[PATCH v8 01/17] libxl: Enhance libxl__sendmsg_fds to deal with EINTR and EWOULDBLOCK

[Anthony PERARD <anthony.perard@citrix.com>]

This patch change the behavior of libxl__sendmsg_fds to retry sendmsg on
EINTR error and return an error on short writes.

This patch allow a caller of libxl__sendmsg_fds to deal with EWOULDBLOCK
and short writes. The function now requires to send only 1 byte of data
so that when dealing with non-blocking fds a EWOULDBLOCK error would
mean that the fds haven't been sent yet. Current caller already send
only 1 byte.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---

Notes:
    v8:
        Acked, but:
            - there is a follow-up patch to get rid of datalen param.
            - commit message have been modified a bit to talk about short
              write in general.
    
    v7:
        always assert datalen == 1, but only fail when sendmsg haven't send
        everything (r != datalen)
        check sendmsg return value on success as well (check for short write)

 tools/libxl/libxl_internal.h |  5 ++++-
 tools/libxl/libxl_utils.c    | 25 ++++++++++++++++++++-----
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 67f3f6ac76..aa9059ffec 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1876,7 +1876,10 @@ _hidden void libxl__qmp_cleanup(libxl__gc *gc, uint32_t domid);
 _hidden int libxl__qmp_initializations(libxl__gc *gc, uint32_t domid,
                                        const libxl_domain_config *guest_config);
 
-/* on failure, logs */
+/* `datalen` should be 1 byte
+ * When dealing with a non-blocking fd, it returns
+ *   ERROR_NOT_READY on EWOULDBLOCK
+ * logs on other failures. */
 int libxl__sendmsg_fds(libxl__gc *gc, int carrier,
                        const void *data, size_t datalen,
                        int nfds, const int fds[], const char *what);
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index 5854717b11..d53db8c37d 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -1070,6 +1070,8 @@ int libxl__sendmsg_fds(libxl__gc *gc, int carrier,
     struct iovec iov;
     int r;
 
+    assert(datalen == 1);
+
     iov.iov_base = (void*)data;
     iov.iov_len  = datalen;
 
@@ -1088,11 +1090,24 @@ int libxl__sendmsg_fds(libxl__gc *gc, int carrier,
 
     msg.msg_controllen = cmsg->cmsg_len;
 
-    r = sendmsg(carrier, &msg, 0);
-    if (r < 0) {
-        LOGE(ERROR, "failed to send fd-carrying message (%s)", what);
-        return ERROR_FAIL;
-    }
+    while (1) {
+        r = sendmsg(carrier, &msg, 0);
+        if (r < 0) {
+            if (errno == EINTR)
+                continue;
+            if (errno == EWOULDBLOCK) {
+                return ERROR_NOT_READY;
+            }
+            LOGE(ERROR, "failed to send fd-carrying message (%s)", what);
+            return ERROR_FAIL;
+        }
+        if (r != datalen) {
+            LOG(ERROR, "sendmsg have written %d instead of %ld",
+                r, datalen);
+            return ERROR_FAIL;
+        }
+        break;
+    };
 
     return 0;
 }
-- 
Anthony PERARD


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