test: Print log of iperf server running in guest

We occasionally see failures of `test_rx_rate_limiting` with the
following error:
```
iperf3: error - unable to connect to server - server may have stopped
running or use a different port, firewall issue, etc.: Connection
refused
```
However, we didn't have any diagnostic info about what happened inside a
guest.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

tests/integration_tests/performance/test_rate_limiter.py

@@ -9,6 +9,9 @@
 # The iperf version to run this tests with
 IPERF_BINARY = "iperf3"
 
+# iperf server side log path inside guest
+GUEST_IPERF_SERVER_LOG = "/tmp/iperf-server.log"
+
 # Interval used by iperf to get maximum bandwidth
 IPERF_TRANSMIT_TIME = 4
 
@@ -127,7 +130,7 @@ def test_rx_rate_limiting_cpu_load(uvm_plain):
         threshold=20,
     )
     with cpu_load_monitor:
-        _run_iperf_on_host(iperf_cmd)
+        _run_iperf_on_host(iperf_cmd, test_microvm)
 
 
 def _check_tx_rate_limiting(test_microvm):
@@ -219,8 +222,7 @@ def _check_rx_rate_limiting(test_microvm):
         BURST_SIZE,
         IPERF_TCP_WINDOW,
     )
-    iperf_out = _run_iperf_on_host(iperf_cmd)
-    print(iperf_out)
+    iperf_out = _run_iperf_on_host(iperf_cmd, test_microvm)
     _, burst_kbps = _process_iperf_output(iperf_out)
     print("RX burst_kbps: {}".format(burst_kbps))
     # Test that the burst bandwidth is at least as two times the rate limit.
@@ -357,9 +359,7 @@ def _get_rx_bandwidth_with_duration(test_microvm, guest_ip, duration):
         duration,
         IPERF_TCP_WINDOW,
     )
-    iperf_out = _run_iperf_on_host(iperf_cmd)
-    print(iperf_out)
-
+    iperf_out = _run_iperf_on_host(iperf_cmd, test_microvm)
     _, observed_kbps = _process_iperf_output(iperf_out)
     print("RX observed_kbps: {}".format(observed_kbps))
     return observed_kbps
@@ -383,7 +383,7 @@ def _patch_iface_bw(test_microvm, iface_id, rx_or_tx, new_bucket_size, new_refil
 
 def _start_iperf_server_on_guest(test_microvm):
     """Start iperf in server mode through an SSH connection."""
-    iperf_cmd = "{} -sD -f KBytes\n".format(IPERF_BINARY)
+    iperf_cmd = f"{IPERF_BINARY} -sD -f KBytes --logfile {GUEST_IPERF_SERVER_LOG}"
     test_microvm.ssh.run(iperf_cmd)
 
     # Wait for the iperf daemon to start.
@@ -411,10 +411,15 @@ def _start_iperf_server_on_host(netns_cmd_prefix):
     time.sleep(1)
 
 
-def _run_iperf_on_host(iperf_cmd):
+def _run_iperf_on_host(iperf_cmd, test_microvm):
     """Execute a client related iperf command locally."""
-    code, stdout, stderr = utils.check_output(iperf_cmd)
-    assert code == 0, f"stdout: {stdout}\nstderr: {stderr}"
+    rc, stdout, stderr = utils.run_cmd(iperf_cmd)
+    assert rc == 0, "stdout:\n{}\nstderr:\n{}\niperf server log:\n{}\n".format(
+        stdout,
+        stderr,
+        test_microvm.ssh.check_output(f"cat {GUEST_IPERF_SERVER_LOG}").stdout,
+    )
+    print(f"iperf log:\n{stdout}")
 
     return stdout