gecko/netwerk/dns/nsHostResolver.cpp

/* vim:set ts=4 sw=4 sts=4 et cin: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#if defined(MOZ_LOGGING)
#define FORCE_PR_LOG
#endif

#if defined(HAVE_RES_NINIT)
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>   
#include <arpa/nameser.h>
#include <resolv.h>
#define RES_RETRY_ON_FAILURE
#endif

#include <stdlib.h>
#include "nsHostResolver.h"
#include "nsError.h"
#include "nsISupportsBase.h"
#include "nsISupportsUtils.h"
#include "nsAutoPtr.h"
#include "pratom.h"
#include "prthread.h"
#include "prerror.h"
#include "prtime.h"
#include "prlong.h"
#include "prlog.h"
#include "pldhash.h"
#include "plstr.h"
#include "nsURLHelper.h"
#include "nsThreadUtils.h"

#include "mozilla/HashFunctions.h"
#include "mozilla/TimeStamp.h"
#include "mozilla/Telemetry.h"

using namespace mozilla;
using namespace mozilla::net;

//----------------------------------------------------------------------------

// Use a persistent thread pool in order to avoid spinning up new threads all the time.
// In particular, thread creation results in a res_init() call from libc which is 
// quite expensive.
//
// The pool dynamically grows between 0 and MAX_RESOLVER_THREADS in size. New requests
// go first to an idle thread. If that cannot be found and there are fewer than MAX_RESOLVER_THREADS
// currently in the pool a new thread is created for high priority requests. If
// the new request is at a lower priority a new thread will only be created if 
// there are fewer than HighThreadThreshold currently outstanding. If a thread cannot be
// created or an idle thread located for the request it is queued.
//
// When the pool is greater than HighThreadThreshold in size a thread will be destroyed after
// ShortIdleTimeoutSeconds of idle time. Smaller pools use LongIdleTimeoutSeconds for a 
// timeout period.

#define HighThreadThreshold     MAX_RESOLVER_THREADS_FOR_ANY_PRIORITY
#define LongIdleTimeoutSeconds  300           // for threads 1 -> HighThreadThreshold
#define ShortIdleTimeoutSeconds 60            // for threads HighThreadThreshold+1 -> MAX_RESOLVER_THREADS

PR_STATIC_ASSERT (HighThreadThreshold <= MAX_RESOLVER_THREADS);

//----------------------------------------------------------------------------

#if defined(PR_LOGGING)
static PRLogModuleInfo *gHostResolverLog = nullptr;
#define LOG(args) PR_LOG(gHostResolverLog, PR_LOG_DEBUG, args)
#else
#define LOG(args)
#endif

//----------------------------------------------------------------------------

static inline void
MoveCList(PRCList &from, PRCList &to)
{
    if (!PR_CLIST_IS_EMPTY(&from)) {
        to.next = from.next;
        to.prev = from.prev;
        to.next->prev = &to;
        to.prev->next = &to;
        PR_INIT_CLIST(&from);
    }             
}

static uint32_t
NowInMinutes()
{
    return uint32_t(PR_Now() / int64_t(60 * PR_USEC_PER_SEC));
}

//----------------------------------------------------------------------------

#if defined(RES_RETRY_ON_FAILURE)

// this class represents the resolver state for a given thread.  if we
// encounter a lookup failure, then we can invoke the Reset method on an
// instance of this class to reset the resolver (in case /etc/resolv.conf
// for example changed).  this is mainly an issue on GNU systems since glibc
// only reads in /etc/resolv.conf once per thread.  it may be an issue on
// other systems as well.

class nsResState
{
public:
    nsResState()
        // initialize mLastReset to the time when this object
        // is created.  this means that a reset will not occur
        // if a thread is too young.  the alternative would be
        // to initialize this to the beginning of time, so that
        // the first failure would cause a reset, but since the
        // thread would have just started up, it likely would
        // already have current /etc/resolv.conf info.
        : mLastReset(PR_IntervalNow())
    {
    }

    bool Reset()
    {
        // reset no more than once per second
        if (PR_IntervalToSeconds(PR_IntervalNow() - mLastReset) < 1)
            return false;

        LOG(("Calling 'res_ninit'.\n"));

        mLastReset = PR_IntervalNow();
        return (res_ninit(&_res) == 0);
    }

private:
    PRIntervalTime mLastReset;
};

