[Libguestfs] [libnbd PATCH v2 2/5] states: Wire in a read callback
Eric Blake
eblake at redhat.com
Fri Jun 21 01:45:05 UTC 2019
When a server supports structured reads, knowing where the holes are
in the responses can aid the client's behavior. Furthermore, since
the server can send data out of order or interleaved with other
replies, a client may be able to start operating on the partial
results as soon as they are available rather than waiting for the
entire buffer to be reconstructed. As such, it makes sense to have an
optional callback invoked each time we finish reading a chunk (for
simple replies, we behave as if everything was a single data chunk).
This also requires special handling of NBD_REPLY_TYPE_ERROR_OFFSET, as
that is the one situation where we want to call the callback to inform
them that the server pointed out a specific offset for the error - a
client can utilize that knowledge to form a wrapper that permits short
reads (rather than all-or-nothing failures). But as that is the only
error with an offset, all other errors continue to affect only the
final pread return value without any use of the callback.
If any callback fails, and if no prior error was set, then the
callback's failure becomes the failure reason for the overall
read. (Note that this is different from the block status callback,
which for now quits using the callback on subsequent chunks if an
earlier chunk failed - we may decide to change one or the other for
consistency before the API is stable.)
Nothing actually passes a callback function yet, so for now this is no
functional change; but this will make it possible for the next patch
to add an 'nbd_aio_pread_structred' API.
---
generator/generator | 4 +++
generator/states-reply-simple.c | 15 +++++++++-
generator/states-reply-structured.c | 44 +++++++++++++++++++++++++++--
lib/internal.h | 4 ++-
4 files changed, 63 insertions(+), 4 deletions(-)
diff --git a/generator/generator b/generator/generator
index 2d1a4e5..b408509 100755
--- a/generator/generator
+++ b/generator/generator
@@ -1934,6 +1934,10 @@ let constants = [
"CMD_FLAG_FUA", 1 lsl 0;
"CMD_FLAG_NO_HOLE", 1 lsl 1;
"CMD_FLAG_REQ_ONE", 1 lsl 3;
+
+ "READ_DATA", 1;
+ "READ_HOLE", 2;
+ "READ_ERROR", 3;
]
(*----------------------------------------------------------------------*)
diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c
index 935f6d2..87622a0 100644
--- a/generator/states-reply-simple.c
+++ b/generator/states-reply-simple.c
@@ -49,9 +49,22 @@
return 0;
REPLY.SIMPLE_REPLY.RECV_READ_PAYLOAD:
+ struct command_in_flight *cmd = h->reply_cmd;
+
switch (recv_into_rbuf (h)) {
case -1: SET_NEXT_STATE (%.DEAD); return -1;
- case 0: SET_NEXT_STATE (%^FINISH_COMMAND);
+ case 0:
+ /* guaranteed by START */
+ assert (cmd);
+ if (cmd->cb.fn.read) {
+ assert (cmd->error == 0);
+ errno = 0;
+ if (cmd->cb.fn.read (cmd->cb.opaque, cmd->data, cmd->count,
+ cmd->offset, 0, LIBNBD_READ_DATA) == -1)
+ cmd->error = errno ? errno : EPROTO;
+ }
+
+ SET_NEXT_STATE (%^FINISH_COMMAND);
}
return 0;
diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c
index 9dcc898..f1d421d 100644
--- a/generator/states-reply-structured.c
+++ b/generator/states-reply-structured.c
@@ -228,13 +228,16 @@
type = be16toh (h->sbuf.sr.structured_reply.type);
assert (cmd); /* guaranteed by CHECK */
+ error = nbd_internal_errno_of_nbd_error (error);
/* The spec requires the server to send a non-zero error */
if (error == NBD_SUCCESS)
debug (h, "server forgot to set error; using EINVAL");
error = nbd_internal_errno_of_nbd_error (error ? error : EINVAL);
- /* Sanity check that any error offset is in range */
+ /* Sanity check that any error offset is in range, then invoke
+ * user callback if present.
+ */
if (type == NBD_REPLY_TYPE_ERROR_OFFSET) {
offset = be64toh (h->sbuf.sr.payload.error.offset);
if (offset < cmd->offset || offset >= cmd->offset + cmd->count) {
@@ -246,6 +249,17 @@
offset, cmd->offset, cmd->count);
return -1;
}
+ if (cmd->cb.fn.read) {
+ /* Different from successful reads: inform the callback about the
+ * current error rather than any earlier one. If the callback fails
+ * without setting errno, then use the server's error below.
+ */
+ errno = 0;
+ if (cmd->cb.fn.read (cmd->cb.opaque, cmd->data + (offset - cmd->offset),
+ 0, offset, error, LIBNBD_READ_ERROR) == -1)
+ if (cmd->error == 0)
+ cmd->error = errno;
+ }
}
/* Preserve first error encountered */
@@ -313,9 +327,27 @@
return 0;
REPLY.STRUCTURED_REPLY.RECV_OFFSET_DATA_DATA:
+ struct command_in_flight *cmd = h->reply_cmd;
+ uint64_t offset;
+ uint32_t length;
+
switch (recv_into_rbuf (h)) {
case -1: SET_NEXT_STATE (%.DEAD); return -1;
- case 0: SET_NEXT_STATE (%FINISH);
+ case 0:
+ length = be32toh (h->sbuf.sr.structured_reply.length);
+ offset = be64toh (h->sbuf.sr.payload.offset_data.offset);
+
+ assert (cmd); /* guaranteed by CHECK */
+ if (cmd->cb.fn.read) {
+ errno = 0;
+ if (cmd->cb.fn.read (cmd->cb.opaque, cmd->data + (offset - cmd->offset),
+ length - sizeof offset, offset, cmd->error,
+ LIBNBD_READ_DATA) == -1)
+ if (cmd->error == 0)
+ cmd->error = errno ? errno : EPROTO;
+ }
+
+ SET_NEXT_STATE (%FINISH);
}
return 0;
@@ -370,6 +402,14 @@
* them as an extension, and this works even when length == 0.
*/
memset (cmd->data + offset, 0, length);
+ if (cmd->cb.fn.read) {
+ errno = 0;
+ if (cmd->cb.fn.read (cmd->cb.opaque, cmd->data + offset, length,
+ cmd->offset + offset, cmd->error,
+ LIBNBD_READ_HOLE) == -1)
+ if (cmd->error == 0)
+ cmd->error = errno ? errno : EPROTO;
+ }
SET_NEXT_STATE(%FINISH);
}
diff --git a/lib/internal.h b/lib/internal.h
index 3756fac..b79c7a9 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -233,12 +233,14 @@ struct socket {
typedef int (*extent_fn) (void *data, const char *metacontext, uint64_t offset,
uint32_t *entries, size_t nr_entries);
+typedef int (*read_fn) (void *data, const void *buf, size_t count,
+ uint64_t offset, int error, int status);
struct command_cb {
void *opaque;
union {
extent_fn extent;
- /* More to come */
+ read_fn read;
} fn;
};
--
2.20.1
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