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vfs_glusterfs: Add comments to the pipe(2) code.

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The guarantees around read(2) and write(2) and pipes are critical
to understanding this code.  Hopefully these comments will help.

Signed-off-by: Ira Cooper <ira@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jan 23 20:58:51 CET 2015 on sn-devel-104
This commit is contained in:
Ira Cooper 2015-01-22 17:14:31 -05:00 committed by Jeremy Allison
parent fba69f4a89
commit 3b2d8bdbb1
1 changed files with 17 additions and 0 deletions
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17
source3/modules/vfs_glusterfs.c
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@ -521,6 +521,13 @@ static void aio_glusterfs_done(glfs_fd_t *fd, ssize_t ret, void *data)
* be called here, as it is not designed to be executed
* in the multithread environment, tevent_req_done() must be
* executed from the smbd main thread.
*
* write(2) on pipes with sizes under _POSIX_PIPE_BUF
* in size is atomic, without this, the use op pipes in this
* code would not work.
*
* sys_write is a thin enough wrapper around write(2)
* that we can trust it here.
*/
sts = sys_write(write_fd, &req, sizeof(struct tevent_req *));
@ -541,6 +548,16 @@ static void aio_tevent_fd_done(struct tevent_context *event_ctx,
struct tevent_req *req = NULL;
int sts = 0;
/*
* read(2) on pipes is atomic if the needed data is available
* in the pipe, per SUS and POSIX. Because we always write
* to the pipe in sizeof(struct tevent_req *) chunks, we can
* always read in those chunks, atomically.
*
* sys_read is a thin enough wrapper around read(2) that we
* can trust it here.
*/
sts = sys_read(read_fd, &req, sizeof(struct tevent_req *));
if (sts < 0) {
DEBUG(0,("\nRead from pipe failed (%s)", strerror(errno)));