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[Xen-devel] [PATCH for-4.7] tools/libxl: Fix legacy migration following COLO backchannel breakage

c/s f5d947bf1b "tools/libxl: add back channel support to read stream"
made a bogus adjustment to libxl__stream_read_start(), including
removing the comment hinting at what was going on, which breaks
conversion of a legacy migration stream.

Symptoms look like:

  root@anonymi:~ # xl migrate domU host
  migration target: Ready to receive domain.
  Saving to migration stream new xl format (info 0x1/0x0/2677)
  xc: error: error polling suspend notification channel: -1: Internal error
  Loading new save file <incoming migration stream> (new xl fmt info 
   Savefile contains xl domain config in JSON format
  Parsing config from <saved>
  libxl: error: libxl_stream_read.c:327:stream_header_done: Invalid ident: 
expected 0x4c6962786c466d74, got 0x01f00f0000000000
  libxl: error: libxl_utils.c:430:libxl_read_exactly: file/stream truncated 
reading ipc msg header from domain 1 save/restore helper stdout pipe

The adjustment is not required for backchannel support (as there is no
interaction between back channels and legacy conversion), and caused
stream->fd to be latched in the datacopier before legacy conversion
substitutes it for the fd which is the output of the conversion script.

This causes libxl to consume data from the legacy stream rather than the
v2 stream, and for the conversion script to encounter an error as the
legacy stream appears to skip ahead.

Undo the adjustments to libxl__stream_read_start(), and introduce a
better description of what is going on.  Introduce some extra assertions
to try and catch similar breakage in the future.

Reported-by: Olaf Hering <olaf@xxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Olaf Hering <olaf@xxxxxxxxx>
CC: Yang Hongyang <hongyang.yang@xxxxxxxxxxxx>
CC: Wen Congyang <wency@xxxxxxxxxxxxxx>
CC: Changlong Xie <xiecl.fnst@xxxxxxxxxxxxxx>
 tools/libxl/libxl_stream_read.c | 33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/tools/libxl/libxl_stream_read.c b/tools/libxl/libxl_stream_read.c
index 9659051..89c2f21 100644
--- a/tools/libxl/libxl_stream_read.c
+++ b/tools/libxl/libxl_stream_read.c
@@ -234,16 +234,16 @@ void libxl__stream_read_start(libxl__egc *egc,
     stream->running = true;
     stream->phase   = SRS_PHASE_NORMAL;
-    dc->ao       = stream->ao;
-    dc->copywhat = "restore v2 stream";
-    dc->readfd = stream->fd;
-    dc->writefd  = -1;
-    if (stream->back_channel)
-        return;
     if (stream->legacy) {
-        /* Convert the legacy stream. */
+        /*
+         * Convert the legacy stream.
+         *
+         * This results in a fork()/exec() of conversion helper script.  It is
+         * passed the exiting stream->fd as an input, and returns the
+         * transformed stream via a new pipe.  The fd of this new pipe then
+         * replaces stream->fd, to make the rest of the stream read code
+         * agnostic to whether legacy conversion is happening or not.
+         */
         libxl__conversion_helper_state *chs = &stream->chs;
         chs->legacy_fd = stream->fd;
@@ -258,10 +258,25 @@ void libxl__stream_read_start(libxl__egc *egc,
             goto err;
+        /* There should be no interaction of COLO backchannels and legacy
+         * stream conversion. */
+        assert(!stream->back_channel);
+        /* Confirm *dc is still zeroed out, while we shuffle stream->fd. */
+        assert(dc->ao == NULL);
         stream->fd = libxl__carefd_fd(stream->chs.v2_carefd);
         stream->dcs->libxc_fd = stream->fd;
+    /* stream->fd is now a v2 stream. */
+    dc->ao       = stream->ao;
+    dc->copywhat = "restore v2 stream";
+    dc->readfd   = stream->fd;
+    dc->writefd  = -1;
+    if (stream->back_channel)
+        return;
     /* Start reading the stream header. */
     rc = setup_read(stream, "stream header",

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