#endif // RES_RETRY_ON_FAILURE

//----------------------------------------------------------------------------

// this macro filters out any flags that are not used when constructing the
// host key.  the significant flags are those that would affect the resulting
// host record (i.e., the flags that are passed down to PR_GetAddrInfoByName).
#define RES_KEY_FLAGS(_f) ((_f) & nsHostResolver::RES_CANON_NAME)

nsHostRecord::nsHostRecord(const nsHostKey *key)
    : addr_info_lock("nsHostRecord.addr_info_lock")
    , addr_info_gencnt(0)
    , addr_info(nullptr)
    , addr(nullptr)
    , negative(false)
    , resolving(false)
    , onQueue(false)
    , usingAnyThread(false)
{
    host = ((char *) this) + sizeof(nsHostRecord);
    memcpy((char *) host, key->host, strlen(key->host) + 1);
    flags = key->flags;
    af = key->af;

    expiration = NowInMinutes();

    PR_INIT_CLIST(this);
    PR_INIT_CLIST(&callbacks);
}

nsresult
nsHostRecord::Create(const nsHostKey *key, nsHostRecord **result)
{
    size_t hostLen = strlen(key->host) + 1;
    size_t size = hostLen + sizeof(nsHostRecord);

    // Use placement new to create the object with room for the hostname
    // allocated after it.
    void *place = ::operator new(size);
    *result = new(place) nsHostRecord(key);
    NS_ADDREF(*result);
    return NS_OK;
}

nsHostRecord::~nsHostRecord()
{
    delete addr_info;
    delete addr;
}

bool
nsHostRecord::Blacklisted(NetAddr *aQuery)
{
    // must call locked
    LOG(("Checking blacklist for host [%s], host record [%p].\n", host, this));

    // skip the string conversion for the common case of no blacklist
    if (!mBlacklistedItems.Length()) {
        return false;
    }

    char buf[kIPv6CStrBufSize];
    if (!NetAddrToString(aQuery, buf, sizeof(buf))) {
        return false;
    }
    nsDependentCString strQuery(buf);

    for (uint32_t i = 0; i < mBlacklistedItems.Length(); i++) {
        if (mBlacklistedItems.ElementAt(i).Equals(strQuery)) {
            LOG(("Address [%s] is blacklisted for host [%s].\n", buf, host));
            return true;
        }
    }

    return false;
}

void
nsHostRecord::ReportUnusable(NetAddr *aAddress)
{
    // must call locked
    LOG(("Adding address to blacklist for host [%s], host record [%p].\n", host, this));

    char buf[kIPv6CStrBufSize];
    if (NetAddrToString(aAddress, buf, sizeof(buf))) {
        LOG(("Successfully adding address [%s] to blacklist for host [%s].\n", buf, host));
        mBlacklistedItems.AppendElement(nsCString(buf));
    }
}

void
nsHostRecord::ResetBlacklist()
{
    // must call locked
    LOG(("Resetting blacklist for host [%s], host record [%p].\n", host, this));
    mBlacklistedItems.Clear();
}

//----------------------------------------------------------------------------

struct nsHostDBEnt : PLDHashEntryHdr
{
    nsHostRecord *rec;
};

static PLDHashNumber
HostDB_HashKey(PLDHashTable *table, const void *key)
{
    const nsHostKey *hk = static_cast<const nsHostKey *>(key);
    return AddToHash(HashString(hk->host), RES_KEY_FLAGS(hk->flags), hk->af);
}

static bool
HostDB_MatchEntry(PLDHashTable *table,
                  const PLDHashEntryHdr *entry,
                  const void *key)
{
    const nsHostDBEnt *he = static_cast<const nsHostDBEnt *>(entry);
    const nsHostKey *hk = static_cast<const nsHostKey *>(key); 

    return !strcmp(he->rec->host, hk->host) &&
            RES_KEY_FLAGS (he->rec->flags) == RES_KEY_FLAGS(hk->flags) &&
            he->rec->af == hk->af;
}

static void
HostDB_MoveEntry(PLDHashTable *table,
                 const PLDHashEntryHdr *from,
                 PLDHashEntryHdr *to)
{
    static_cast<nsHostDBEnt *>(to)->rec =
            static_cast<const nsHostDBEnt *>(from)->rec;
}

static void
HostDB_ClearEntry(PLDHashTable *table,
                  PLDHashEntryHdr *entry)
{
    nsHostDBEnt *he = static_cast<nsHostDBEnt*>(entry);
    MOZ_ASSERT(he, "nsHostDBEnt is null!");

    nsHostRecord *hr = he->rec;
    MOZ_ASSERT(hr, "nsHostDBEnt has null host record!");

    LOG(("Clearing cache db entry for host [%s].\n", hr->host));
#if defined(DEBUG) && defined(PR_LOGGING)
    {
        MutexAutoLock lock(hr->addr_info_lock);
        if (!hr->addr_info) {
            LOG(("No address info for host [%s].\n", hr->host));
        } else {
            int32_t now = (int32_t) NowInMinutes();
            int32_t diff = (int32_t) hr->expiration - now;
            LOG(("Record for [%s] expires in %d minute(s).\n", hr->host, diff));

