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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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package jdk.dns.client.internal;

import jdk.dns.client.AddressFamily;

import jdk.dns.client.internal.util.ReloadTracker;

import sun.net.util.IPAddressUtil;

import java.net.InetAddress;

import java.net.UnknownHostException;

import java.nio.charset.StandardCharsets;

import java.nio.file.Files;

import java.nio.file.Path;

import java.nio.file.Paths;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.security.PrivilegedActionException;

import java.security.PrivilegedExceptionAction;

import java.util.Arrays;

import java.util.Collections;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.Objects;

import java.util.concurrent.locks.ReadWriteLock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

import java.util.function.Predicate;

import java.util.stream.Stream;

public class HostsFileResolver {

    private static final String HOSTS_FILE_LOCATION_PROPERTY_VALUE =

            AccessController.doPrivileged((PrivilegedAction<String>)

                    () -> System.getProperty("jdk.net.hosts.file", "/etc/hosts")

            );

    private static final ReadWriteLock LOCK = new ReentrantReadWriteLock();

    // 300 seconds, similar to DnsResolverConfiguration in millis since Epoch

    private static final long REFRESH_TIMEOUT_MILLIS = 300_000;

    private static final ReloadTracker HOSTS_FILE_TRACKER;

    private static volatile Map<String, HostFileEntry> HOST_ADDRESSES = Collections.emptyMap();

    void loadHostsAddresses() {

        LOCK.readLock().lock();

        var rsf = HOSTS_FILE_TRACKER.getReloadStatus();

        try {

            if (!HOSTS_FILE_TRACKER.getReloadStatus().isReloadNeeded()) {

                return;

            }

        } finally {

            LOCK.readLock().unlock();

        }

        LOCK.writeLock().lock();

        try {

            var rs = HOSTS_FILE_TRACKER.getReloadStatus();

            // Check if reload is still needed

            if (rs.isReloadNeeded()) {

                if (rs.isFileExists()) {

                    HOST_ADDRESSES = parseHostsFile();

                    HOSTS_FILE_TRACKER.updateTimestamps(rs);

                } else {

                    HOST_ADDRESSES = Collections.emptyMap();

                }

            }

        } finally {

            LOCK.writeLock().unlock();

        }

    }

    private static String removeComments(String hostsEntry) {

        String filteredEntry = hostsEntry;

        int hashIndex;

        if ((hashIndex = hostsEntry.indexOf("#")) != -1) {

            filteredEntry = hostsEntry.substring(0, hashIndex);

        }

        return filteredEntry;

    }

    private static class HostFileEntry {

        final List<String> names; // Might need to split into aliases and name

        final InetAddress address;

        final boolean isValid;

        final boolean isHostname;

        HostFileEntry(String[] data) {

            assert data.length > 1;

            List<String> ln = List.of(Arrays.copyOfRange(data, 1, data.length));

            String addressString = data[0];

            names = ln;

            address = parseAddress(ln.isEmpty() ? null : ln.get(0), addressString);

            isValid = address != null;

            isHostname = false;

        }

        @Override

        public String toString() {

            return names+"/"+address;

        }

        private HostFileEntry(String name, InetAddress address, boolean isHostname) {

            this.names = List.of(name);

            this.address = address;

            this.isValid = address != null;

            this.isHostname = isHostname;

        }

        boolean isValid() {

            return isValid;

        }

        boolean isHostname() {return isHostname;}

        String getHostName() {

            return names.get(0);

        }

        Stream<HostFileEntry> oneNameStream() {

            HostFileEntry hostName = new HostFileEntry(names.get(0), address, true);

            Stream<HostFileEntry> aliases = names.stream()

                    .skip(1)

                    .map(n -> new HostFileEntry(n, address, false));

            return Stream.concat(Stream.of(hostName), aliases);

        }

        private InetAddress parseAddress(String hostName, String addressString) {

            // TODO: Revisit

            Objects.requireNonNull(hostName);

            // IPAddressUtil is from

            var pa = (PrivilegedAction<byte[]>) () -> {

                if (IPAddressUtil.isIPv4LiteralAddress(addressString)) {

                    return IPAddressUtil.textToNumericFormatV4(addressString);

