Correctly parse /etc/resolv.conf when contain multiple entries for searchdomain. (#8351)

Motivation:

ba594bcf4a added a utility to parse searchdomains defined in /etc/resolv.conf but did not correctly handle the case when multiple are defined that are seperated by either whitespace or tab.

Modifications:

- Correctly parse multiple entries
- Add unit test.

Result:

Correctly parse multiple searchdomain entries.

resolver-dns/src/main/java/io/netty/resolver/dns/UnixResolverDnsServerAddressStreamProvider.java

@@ -27,12 +27,12 @@ import java.io.FileReader;
 import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.regex.Pattern;
 
 import static io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.DNS_PORT;
 import static io.netty.util.internal.ObjectUtil.checkNotNull;
@@ -59,6 +59,7 @@ public final class UnixResolverDnsServerAddressStreamProvider implements DnsServ
     static final int DEFAULT_NDOTS = 1;
     private final DnsServerAddresses defaultNameServerAddresses;
     private final Map<String, DnsServerAddresses> domainToNameServerStreamMap;
+    private static final Pattern SEARCH_DOMAIN_PATTERN = Pattern.compile("\\s+");
 
     /**
      * Attempt to parse {@code /etc/resolv.conf} and files in the {@code /etc/resolver} directory by default.
@@ -323,7 +324,10 @@ public final class UnixResolverDnsServerAddressStreamProvider implements DnsServ
                 } else if (line.startsWith(SEARCH_ROW_LABEL)) {
                     int i = indexOfNonWhiteSpace(line, SEARCH_ROW_LABEL.length());
                     if (i >= 0) {
-                        searchDomains.add(line.substring(i));
+                        // May contain more then one entry, either seperated by whitespace or tab.
+                        // See https://linux.die.net/man/5/resolver
+                        String[] domains = SEARCH_DOMAIN_PATTERN.split(line.substring(i));
+                        Collections.addAll(searchDomains, domains);
                     }
                 }
             }

resolver-dns/src/test/java/io/netty/resolver/dns/UnixResolverDnsServerAddressStreamProviderTest.java

@@ -139,6 +139,22 @@ public class UnixResolverDnsServerAddressStreamProviderTest {
         assertEquals(Arrays.asList("linecorp.local", "squarecorp.local"), domains);
     }
 
+    @Test
+    public void searchDomainsWithMultipleSearchSeperatedByWhitespace() throws IOException {
+        File f = buildFile("search linecorp.local squarecorp.local\n" +
+                           "nameserver 127.0.0.2\n");
+        List<String> domains = UnixResolverDnsServerAddressStreamProvider.parseEtcResolverSearchDomains(f);
+        assertEquals(Arrays.asList("linecorp.local", "squarecorp.local"), domains);
+    }
+
+    @Test
+    public void searchDomainsWithMultipleSearchSeperatedByTab() throws IOException {
+        File f = buildFile("search linecorp.local\tsquarecorp.local\n" +
+                "nameserver 127.0.0.2\n");
+        List<String> domains = UnixResolverDnsServerAddressStreamProvider.parseEtcResolverSearchDomains(f);
+        assertEquals(Arrays.asList("linecorp.local", "squarecorp.local"), domains);
+    }
+
     @Test
     public void searchDomainsPrecedence() throws IOException {
         File f = buildFile("domain linecorp.local\n" +