            NetAddrElement *addrElement = nullptr;
            char buf[kIPv6CStrBufSize];
            do {
                if (!addrElement) {
                    addrElement = hr->addr_info->mAddresses.getFirst();
                } else {
                    addrElement = addrElement->getNext();
                }

                if (addrElement) {
                    NetAddrToString(&addrElement->mAddress, buf, sizeof(buf));
                    LOG(("  [%s]\n", buf));
                }
            }
            while (addrElement);
        }
    }
#endif
    NS_RELEASE(he->rec);
}

static bool
HostDB_InitEntry(PLDHashTable *table,
                 PLDHashEntryHdr *entry,
                 const void *key)
{
    nsHostDBEnt *he = static_cast<nsHostDBEnt *>(entry);
    nsHostRecord::Create(static_cast<const nsHostKey *>(key), &he->rec);
    return true;
}

static PLDHashTableOps gHostDB_ops =
{
    PL_DHashAllocTable,
    PL_DHashFreeTable,
    HostDB_HashKey,
    HostDB_MatchEntry,
    HostDB_MoveEntry,
    HostDB_ClearEntry,
    PL_DHashFinalizeStub,
    HostDB_InitEntry,
};

static PLDHashOperator
HostDB_RemoveEntry(PLDHashTable *table,
                   PLDHashEntryHdr *hdr,
                   uint32_t number,
                   void *arg)
{
    return PL_DHASH_REMOVE;
}

//----------------------------------------------------------------------------

nsHostResolver::nsHostResolver(uint32_t maxCacheEntries,
                               uint32_t maxCacheLifetime,
                               uint32_t lifetimeGracePeriod)
    : mMaxCacheEntries(maxCacheEntries)
    , mMaxCacheLifetime(maxCacheLifetime)
    , mGracePeriod(lifetimeGracePeriod)
    , mLock("nsHostResolver.mLock")
    , mIdleThreadCV(mLock, "nsHostResolver.mIdleThreadCV")
    , mNumIdleThreads(0)
    , mThreadCount(0)
    , mActiveAnyThreadCount(0)
    , mEvictionQSize(0)
    , mPendingCount(0)
    , mShutdown(true)
{
    mCreationTime = PR_Now();
    PR_INIT_CLIST(&mHighQ);
    PR_INIT_CLIST(&mMediumQ);
    PR_INIT_CLIST(&mLowQ);
    PR_INIT_CLIST(&mEvictionQ);

    mLongIdleTimeout  = PR_SecondsToInterval(LongIdleTimeoutSeconds);
    mShortIdleTimeout = PR_SecondsToInterval(ShortIdleTimeoutSeconds);
}

nsHostResolver::~nsHostResolver()
{
    PL_DHashTableFinish(&mDB);
}

nsresult
nsHostResolver::Init()
{
    PL_DHashTableInit(&mDB, &gHostDB_ops, nullptr, sizeof(nsHostDBEnt), 0);

    mShutdown = false;

#if defined(HAVE_RES_NINIT)
    // We want to make sure the system is using the correct resolver settings,
    // so we force it to reload those settings whenever we startup a subsequent
    // nsHostResolver instance.  We assume that there is no reason to do this
    // for the first nsHostResolver instance since that is usually created
    // during application startup.
    static int initCount = 0;
    if (initCount++ > 0) {
        LOG(("Calling 'res_ninit'.\n"));
        res_ninit(&_res);
    }
#endif
    return NS_OK;
}

void
nsHostResolver::ClearPendingQueue(PRCList *aPendingQ)
{
    // loop through pending queue, erroring out pending lookups.
    if (!PR_CLIST_IS_EMPTY(aPendingQ)) {
        PRCList *node = aPendingQ->next;
        while (node != aPendingQ) {
            nsHostRecord *rec = static_cast<nsHostRecord *>(node);
            node = node->next;
            OnLookupComplete(rec, NS_ERROR_ABORT, nullptr);
        }
    }
}

void
nsHostResolver::Shutdown()
{
    LOG(("Shutting down host resolver.\n"));

    PRCList pendingQHigh, pendingQMed, pendingQLow, evictionQ;
    PR_INIT_CLIST(&pendingQHigh);
    PR_INIT_CLIST(&pendingQMed);
    PR_INIT_CLIST(&pendingQLow);
    PR_INIT_CLIST(&evictionQ);

    {
        MutexAutoLock lock(mLock);
        
        mShutdown = true;

