File 3492-Rewrite-to-use-bag-table.patch of Package erlang

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File 3492-Rewrite-to-use-bag-table.patch of Package erlang

From b2e6a28ea619212260c70f8de8603a72bb6e4062 Mon Sep 17 00:00:00 2001
From: Raimo Niskanen <raimo@erlang.org>
Date: Thu, 8 Apr 2021 17:27:23 +0200
Subject: [PATCH 2/4] Rewrite to use bag table

Using a set table broke the desired behaviour to get multiple
addresses for a domain.  Revert to using a bag table instead,
but still do lookups on the ets table without going through
the server.

The cache is now a bag table containing #dns_rr{} records
with domain as key.  A lookup uses ets:match_object for domain,
class and type so the key is in the match expression.  This is
a rather fast lookup, but it has to check all objects with
the domain (key) for matching class and type.

It might be faster to store tuples with a composite key
{{Domain, Class, Type} = Key, #dns_rr{}} since then ets:lookup
for Key could be used instead.  Due to how bag tables are
implemented internally it is not certain that this would be
very much faster.

After a lookup for all matching records, they need to be updated
with a new cnt time value.  Since we are using a bag table this
has to be done by inserting a new record and deleting the old.
If we insert before delete we always have a record in the cache
so parallel lookups will not accidentaly find no record.

This might create duplicate records i.e records with the same
(domain, class, type) but different e.g cnt field since parallel
processes might insert different new records and then delete
the old.  Therefore we have to weed out duplicate records
when returning the result.

Try again to use a set table?
-----------------------------
Faster lookups would be achieved by storing
{{Domain, Class, Type} = Key, [#dns_rr{}]} tuples in a set table.

Updates of the cnt value could then be done by overwriting
the tuple with a new [#dns_rr{}] list.  Then parallel processes
might overwrite each others' changes, which might accedentally
delete a cache record written by a recent parallel lookup.
This might actually be acceptable since it only would cause
another cache miss.
---
 lib/kernel/src/inet_db.erl | 242 ++++++++++++++++++++++++-------------
 1 file changed, 155 insertions(+), 87 deletions(-)

diff --git a/lib/kernel/src/inet_db.erl b/lib/kernel/src/inet_db.erl
index f272d0d115..7cf95dda8a 100644
--- a/lib/kernel/src/inet_db.erl
+++ b/lib/kernel/src/inet_db.erl
@@ -584,11 +584,10 @@ add_rr(RR) ->
     call({add_rr, RR}).
 
 add_rr(Domain, Class, Type, TTL, Data) ->
-    call({add_rr, #dns_rr { domain = Domain, class = Class,
-		       type = Type, ttl = TTL, data = Data}}).
+    call({add_rr, dns_rr_add(Domain, Class, Type, TTL, Data)}).
 
-del_rr(Domain, Class, Type, _Data) ->
-    call({del_rr, {Domain, Class, Type}}).
+del_rr(Domain, Class, Type, Data) ->
+    call({del_rr, dns_rr_match(Domain, Class, Type, Data)}).
 
 res_cache_answer(Rec) ->
     lists:foreach( fun(RR) -> add_rr(RR) end, Rec#dns_rec.anlist).
@@ -694,10 +693,9 @@ lookup_type(Domain, Type) ->
      _ -> []
     end.
 
-%% Have to do all lookups (changes to the db) in the
-%% process in order to make it possible to refresh the cache.
+%% lookup resource record
 lookup_rr(Domain, Class, Type) ->
-    match_rr({Domain, Class, Type}).
+    match_rr(dns_rr_match(tolower(Domain), Class, Type)).
 
 %%
 %% hostent_by_domain (newly resolved version)
@@ -743,7 +741,7 @@ res_lookup_type(Domain,Type,RRs) ->
 gethostbyaddr(IP) ->
     case dnip(IP) of
 	{ok, {IP1, HType, HLen, DnIP}} ->
-	    RRs = match_rr({DnIP, in, ptr}),
+            RRs = match_rr(dns_rr_match(DnIP, in, ptr)),
 	    ent_gethostbyaddr(RRs,  IP1, HType, HLen);
 	Error -> Error
     end.
@@ -888,7 +886,8 @@ init([]) ->
     end,
     Db = ets:new(inet_db, [public, named_table]),
     reset_db(Db),
-    Cache = ets:new(inet_cache, [public, set, named_table]),
+    CacheOpts = [public, bag, {keypos,#dns_rr.domain}, named_table],
+    Cache = ets:new(inet_cache, CacheOpts),
     HostsByname = ets:new(inet_hosts_byname, [named_table]),
     HostsByaddr = ets:new(inet_hosts_byaddr, [named_table]),
     HostsFileByname = ets:new(inet_hosts_file_byname, [named_table]),
@@ -977,9 +976,9 @@ handle_call(Request, From, #state{db=Db}=State) ->
 	    do_add_rr(RR, Db, State),
 	    {reply, ok, State};
 
