test: fix flaky testcase_15_wait_online_dns in TEST-75-RESOLVED

The test used `timeout 30 bash -c "journalctl -b -u $unit -f | grep -m1 ..."`
to wait for systemd-networkd-wait-online to log that no DNS server is
accessible. The expected message is actually emitted ~1s after the unit
starts, but `grep -m1` exiting doesn't tear down `journalctl -f`: journalctl
only notices the closed pipe on its next write, which may never happen for
an otherwise idle unit. The pipeline therefore hangs until the 30s timeout
fires, eventually causing the test to fail.

Replace the follow+pipe with a polling `journalctl --grep` loop, which
exits cleanly as soon as the message lands in the journal.

Logs from the failing run:

  [ 2650.871441] systemd-networkd-wait-online[2190]: dns0: No DNS configuration yet
  [ 2651.723180] systemd-networkd-wait-online[2190]: dns0: No DNS server is accessible.
  [ 2680.909048] systemd-networkd-wait-online[2190]: json-stream: Got POLLHUP from socket.
  [ 2680.909092] systemd-networkd-wait-online[2190]: DNS configuration monitor disconnected, reconnecting...
  [ 2680.914368] systemd-networkd-wait-online[2190]: Failed to connect to io.systemd.Resolve.Monitor: Connection refused
  [ 2681.966674] systemd-networkd-wait-online[2190]: dns0: No DNS server is accessible.
  [ 2681.969527] systemd-networkd-wait-online[2190]: Failed to connect to io.systemd.Resolve.Monitor: Connection refused
  [ 2682.077032] systemd[1]: Stopping wait-online-dns-0f9e4f6d-8b34-4cff-b2da-03612ca731e8.service - [systemd-run] /usr/lib/systemd/systemd-networkd-wait-online --timeout=0 --dns --interface=dns0...

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>

diff --git a/test/units/TEST-75-RESOLVED.sh b/test/units/TEST-75-RESOLVED.sh
index bb1cf9576c2924d33f074ced917f95c7ab4d22d2..7b5670a4bc72677805a557993fc30d62a1f5b626 100755 (executable)
--- a/test/units/TEST-75-RESOLVED.sh
+++ b/test/units/TEST-75-RESOLVED.sh
@@ -1421,7 +1421,7 @@ testcase_15_wait_online_dns() {
         /usr/lib/systemd/systemd-networkd-wait-online --timeout=0 --dns --interface=dns0
 
     # Wait until it blocks waiting for updated DNS config
-    timeout 30 bash -c "journalctl -b -u $unit -f | grep -m1 'dns0: No.*DNS server is accessible'" >/dev/null
+    timeout 30 bash -c "until journalctl -b -u $unit --grep 'dns0: No.*DNS server is accessible' >/dev/null 2>&1; do sleep 0.5; done"
 
     # Update the global configuration. Restart rather than reload systemd-resolved so that
     # systemd-networkd-wait-online has to re-connect to the varlink service.