        MoveCList(mHighQ, pendingQHigh);
        MoveCList(mMediumQ, pendingQMed);
        MoveCList(mLowQ, pendingQLow);
        MoveCList(mEvictionQ, evictionQ);
        mEvictionQSize = 0;
        mPendingCount = 0;
        
        if (mNumIdleThreads)
            mIdleThreadCV.NotifyAll();
        
        // empty host database
        PL_DHashTableEnumerate(&mDB, HostDB_RemoveEntry, nullptr);
    }
    
    ClearPendingQueue(&pendingQHigh);
    ClearPendingQueue(&pendingQMed);
    ClearPendingQueue(&pendingQLow);

    if (!PR_CLIST_IS_EMPTY(&evictionQ)) {
        PRCList *node = evictionQ.next;
        while (node != &evictionQ) {
            nsHostRecord *rec = static_cast<nsHostRecord *>(node);
            node = node->next;
            NS_RELEASE(rec);
        }
    }

#ifdef NS_BUILD_REFCNT_LOGGING
    
    // Logically join the outstanding worker threads with a timeout.
    // Use this approach instead of PR_JoinThread() because that does 
    // not allow a timeout which may be necessary for a semi-responsive 
    // shutdown if the thread is blocked on a very slow DNS resolution.
    // mThreadCount is read outside of mLock, but the worst case 
    // scenario for that race is one extra 25ms sleep.

    PRIntervalTime delay = PR_MillisecondsToInterval(25);
    PRIntervalTime stopTime = PR_IntervalNow() + PR_SecondsToInterval(20);
    while (mThreadCount && PR_IntervalNow() < stopTime)
        PR_Sleep(delay);
#endif
}

static inline bool
IsHighPriority(uint16_t flags)
{
    return !(flags & (nsHostResolver::RES_PRIORITY_LOW | nsHostResolver::RES_PRIORITY_MEDIUM));
}

static inline bool
IsMediumPriority(uint16_t flags)
{
    return flags & nsHostResolver::RES_PRIORITY_MEDIUM;
}

static inline bool
IsLowPriority(uint16_t flags)
{
    return flags & nsHostResolver::RES_PRIORITY_LOW;
}

void 
nsHostResolver::MoveQueue(nsHostRecord *aRec, PRCList &aDestQ)
{
    NS_ASSERTION(aRec->onQueue, "Moving Host Record Not Currently Queued");
    
    PR_REMOVE_LINK(aRec);
    PR_APPEND_LINK(aRec, &aDestQ);
}

nsresult
nsHostResolver::ResolveHost(const char            *host,
                            uint16_t               flags,
                            uint16_t               af,
                            nsResolveHostCallback *callback)
{
    NS_ENSURE_TRUE(host && *host, NS_ERROR_UNEXPECTED);

    LOG(("Resolving host [%s].\n", host));

    // ensure that we are working with a valid hostname before proceeding.  see
    // bug 304904 for details.
    if (!net_IsValidHostName(nsDependentCString(host)))
        return NS_ERROR_UNKNOWN_HOST;

    // if result is set inside the lock, then we need to issue the
    // callback before returning.
    nsRefPtr<nsHostRecord> result;
    nsresult status = NS_OK, rv = NS_OK;
    {
        MutexAutoLock lock(mLock);

        if (mShutdown)
            rv = NS_ERROR_NOT_INITIALIZED;
        else {
            // Used to try to parse to an IP address literal.
            PRNetAddr tempAddr;
            // Unfortunately, PR_StringToNetAddr does not properly initialize
            // the output buffer in the case of IPv6 input. See bug 223145.
            memset(&tempAddr, 0, sizeof(PRNetAddr));
            
            // check to see if there is already an entry for this |host|
            // in the hash table.  if so, then check to see if we can't
            // just reuse the lookup result.  otherwise, if there are
            // any pending callbacks, then add to pending callbacks queue,
            // and return.  otherwise, add ourselves as first pending
            // callback, and proceed to do the lookup.

            nsHostKey key = { host, flags, af };
            nsHostDBEnt *he = static_cast<nsHostDBEnt *>
                                         (PL_DHashTableOperate(&mDB, &key, PL_DHASH_ADD));

            // if the record is null, then HostDB_InitEntry failed.
            if (!he || !he->rec)
                rv = NS_ERROR_OUT_OF_MEMORY;
            // do we have a cached result that we can reuse?
            else if (!(flags & RES_BYPASS_CACHE) &&
                     he->rec->HasResult() &&
                     NowInMinutes() <= he->rec->expiration + mGracePeriod) {
                        
