[Xen-devel] [PATCH 2/3] libxl: Cope with pipes which signal POLLHUP|POLLIN on read eof

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Some operating systems (including FreeBSD[1]) signal not just POLLIN
when a reading pipe reaches EOF, but also POLLHUP.  This is
permitted[2].  But datacopiers mishandle this, because they always
treat POLLHUP exceptionally (either reporting it via callback_pollhup,
or treating it as an error).  datacopiers reading from pipes on such
OSs can fail (perhaps leaving some data unprocessed) rather than
completing successfully.

[1] http://www.greenend.org.uk/rjk/tech/poll.html
[2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html

Most callers in libxl do not care about POLLHUP except as an error
condition.

So change the datacopier semantics so that unless callback_pollhup is
specified we treat POLLHUP|POLLIN as if it were POLLIN - and, read all
available data).

This fixes the problem which 7e9ec50b0535 was aimed at.

Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 tools/libxl/libxl_aoutils.c  |    2 +-
 tools/libxl/libxl_internal.h |    3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl_aoutils.c b/tools/libxl/libxl_aoutils.c
index 3942634..c3232a6 100644
--- a/tools/libxl/libxl_aoutils.c
+++ b/tools/libxl/libxl_aoutils.c
@@ -208,7 +208,7 @@ static void datacopier_readable(libxl__egc *egc, 
libxl__ev_fd *ev,
     if (datacopier_pollhup_handled(egc, dc, revents, 0))
         return;
 
-    if (revents & ~POLLIN) {
+    if (revents & ~POLLIN && revents != (POLLIN|POLLHUP)) {
         LOG(ERROR, "unexpected poll event 0x%x (should be POLLIN)"
             " on %s during copy of %s", revents, dc->readwhat, dc->copywhat);
         datacopier_callback(egc, dc, -1, 0);
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 7f81dd3..c6969f0 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -2535,7 +2535,8 @@ typedef struct libxl__datacopier_buf 
libxl__datacopier_buf;
  *     errnoval!=0 means we had a read error, logged
  * onwrite==-1 means some other internal failure, errnoval not valid, logged
  * If we get POLLHUP, we call callback_pollhup(..., onwrite, -1);
- * or if callback_pollhup==0 this is an internal failure, as above.
+ * or if callback_pollhup==0 this is treated as eof (if POLLIN|POLLHUP
+ * on the reading fd) or an internal failure (otherwise), as above.
  * In all cases copier is killed before calling this callback */
 typedef void libxl__datacopier_callback(libxl__egc *egc,
      libxl__datacopier_state *dc, int onwrite, int errnoval);
-- 
1.7.10.4


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