[PATCH v2 12/17] virshStreamSkip: Emulate skip for block devices

[Michal Privoznik]

This callback is called when the server sends us STREAM_HOLE
meaning there is no real data, only zeroes. For regular files
we would just seek() beyond EOF and ftruncate() to create the
hole. But for block devices this won't work. Not only we can't
seek() beyond EOF, and ftruncate() will fail, this approach won't
fill the device with zeroes. We have to do it manually.

Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1852528

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 tools/virsh-util.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/tools/virsh-util.c b/tools/virsh-util.c
index 89f15efd08..884261eb49 100644
--- a/tools/virsh-util.c
+++ b/tools/virsh-util.c
@@ -189,11 +189,33 @@ virshStreamSkip(virStreamPtr st G_GNUC_UNUSED,
     virshStreamCallbackDataPtr cbData = opaque;
     off_t cur;
 
-    if ((cur = lseek(cbData->fd, offset, SEEK_CUR)) == (off_t) -1)
-        return -1;
+    if (cbData->isBlock) {
+        g_autofree char * buf = NULL;
+        const size_t buflen = 1 * 1024 * 1024; /* 1MiB */
 
-    if (ftruncate(cbData->fd, cur) < 0)
-        return -1;
+        /* 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 (offset) {
+            size_t count = MIN(offset, buflen);
+            ssize_t r;
+
+            if ((r = safewrite(cbData->fd, buf, count)) < 0)
+                return -1;
+
+            offset -= r;
+        }
+    } else {
+        if ((cur = lseek(cbData->fd, offset, SEEK_CUR)) == (off_t) -1)
+            return -1;
+
+        if (ftruncate(cbData->fd, cur) < 0)
+            return -1;
+    }
 
     return 0;
 }
-- 
2.26.2