                LOG(("Using cached record for host [%s].\n", host));
                // put reference to host record on stack...
                result = he->rec;
                Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2, METHOD_HIT);

                // For entries that are in the grace period with a failed connect,
                // or all cached negative entries, use the cache but start a new lookup in
                // the background
                if ((((NowInMinutes() > he->rec->expiration) &&
                      he->rec->mBlacklistedItems.Length()) ||
                     he->rec->negative) && !he->rec->resolving) {
                    LOG(("Using %s cache entry for host [%s] but starting async renewal.",
                         he->rec->negative ? "negative" :"positive", host));
                    IssueLookup(he->rec);

                    if (!he->rec->negative) {
                        // negative entries are constantly being refreshed, only
                        // track positive grace period induced renewals
                        Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                              METHOD_RENEWAL);
                    }
                }
                
                if (he->rec->negative) {
                    Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                          METHOD_NEGATIVE_HIT);
                    status = NS_ERROR_UNKNOWN_HOST;
                }
            }
            // if the host name is an IP address literal and has been parsed,
            // go ahead and use it.
            else if (he->rec->addr) {
                Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                      METHOD_LITERAL);
                result = he->rec;
            }
            // try parsing the host name as an IP address literal to short
            // circuit full host resolution.  (this is necessary on some
            // platforms like Win9x.  see bug 219376 for more details.)
            else if (PR_StringToNetAddr(host, &tempAddr) == PR_SUCCESS) {
                // ok, just copy the result into the host record, and be done
                // with it! ;-)
                he->rec->addr = new NetAddr();
                PRNetAddrToNetAddr(&tempAddr, he->rec->addr);
                // put reference to host record on stack...
                Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                      METHOD_LITERAL);
                result = he->rec;
            }
            else if (mPendingCount >= MAX_NON_PRIORITY_REQUESTS &&
                     !IsHighPriority(flags) &&
                     !he->rec->resolving) {
                Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                      METHOD_OVERFLOW);
                // This is a lower priority request and we are swamped, so refuse it.
                rv = NS_ERROR_DNS_LOOKUP_QUEUE_FULL;
            }
            else if (flags & RES_OFFLINE) {
                rv = NS_ERROR_OFFLINE;
            }

            // otherwise, hit the resolver...
            else {
                // Add callback to the list of pending callbacks.
                PR_APPEND_LINK(callback, &he->rec->callbacks);

                if (!he->rec->resolving) {
                    he->rec->flags = flags;
                    rv = IssueLookup(he->rec);
                    Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                          METHOD_NETWORK_FIRST);
                    if (NS_FAILED(rv))
                        PR_REMOVE_AND_INIT_LINK(callback);
                    else
                        LOG(("DNS lookup for host [%s] blocking pending 'getaddrinfo' query.", host));
                }
                else if (he->rec->onQueue) {
                    Telemetry::Accumulate(Telemetry::DNS_LOOKUP_METHOD2,
                                          METHOD_NETWORK_SHARED);

                    // Consider the case where we are on a pending queue of 
                    // lower priority than the request is being made at.
                    // In that case we should upgrade to the higher queue.

                    if (IsHighPriority(flags) && !IsHighPriority(he->rec->flags)) {
                        // Move from (low|med) to high.
                        MoveQueue(he->rec, mHighQ);
                        he->rec->flags = flags;
                        ConditionallyCreateThread(he->rec);
                    } else if (IsMediumPriority(flags) && IsLowPriority(he->rec->flags)) {
                        // Move from low to med.
                        MoveQueue(he->rec, mMediumQ);
                        he->rec->flags = flags;
                        mIdleThreadCV.Notify();
                    }
                }
            }
        }
    }
    if (result)
        callback->OnLookupComplete(this, result, status);
    return rv;
}

void
nsHostResolver::DetachCallback(const char            *host,
                               uint16_t               flags,
                               uint16_t               af,
                               nsResolveHostCallback *callback,
                               nsresult               status)
{
    nsRefPtr<nsHostRecord> rec;
    {
        MutexAutoLock lock(mLock);

        nsHostKey key = { host, flags, af };
        nsHostDBEnt *he = static_cast<nsHostDBEnt *>
                                     (PL_DHashTableOperate(&mDB, &key, PL_DHASH_LOOKUP));
        if (he && he->rec) {
            // walk list looking for |callback|... we cannot assume
            // that it will be there!
            PRCList *node = he->rec->callbacks.next;
            while (node != &he->rec->callbacks) {
                if (static_cast<nsResolveHostCallback *>(node) == callback) {
                    PR_REMOVE_LINK(callback);
                    rec = he->rec;
                    break;
                }
                node = node->next;
            }
        }
    }

