block devices
+ * we have to write zeroes to read() zeroes. And we have to write
+ * @got bytes of zeroes. Do that in smaller chunks though.*/
+
+ buf = g_new0(char, buflen);
+
+ while (toWrite) {
+ size_t count = MIN(toWrite, buflen);
+ ssize_t r;
+
+ if ((r = safewrite(fdout, buf, count)) < 0) {
+ virReportSystemError(errno,
+ _("Unable to write %s"),
+ fdoutname);
+ return -1;
+ }
+
+ toWrite -= r;
+ }
+ } else {
+ off_t off;
+
+ off = lseek(fdout, got, SEEK_CUR);
+ if (off == (off_t) -1) {
+ virReportSystemError(errno,
+ _("unable to seek in %s"),
+ fdoutname);
+ return -1;
+ }
+
+ if (ftruncate(fdout, off) < 0) {
+ virReportSystemError(errno,
+ _("unable to truncate %s"),
+ fdoutname);
+ return -1;
+ }
}
pop = true;
@@ -618,7 +647,7 @@ virFDStreamThread(void *opaque)
length, total,
&dataLen, buflen);
else
- got = virFDStreamThreadDoWrite(fdst, sparse,
+ got = virFDStreamThreadDoWrite(fdst, sparse, isBlock,
fdin, fdout,
fdinname, fdoutname);
--
2.26.2