iotests: Let 233 run concurrently

common.nbd's nbd_server_set_tcp_port() tries to find a free port, and
then uses it for the whole test run.  However, this is racy because even
if the port was free at the beginning, there is no guarantee it will
continue to be available.  Therefore, 233 currently cannot reliably be
run concurrently with other NBD TCP tests.

This patch addresses the problem by dropping nbd_server_set_tcp_port(),
and instead finding a new port every time nbd_server_start_tcp_socket()
is invoked.  For this, we run qemu-nbd with --fork and on error evaluate
the output to see whether it contains "Address already in use".  If so,
we try the next port.

On success, we still want to continually redirect the output from
qemu-nbd to stderr.  To achieve both, we redirect qemu-nbd's stderr to a
FIFO that we then open in bash.  If the parent process exits with status
0 (which means that the server has started successfully), we launch a
background cat process that copies the FIFO to stderr.  On failure, we
read the whole content into a variable and then evaluate it.

While at it, use --fork in nbd_server_start_unix_socket(), too.  Doing
so allows us to drop nbd_server_wait_for_*_socket().

Note that the reason common.nbd did not use --fork before is that
qemu-nbd did not have --pid-file.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190508211820.17851-6-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/233

@@ -49,7 +49,6 @@ _supported_proto file
 # If porting to non-Linux, consider using socat instead of ss in common.nbd
 _require_command QEMU_NBD
 
-nbd_server_set_tcp_port
 tls_x509_init
 
 echo

tests/qemu-iotests/common.nbd

@@ -22,6 +22,11 @@
 nbd_unix_socket="${TEST_DIR}/qemu-nbd.sock"
 nbd_tcp_addr="127.0.0.1"
 nbd_pid_file="${TEST_DIR}/qemu-nbd.pid"
+nbd_stderr_fifo="${TEST_DIR}/qemu-nbd.fifo"
+
+# If bash version is >= 4.1, this will be overwritten by a dynamically
+# assigned file descriptor value.
+nbd_fifo_fd=10
 
 nbd_server_stop()
 {
@@ -33,77 +38,62 @@ nbd_server_stop()
             kill "$NBD_PID"
         fi
     fi
-    rm -f "$nbd_unix_socket"
-}
-
-nbd_server_wait_for_unix_socket()
-{
-    pid=$1
-
-    for ((i = 0; i < 300; i++))
-    do
-        if [ -r "$nbd_unix_socket" ]; then
-            return
-        fi
-        kill -s 0 $pid 2>/dev/null
-        if test $? != 0
-        then
-            echo "qemu-nbd unexpectedly quit"
-            exit 1
-        fi
-        sleep 0.1
-    done
-    echo "Failed in check of unix socket created by qemu-nbd"
-    exit 1
+    rm -f "$nbd_unix_socket" "$nbd_stderr_fifo"
 }
 
 nbd_server_start_unix_socket()
 {
     nbd_server_stop
-    $QEMU_NBD -v -t -k "$nbd_unix_socket" "$@" &
-    nbd_server_wait_for_unix_socket $!
-}
-
-nbd_server_set_tcp_port()
-{
-    (ss --help) >/dev/null 2>&1 || _notrun "ss utility not found, skipping test"
-
-    for ((port = 10809; port <= 10909; port++))
-    do
-        if ! ss -tln | grep -sqE ":$port\b"; then
-            nbd_tcp_port=$port
-            return
-        fi
-    done
-
-    echo "Cannot find free TCP port for nbd in range 10809-10909"
-    exit 1
-}
-
-nbd_server_wait_for_tcp_socket()
-{
-    pid=$1
-
-    for ((i = 0; i < 300; i++))
-    do
-        if ss -tln | grep -sqE ":$nbd_tcp_port\b"; then
-            return
-        fi
-        kill -s 0 $pid 2>/dev/null
-        if test $? != 0
-        then
-            echo "qemu-nbd unexpectedly quit"
-            exit 1
-        fi
-        sleep 0.1
-    done
-    echo "Failed in check of TCP socket created by qemu-nbd"
-    exit 1
+    $QEMU_NBD -v -t -k "$nbd_unix_socket" --fork "$@"
 }
 
 nbd_server_start_tcp_socket()
 {
     nbd_server_stop
-    $QEMU_NBD -v -t -b $nbd_tcp_addr -p $nbd_tcp_port "$@" &
-    nbd_server_wait_for_tcp_socket $!
+
+    mkfifo "$nbd_stderr_fifo"
+    for ((port = 10809; port <= 10909; port++))
+    do
+        # Redirect stderr to FIFO, so we can later decide whether we
+        # want to read it or to redirect it to our stderr, depending
+        # on whether the command fails or not
+        $QEMU_NBD -v -t -b $nbd_tcp_addr -p $port --fork "$@" \
+            2> "$nbd_stderr_fifo" &
+
+        # Taken from common.qemu
+        if [[ "${BASH_VERSINFO[0]}" -ge "5" ||
+            ("${BASH_VERSINFO[0]}" -ge "4" && "${BASH_VERSINFO[1]}" -ge "1") ]]
+        then
+            exec {nbd_fifo_fd}<"$nbd_stderr_fifo"
+        else
+            let _nbd_fifo_fd++
+            eval "exec ${_nbd_fifo_fd}<'$nbd_stderr_fifo'"
+        fi
+        wait $!
+
+        if test $? == 0
+        then
+            # Success, redirect qemu-nbd's stderr to our stderr
+            nbd_tcp_port=$port
+            (cat <&$nbd_fifo_fd >&2) &
+            eval "exec $nbd_fifo_fd>&-"
+            return
+        fi
+
+        # Failure, read the output
+        output=$(cat <&$nbd_fifo_fd)
+        eval "exec $nbd_fifo_fd>&-"
+
+        if ! echo "$output" | grep -q "Address already in use"
+        then
+            # Unknown error, print it
+            echo "$output" >&2
+            rm -f "$nbd_stderr_fifo"
+            exit 1
+        fi
+    done
+
+    echo "Cannot find free TCP port for nbd in range 10809-10909"
+    rm -f "$nbd_stderr_fifo"
+    exit 1
 }