    // complete callback with the given status code; this would only be done if
    // the record was in the process of being resolved.
    if (rec)
        callback->OnLookupComplete(this, rec, status);
}

nsresult
nsHostResolver::ConditionallyCreateThread(nsHostRecord *rec)
{
    if (mNumIdleThreads) {
        // wake up idle thread to process this lookup
        mIdleThreadCV.Notify();
    }
    else if ((mThreadCount < HighThreadThreshold) ||
             (IsHighPriority(rec->flags) && mThreadCount < MAX_RESOLVER_THREADS)) {
        // dispatch new worker thread
        NS_ADDREF_THIS(); // owning reference passed to thread

        mThreadCount++;
        PRThread *thr = PR_CreateThread(PR_SYSTEM_THREAD,
                                        ThreadFunc,
                                        this,
                                        PR_PRIORITY_NORMAL,
                                        PR_GLOBAL_THREAD,
                                        PR_UNJOINABLE_THREAD,
                                        0);
        if (!thr) {
            mThreadCount--;
            NS_RELEASE_THIS();
            return NS_ERROR_OUT_OF_MEMORY;
        }
    }
#if defined(PR_LOGGING)
    else
      LOG(("Unable to find a thread for looking up host [%s].\n", rec->host));
#endif
    return NS_OK;
}

nsresult
nsHostResolver::IssueLookup(nsHostRecord *rec)
{
    nsresult rv = NS_OK;
    NS_ASSERTION(!rec->resolving, "record is already being resolved");

    // Add rec to one of the pending queues, possibly removing it from mEvictionQ.
    // If rec is on mEvictionQ, then we can just move the owning
    // reference over to the new active queue.
    if (rec->next == rec)
        NS_ADDREF(rec);
    else {
        PR_REMOVE_LINK(rec);
        mEvictionQSize--;
    }
    
    if (IsHighPriority(rec->flags))
        PR_APPEND_LINK(rec, &mHighQ);
    else if (IsMediumPriority(rec->flags))
        PR_APPEND_LINK(rec, &mMediumQ);
    else
        PR_APPEND_LINK(rec, &mLowQ);
    mPendingCount++;
    
    rec->resolving = true;
    rec->onQueue = true;

    rv = ConditionallyCreateThread(rec);
    
    LOG (("DNS thread counters: total=%d any-live=%d idle=%d pending=%d\n",
          mThreadCount,
          mActiveAnyThreadCount,
          mNumIdleThreads,
          mPendingCount));

    return rv;
}

void
nsHostResolver::DeQueue(PRCList &aQ, nsHostRecord **aResult)
{
    *aResult = static_cast<nsHostRecord *>(aQ.next);
    PR_REMOVE_AND_INIT_LINK(*aResult);
    mPendingCount--;
    (*aResult)->onQueue = false;
}

bool
nsHostResolver::GetHostToLookup(nsHostRecord **result)
{
    bool timedOut = false;
    PRIntervalTime epoch, now, timeout;
    
    MutexAutoLock lock(mLock);

    timeout = (mNumIdleThreads >= HighThreadThreshold) ? mShortIdleTimeout : mLongIdleTimeout;
    epoch = PR_IntervalNow();

    while (!mShutdown) {
        // remove next record from Q; hand over owning reference. Check high, then med, then low
        
        if (!PR_CLIST_IS_EMPTY(&mHighQ)) {
            DeQueue (mHighQ, result);
            return true;
        }

        if (mActiveAnyThreadCount < HighThreadThreshold) {
            if (!PR_CLIST_IS_EMPTY(&mMediumQ)) {
                DeQueue (mMediumQ, result);
                mActiveAnyThreadCount++;
                (*result)->usingAnyThread = true;
                return true;
            }
            
            if (!PR_CLIST_IS_EMPTY(&mLowQ)) {
                DeQueue (mLowQ, result);
                mActiveAnyThreadCount++;
                (*result)->usingAnyThread = true;
                return true;
            }
        }
        
        // Determining timeout is racy, so allow one cycle through checking the queues
        // before exiting.
        if (timedOut)
            break;

        // wait for one or more of the following to occur:
        //  (1) the pending queue has a host record to process
        //  (2) the shutdown flag has been set
        //  (3) the thread has been idle for too long
        
        mNumIdleThreads++;
        mIdleThreadCV.Wait(timeout);
        mNumIdleThreads--;
        
        now = PR_IntervalNow();
        
        if ((PRIntervalTime)(now - epoch) >= timeout)
            timedOut = true;
        else {
            // It is possible that PR_WaitCondVar() was interrupted and returned early,
            // in which case we will loop back and re-enter it. In that case we want to
            // do so with the new timeout reduced to reflect time already spent waiting.
            timeout -= (PRIntervalTime)(now - epoch);
            epoch = now;
        }
    }
    
