test: use socat in unidirectional mode

By default socat open a separate r/w channel for each specified address,
and terminates the connection after .5s from receiving EOF on _either_
side. And since one side of that connection is an empty stdin, we reach
that EOF pretty quickly. Let's avoid this by using socat in
"reversed unidirectional" mode, where the first address is used only for
writing, and the second one is used only for reading.

Addresses:
  - https://github.com/systemd/systemd/issues/31500
  - https://github.com/systemd/systemd/issues/31493

Follow-up for 3456c89ac2.

test/units/testsuite-74.socket.sh

@@ -9,21 +9,20 @@ set -o pipefail
 
 at_exit() {
     systemctl stop per-source-limit.socket
-    rm -f /run/systemd/system/per-source-limit.socket /run/systemd/system/per-source-limit@.service
-    rm -f /tmp/foo.conn1 /tmp/foo.conn2 /tmp/foo.conn3 /tmp/foo.conn4
+    rm -f /run/systemd/system/per-source-limit{@.service,.socket} /run/foo.conn{1..4}
     systemctl daemon-reload
 }
 
 trap at_exit EXIT
 
-cat > /run/systemd/system/per-source-limit.socket <<EOF
+cat >/run/systemd/system/per-source-limit.socket <<EOF
 [Socket]
 ListenStream=/run/per-source-limit.sk
 MaxConnectionsPerSource=2
 Accept=yes
 EOF
 
-cat > /run/systemd/system/per-source-limit@.service <<EOF
+cat >/run/systemd/system/per-source-limit@.service <<EOF
 [Unit]
 BindsTo=per-source-limit.socket
 After=per-source-limit.socket
@@ -36,17 +35,27 @@ EOF
 
 systemctl daemon-reload
 systemctl start per-source-limit.socket
+systemctl status per-source-limit.socket
 
 # So these two should take up the first two connection slots
-socat - UNIX-CONNECT:/run/per-source-limit.sk > /tmp/foo.conn1 &
+socat -U - UNIX-CONNECT:/run/per-source-limit.sk | tee /tmp/foo.conn1 &
 J1="$!"
-socat - UNIX-CONNECT:/run/per-source-limit.sk > /tmp/foo.conn2 &
+socat -U - UNIX-CONNECT:/run/per-source-limit.sk | tee /tmp/foo.conn2 &
 J2="$!"
 
 waitfor() {
-    while ! grep -q "waldo" "$1" ; do
-        sleep .2
+    local file="${1:?}"
+
+    for _ in {0..20}; do
+        if grep -q waldo "$file"; then
+            return 0
+        fi
+
+        sleep .5
     done
+
+    echo >&2 "Timeout while waiting for the expected output"
+    return 1
 }
 
 # Wait until the word "waldo" shows in the output files
@@ -54,11 +63,11 @@ waitfor /tmp/foo.conn1
 waitfor /tmp/foo.conn2
 
 # The next connection should fail, because the limit is hit
-socat - UNIX-CONNECT:/run/per-source-limit.sk > /tmp/foo.conn3 &
+socat -U - UNIX-CONNECT:/run/per-source-limit.sk | tee /tmp/foo.conn3 &
 J3="$!"
 
 # But this one should work, because done under a different UID
-setpriv --reuid=1 socat - UNIX-CONNECT:/run/per-source-limit.sk > /tmp/foo.conn4 &
+setpriv --reuid=1 socat -U - UNIX-CONNECT:/run/per-source-limit.sk | tee /tmp/foo.conn4 &
 J4="$!"
 
 waitfor /tmp/foo.conn4