Pre-test bind-source-addr before running test. (#9214)

This attempts to catch any non-standard configuration where the test may
fail and produce a false positive.

tests/unit/networking.tcl

@@ -37,8 +37,40 @@ test {CONFIG SET bind address} {
     }
 } {} {external:skip}
 
+# Attempt to connect to host using a client bound to bindaddr,
+# and return a non-zero value if successful within specified
+# millisecond timeout, or zero otherwise.
+proc test_loopback {host bindaddr timeout} {
+    if {[exec uname] != {Linux}} {
+        return 0
+    }
+
+    after $timeout set ::test_loopback_state timeout
+    if {[catch {
+        set server_sock [socket -server accept 0]
+        set port [lindex [fconfigure $server_sock -sockname] 2] } err]} {
+            return 0
+    }
+
+    proc accept {channel clientaddr clientport} {
+        set ::test_loopback_state "connected"
+        close $channel
+    }
+
+    if {[catch {set client_sock [socket -async -myaddr $bindaddr $host $port]} err]} {
+        puts "test_loopback: Client connect failed: $err"
+    } else {
+        close $client_sock
+    }
+
+    vwait ::test_loopback_state
+    close $server_sock
+
+    return [expr {$::test_loopback_state == {connected}}]
+}
+
 test {CONFIG SET bind-source-addr} {
-    if {[exec uname] == {Linux}} {
+    if {[test_loopback 127.0.0.1 127.0.0.2 1000]} {
         start_server {} {
             start_server {} {
                 set replica [srv 0 client]
@@ -58,6 +90,8 @@ test {CONFIG SET bind-source-addr} {
                 assert_match {*ip=127.0.0.2*} [s -1 slave0]
             }
         }
+    } else {
+        if {$::verbose} { puts "Skipping bind-source-addr test." }
     }
 } {} {external:skip}