    // tell thread to exit...
    mThreadCount--;
    return false;
}

void
nsHostResolver::OnLookupComplete(nsHostRecord *rec, nsresult status, AddrInfo *result)
{
    // get the list of pending callbacks for this lookup, and notify
    // them that the lookup is complete.
    PRCList cbs;
    PR_INIT_CLIST(&cbs);
    {
        MutexAutoLock lock(mLock);

        // grab list of callbacks to notify
        MoveCList(rec->callbacks, cbs);

        // update record fields.  We might have a rec->addr_info already if a
        // previous lookup result expired and we're reresolving it..
        AddrInfo  *old_addr_info;
        {
            MutexAutoLock lock(rec->addr_info_lock);
            old_addr_info = rec->addr_info;
            rec->addr_info = result;
            rec->addr_info_gencnt++;
        }
        delete old_addr_info;

        rec->expiration = NowInMinutes();
        if (result) {
            rec->expiration += mMaxCacheLifetime;
            rec->negative = false;
        }
        else {
            rec->expiration += 1;                 /* one minute for negative cache */
            rec->negative = true;
        }
        rec->resolving = false;
        
        if (rec->usingAnyThread) {
            mActiveAnyThreadCount--;
            rec->usingAnyThread = false;
        }

        if (rec->addr_info && !mShutdown) {
            // add to mEvictionQ
            PR_APPEND_LINK(rec, &mEvictionQ);
            NS_ADDREF(rec);
            if (mEvictionQSize < mMaxCacheEntries)
                mEvictionQSize++;
            else {
                // remove first element on mEvictionQ
                nsHostRecord *head =
                    static_cast<nsHostRecord *>(PR_LIST_HEAD(&mEvictionQ));
                PR_REMOVE_AND_INIT_LINK(head);
                PL_DHashTableOperate(&mDB, (nsHostKey *) head, PL_DHASH_REMOVE);

                if (!head->negative) {
                    // record the age of the entry upon eviction.
                    uint32_t age =
                        NowInMinutes() - (head->expiration - mMaxCacheLifetime);
                    Telemetry::Accumulate(Telemetry::DNS_CLEANUP_AGE, age);
                }

                // release reference to rec owned by mEvictionQ
                NS_RELEASE(head);
            }
        }
    }

    if (!PR_CLIST_IS_EMPTY(&cbs)) {
        PRCList *node = cbs.next;
        while (node != &cbs) {
            nsResolveHostCallback *callback =
                    static_cast<nsResolveHostCallback *>(node);
            node = node->next;
            callback->OnLookupComplete(this, rec, status);
            // NOTE: callback must not be dereferenced after this point!!
        }
    }

    NS_RELEASE(rec);
}

void
nsHostResolver::CancelAsyncRequest(const char            *host,
                                   uint16_t               flags,
                                   uint16_t               af,
                                   nsIDNSListener        *aListener,
                                   nsresult               status)

{
    MutexAutoLock lock(mLock);

    // Lookup the host record associated with host, flags & address family
    nsHostKey key = { host, flags, af };
    nsHostDBEnt *he = static_cast<nsHostDBEnt *>
                      (PL_DHashTableOperate(&mDB, &key, PL_DHASH_LOOKUP));
    if (he && he->rec) {
        nsHostRecord* recPtr = NULL;
        PRCList *node = he->rec->callbacks.next;
        // Remove the first nsDNSAsyncRequest callback which matches the
        // supplied listener object
        while (node != &he->rec->callbacks) {
            nsResolveHostCallback *callback
                = static_cast<nsResolveHostCallback *>(node);
            if (callback && (callback->EqualsAsyncListener(aListener))) {
                // Remove from the list of callbacks
                PR_REMOVE_LINK(callback);
                recPtr = he->rec;
                callback->OnLookupComplete(this, recPtr, status);
                break;
            }
            node = node->next;
        }

        // If there are no more callbacks, remove the hash table entry
        if (recPtr && PR_CLIST_IS_EMPTY(&recPtr->callbacks)) {
            PL_DHashTableOperate(&mDB, (nsHostKey *)recPtr, PL_DHASH_REMOVE);
            // If record is on a Queue, remove it and then deref it
            if (recPtr->next != recPtr) {
                PR_REMOVE_LINK(recPtr);
                NS_RELEASE(recPtr);
            }
        }
    }
}

//----------------------------------------------------------------------------

void
nsHostResolver::ThreadFunc(void *arg)
{
    LOG(("DNS lookup thread starting execution.\n"));

    static nsThreadPoolNaming naming;
    naming.SetThreadPoolName(NS_LITERAL_CSTRING("DNS Resolver"));