-	{del_rr, {_Domain, _Class, _Type} = Key} ->
+	{del_rr, RR} when is_record(RR, dns_rr) ->
 	    Cache = State#state.cache,
-	    ets:delete(Cache, Key),
+            ets:match_delete(Cache, RR),
 	    {reply, ok, State};
 
 	{listop, Opt, Op, E} ->
@@ -1599,55 +1598,132 @@ is_reqname(clear_cache) -> true;
 is_reqname(clear_hosts) -> true;
 is_reqname(_) -> false.
 
-%% Add a resource record to the cache if there are space left.
+%% Add a resource record to the cache if there is space left.
 %% If the cache is full this function first deletes old entries,
-%% i.e. entries with oldest latest access time.
+%% i.e. entries with the oldest access time.
+%%
+%% #dns_rr.cnt is used to store the access time
+%% instead of number of accesses.
+%%
 do_add_rr(RR, Db, State) ->
     CacheDb = State#state.cache,
     TM = times(),
     case alloc_entry(Db, CacheDb, TM) of
 	true ->
-	    cache_rr(CacheDb, RR#dns_rr{tm = TM, cnt = TM}, TM);
-	_ ->
+            %% Add to cache
+            #dns_rr{
+               domain = Domain, class = Class, type = Type,
+               data = Data} = RR,
+            DeleteRRs =
+                ets:match_object(
+                  CacheDb, dns_rr_match(Domain, Class, Type, Data)),
+            InsertRR = RR#dns_rr{tm = TM, cnt = TM},
+            %% Insert before delete to always have an RR present.
+            %% Watch out to not delete what we insert.
+            case lists:member(InsertRR, DeleteRRs) of
+                true ->
+                    _ = [ets:delete_object(CacheDb, DelRR) ||
+                            DelRR <- DeleteRRs,
+                            DelRR =/= InsertRR],
+                    true;
+                false ->
+                    ets:insert(CacheDb, InsertRR),
+                    _ = [ets:delete_object(CacheDb, DelRR) ||
+                            DelRR <- DeleteRRs],
+                    true
+            end;
+	false ->
 	    false
     end.
 
-cache_rr(Cache, RR, Count) ->
-    ets:insert(Cache, {cache_key(RR), RR, Count}).
 