                } else if (IPAddressUtil.isIPv6LiteralAddress(addressString)) {

                    return IPAddressUtil.textToNumericFormatV6(addressString);

                }

                return null;

            };

            byte[] addr = System.getSecurityManager() == null ?

                    pa.run() : AccessController.doPrivileged(pa);

            if (addr != null) {

                try {

                    // if (hostName == null) hostName = addressString

                    return InetAddress.getByAddress(hostName, addr);

                } catch (UnknownHostException e) {

                }

            }

            return null;

        }

    }

    private Map<String, HostFileEntry> parseHostsFile() {

        Path hf = Paths.get(HOSTS_FILE_LOCATION_PROPERTY_VALUE);

        try {

            // TODO: Revisit

            var pea = (PrivilegedExceptionAction<Boolean>) () -> Files.isRegularFile(hf);

            boolean isRegularFile = System.getSecurityManager() == null ? pea.run()

                    : AccessController.doPrivileged(pea);

            if (isRegularFile) {

                var result = new HashMap<String, HostFileEntry>();

                var pea2 = (PrivilegedExceptionAction<List<String>>) () -> Files.readAllLines(hf, StandardCharsets.UTF_8);

                var lines = System.getSecurityManager() == null ? pea2.run()

                        : AccessController.doPrivileged(pea2);

                lines.stream()

                        .map(HostsFileResolver::removeComments)

                        .filter(Predicate.not(String::isBlank))

                        .map(s -> s.split("\\s+"))

                        .filter(a -> a.length > 1)

                        .map(HostFileEntry::new)

                        .filter(HostFileEntry::isValid)

                        .flatMap(HostFileEntry::oneNameStream)

                        .forEachOrdered(

                                // If the same host name is listed multiple times then

                                // use the first encountered line

                                hfe -> result.putIfAbsent(hfe.names.get(0), hfe)

                        );

                return Map.copyOf(result);

            }

        } catch (PrivilegedActionException pae) {

            throw new RuntimeException("Can't read hosts file", pae.getCause());

        } catch (Exception e) {

            throw new RuntimeException("Can't read hosts file", e);

        }

        return Collections.emptyMap();

    }

    public InetAddress getHostAddress(String hostName) throws UnknownHostException {

        return getHostAddress(hostName, AddressFamily.ANY);

    }

    public InetAddress getHostAddress(String hostName, AddressFamily family) throws UnknownHostException {

        loadHostsAddresses();

        var map = HOST_ADDRESSES;

        var he = map.get(hostName);

        if (he == null) {

            throw new UnknownHostException(hostName);

        }

        var addr = he.address;

        if (!family.sameFamily(addr)) {

            throw new UnknownHostException(hostName);

        }

        return addr;

    }

    public String getByAddress(final InetAddress ha) throws UnknownHostException {

        loadHostsAddresses();

        var map = HOST_ADDRESSES;

        var entry = map.values().stream()

                .filter(HostFileEntry::isHostname)

                .filter(e -> isAddressBytesTheSame(ha.getAddress(), e.address.getAddress()))

                .findFirst();

        if (entry.isEmpty()) {

            throw new UnknownHostException(ha.toString());

        }

        return entry.get().getHostName();

    }

    private static boolean isAddressBytesTheSame(byte [] addr1, byte [] addr2) {

        if (addr1 == null || addr2 == null) {

            return false;

        }

        if (addr1.length != addr2.length) {

            return false;

        }

        for (int i=0; i<addr1.length; i++) {

            if (addr1[i] != addr2[i])

                return false;

        }

        return true;

    }

    static {

        // TODO: Revisit

        try {

            var pea = (PrivilegedExceptionAction<ReloadTracker>) () ->

                    ReloadTracker.newInstance(Paths.get(HOSTS_FILE_LOCATION_PROPERTY_VALUE), REFRESH_TIMEOUT_MILLIS);

            HOSTS_FILE_TRACKER = System.getSecurityManager() == null ? pea.run() :

                    AccessController.doPrivileged(pea);

        } catch (PrivilegedActionException pae) {

            throw new RuntimeException("Error registering hosts file watch service", pae.getCause());

        } catch (Exception e) {

            throw new RuntimeException("Error registering hosts file watch service", e);

        }

    }

}