#if defined(RES_RETRY_ON_FAILURE)
    nsResState rs;
#endif
    nsHostResolver *resolver = (nsHostResolver *)arg;
    nsHostRecord *rec;
    PRAddrInfo *prai = nullptr;
    while (resolver->GetHostToLookup(&rec)) {
        LOG(("Calling getaddrinfo for host [%s].\n", rec->host));

        int flags = PR_AI_ADDRCONFIG;
        if (!(rec->flags & RES_CANON_NAME))
            flags |= PR_AI_NOCANONNAME;

        TimeStamp startTime = TimeStamp::Now();

        prai = PR_GetAddrInfoByName(rec->host, rec->af, flags);
#if defined(RES_RETRY_ON_FAILURE)
        if (!prai && rs.Reset())
            prai = PR_GetAddrInfoByName(rec->host, rec->af, flags);
#endif

        TimeDuration elapsed = TimeStamp::Now() - startTime;
        uint32_t millis = static_cast<uint32_t>(elapsed.ToMilliseconds());

        // convert error code to nsresult
        nsresult status;
        AddrInfo *ai = nullptr;
        if (prai) {
            ai = new AddrInfo(rec->host, prai);
            PR_FreeAddrInfo(prai);

            status = NS_OK;

            Telemetry::Accumulate(!rec->addr_info_gencnt ?
                                    Telemetry::DNS_LOOKUP_TIME :
                                    Telemetry::DNS_RENEWAL_TIME,
                                  millis);
        }
        else {
            status = NS_ERROR_UNKNOWN_HOST;
            Telemetry::Accumulate(Telemetry::DNS_FAILED_LOOKUP_TIME, millis);
        }

        // OnLookupComplete may release "rec", log before we lose it.
        LOG(("Lookup completed for host [%s].\n", rec->host));
        resolver->OnLookupComplete(rec, status, ai);
    }
    NS_RELEASE(resolver);
    LOG(("DNS lookup thread ending execution.\n"));
}

//----------------------------------------------------------------------------

nsresult
nsHostResolver::Create(uint32_t         maxCacheEntries,
                       uint32_t         maxCacheLifetime,
                       uint32_t         lifetimeGracePeriod,
                       nsHostResolver **result)
{
#if defined(PR_LOGGING)
    if (!gHostResolverLog)
        gHostResolverLog = PR_NewLogModule("nsHostResolver");
#endif

    nsHostResolver *res = new nsHostResolver(maxCacheEntries,
                                             maxCacheLifetime,
                                             lifetimeGracePeriod);
    if (!res)
        return NS_ERROR_OUT_OF_MEMORY;
    NS_ADDREF(res);

    nsresult rv = res->Init();
    if (NS_FAILED(rv))
        NS_RELEASE(res);

    *result = res;
    return rv;
}

PLDHashOperator
CacheEntryEnumerator(PLDHashTable *table, PLDHashEntryHdr *entry,
                     uint32_t number, void *arg)
{
    // We don't pay attention to address literals, only resolved domains.
    // Also require a host.
    nsHostRecord *rec = static_cast<nsHostDBEnt*>(entry)->rec;
    if (!rec->addr_info || !rec->host) {
        return PL_DHASH_NEXT;
    }

    DNSCacheEntries info;
    info.hostname = rec->host;
    info.family = rec->af;
    info.expiration = ((int64_t)rec->expiration - NowInMinutes()) * 60;
    if (info.expiration <= 0) {
        // We only need valid DNS cache entries
        return PL_DHASH_NEXT;
    }

    {
        MutexAutoLock lock(rec->addr_info_lock);

        NetAddr *addr = nullptr;
        NetAddrElement *addrElement = rec->addr_info->mAddresses.getFirst();
        if (addrElement) {
            addr = &addrElement->mAddress;
        }
        while (addr) {
            char buf[kIPv6CStrBufSize];
            if (NetAddrToString(addr, buf, sizeof(buf))) {
                info.hostaddr.AppendElement(buf);
            }
            addr = nullptr;
            addrElement = addrElement->getNext();
            if (addrElement) {
                addr = &addrElement->mAddress;
            }
        }
    }

    nsTArray<DNSCacheEntries> *args = static_cast<nsTArray<DNSCacheEntries> *>(arg);
    args->AppendElement(info);

    return PL_DHASH_NEXT;
}

void
nsHostResolver::GetDNSCacheEntries(nsTArray<DNSCacheEntries> *args)
{
    PL_DHashTableEnumerate(&mDB, CacheEntryEnumerator, args);
}