 times() ->
     erlang:monotonic_time(second).
-    %% erlang:convert_time_unit(erlang:monotonic_time() - erlang:system_info(start_time),
-    %%     		     native, second).
-
-%% match and remove old entries
-
-match_rr({_, _, _} = Key) ->
-    Time = times(),
-    case ets:lookup(inet_cache, Key) of
-	[{_,RR,_}] when RR#dns_rr.ttl =:= 0 ->
-	    %% ttl=0 is served once (by the current process) as we request
-	    %% a deletion immediately. It is served at least once concurrently.
-	    %% Deletions are safe thanks to the use of safe_fixtable.
-	    ets:delete(inet_cache, Key),
-	    [RR];
-	[{_,RR,_}] when RR#dns_rr.tm + RR#dns_rr.ttl < Time ->
-	    ets:delete(inet_cache, Key),
-	    [];
-	[{_,RR,_}] ->
-	    %% Update the entry concurrently. If the server deletes it, we ignore
-	    %% the result. If the server updates it, we will race on the time part
-	    %% which should be close enough to each other (as we are racing).
-	    _ = ets:update_element(inet_cache, Key, {3, Time}),
-	    [RR];
-	[] ->
-	    []
+
+
+%% ETS match expressions
+%%
+-compile({inline, [dns_rr_match/3, dns_rr_match/4]}).
+dns_rr_match(Domain, Class, Type) ->
+    #dns_rr{
+       domain = Domain, class = Class, type = Type, data = '_',
+       cnt = '_', tm = '_', ttl = '_', bm = '_', func = '_'}.
+%%
+dns_rr_match(Domain, Class, Type, Data) ->
+    #dns_rr{
+       domain = Domain, class = Class, type = Type, data = Data,
+       cnt = '_', tm = '_', ttl = '_', bm = '_', func = '_'}.
+
+%% RR creation
+-compile({inline, [dns_rr_add/5]}).
+dns_rr_add(Domain, Class, Type, TTL, Data) ->
+    #dns_rr{
+       domain = Domain, class = Class, type = Type,
+       ttl = TTL, data = Data}.
+
+
+%% We are simultaneously updating the table from all clients
+%% and the server, so we might get duplicate recource records
+%% in the table, i.e identical domain, class, type and data.
+%% We embrace that and eliminate duplicates here.
+%%
+%% Look up all matching objects.  The still valid ones
+%% should be returned, and updated with a new cnt time.
+%% All expired ones should be deleted.  We count TTL 0
+%% RRs as valid but immediately expired.
+%%
+match_rr(MatchRR) ->
+    CacheDb = inet_cache,
+    RRs = ets:match_object(CacheDb, MatchRR),
+    match_rr(CacheDb, RRs, times(), #{}, #{}, []).
+%%
+match_rr(CacheDb, [], _Time, ResultRRs, InsertRRs, DeleteRRs) ->
+    %% We insert first so an RR always is present,
+    %% which may create duplicates
+    _ = [ets:insert(CacheDb, RR) || RR <- maps:values(InsertRRs)],
+    _ = [ets:delete_object(CacheDb, RR) || RR <- DeleteRRs],
+    maps:values(ResultRRs);
+match_rr(CacheDb, [RR | RRs], Time, ResultRRs, InsertRRs, DeleteRRs) ->
+    %%
+    #dns_rr{ttl = TTL, tm = TM, cnt = Cnt} = RR,
+    if
+        TTL =:= 0 ->
+            %% Valid, immediately expired; return and delete
+            Key = match_rr_key(RR),
+            match_rr(
+              CacheDb, RRs, Time,
+              ResultRRs#{Key => RR}, InsertRRs, [RRs | DeleteRRs]);
+        TM + TTL < Time ->
+            %% Expired, delete
+            match_rr(
+              CacheDb, RRs, Time,
+              ResultRRs, InsertRRs, [RRs | DeleteRRs]);
+        Time =< Cnt ->
+            %% Valid and just updated, return and do not update
+            Key = match_rr_key(RR),
+            match_rr(
+              CacheDb, RRs, Time,
+              ResultRRs#{Key => RR}, InsertRRs, DeleteRRs);
+        true ->
+            %% Valid; return and re-insert with updated cnt time.
+            %% The clause above ensures that the cnt field is changed
+            %% which is essential to not accidentally delete
+            %% a record we also insert.
+            Key = match_rr_key(RR),
+            match_rr(
+              CacheDb, RRs, Time,
+              ResultRRs#{Key => RR},
+              InsertRRs#{Key => RR#dns_rr{cnt = Time}},
+              [RR | DeleteRRs])
     end.
 
-cache_key(#dns_rr{domain = Domain, class = Class, type = Type}) ->
-    {Domain, Class, Type}.
+-compile({inline, [match_rr_key/1]}).
+match_rr_key(
+  #dns_rr{domain = Domain, class = Class, type = Type, data = Data}) ->
+    {Domain, Class, Type, Data}.
+
 
-%% Lower case the domain name before storage.
+%% Lowercase the domain name before storage.
 %%
 lower_rr(#dns_rr{domain=Domain}=RR) when is_list(Domain) ->
     RR#dns_rr { domain = tolower(Domain) };
@@ -1657,7 +1733,7 @@ lower_rr(RR) -> RR.
 %% Case fold upper-case to lower-case according to RFC 4343
 %% "Domain Name System (DNS) Case Insensitivity Clarification".
 %%
-%% NOTE: this code is in kernel and we don't want to relay
+%% NOTE: this code is in kernel and we don't want to rely
 %% to much on stdlib. Furthermore string:to_lower/1
 %% does not follow RFC 4343.
 %%
@@ -1726,26 +1802,20 @@ cache_refresh() ->
 %% in the cache.
 do_refresh_cache(CacheDb) ->
     Now = times(),
-    true = ets:safe_fixtable(CacheDb, true),
-    Res = do_refresh_cache(ets:first(CacheDb), CacheDb, Now, Now),
-    true = ets:safe_fixtable(CacheDb, false),
-    Res.
-
-do_refresh_cache('$end_of_table', _, _, OldestT) ->
-    OldestT;
-do_refresh_cache(Key, CacheDb, Now, OldestT) ->
-    Next = ets:next(CacheDb, Key),
-    OldT =
-        case ets:lookup(CacheDb, Key) of
-	    [{_,RR,_}] when RR#dns_rr.tm + RR#dns_rr.ttl < Now ->
-		ets:delete(CacheDb, Key),
-		OldestT;
-	     [{_,#dns_rr{},C}] when C < OldestT ->
-		C;
-	     _ ->
-		OldestT
-	end,
-    do_refresh_cache(Next, CacheDb, Now, OldT).
+    [OldestT | RRsToDelete] =
+        ets:foldl(
+          fun (#dns_rr{tm = TM, ttl = TTL} = RR, [T | RRs])
+                when TM + TTL < