[PATCH 1/2] tests: fix hang in virshtest 'read-big-pipe' case

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

The virshtest program testPipeFeeder method is doing this:

  mkfifo("test.fifo", 0600) ;

  int fd = open("test.fifo", O_RDWR);

  char buf[...];
  memset(buf, 'a', sizeof(buf));
  write(fd, buf, sizeof(buf)) == sizeof(buf));
  close(fd);

while the the 'virsh' child process then ends up doing:

  fd = open("test.fifo", O_RDONLY);
  read(fd, buf, sizeof(buf)) == sizeof(buf));
  close(fd);

The 'virsh' code hangs on open() on at least ppc64 and some other
arches. It can be provoked to hang even on x86 by reducing the size of
the buffer. It can be prevented from hanging on ppc64 by increasing the
size of the buffer.

What is happening is a result of differing page sizes, altering the
overall pipe capacity size, since pipes on linux default to 16 pages
in size and thus have architecture specific capacity when measured
in bytes.

 * On x86, testPipeFeeder opens R+W, tries to write 140kb and
   write() blocks because the pipe is full. This gives time for
   virsh to start up, and it can open the pipe for O_RDONLY
   since testPipeFeeder still has it open for write. Everything
   works as intended.

 * On ppc64,  testPipeFeeder opens R+W, tries to write 140kb
   and write() succeeds because the larger 64kb page size
   resulted in greater buffer capacity for the pipe. It thus
   quickly closes the pipe, removing the writer, and triggering
   discard of all the unread data. Now virsh starts up, tries
   to open the pipe for O_RDONLY and blocks waiting for a new
   writer to open it, which will never happen. Meson kills it
   the test after 30 seconds.

   NB, every now & then, it will not block because virsh starts
   up quickly enough that testPipeFeeder has not yet closed the
   write end of the pipe, giving the illusion of correctness.

The key flaw here is that it should not have been using O_RDWR
in testPipeFeeder. Synchronization is required such that both
virsh and testPipeFeeder have their respective ends of the pipe
open before any data is sent. This is trivially arranged by
using O_WRONLY in testPipeFeeder.

Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
---
 tests/virshtest.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/virshtest.c b/tests/virshtest.c
index a1ae481316..7a7797647c 100644
--- a/tests/virshtest.c
+++ b/tests/virshtest.c
@@ -145,7 +145,7 @@ testPipeFeeder(void *opaque)
     g_autofree char *doc = g_new0(char, emptyspace + xmlsize + 1);
     VIR_AUTOCLOSE fd = -1;
 
-    if ((fd = open(pipepath, O_RDWR)) < 0) {
+    if ((fd = open(pipepath, O_WRONLY)) < 0) {
         fprintf(stderr, "\nfailed to open pipe '%s': %s\n", pipepath, g_strerror(errno));
         return;
     }
-- 
2.43.0
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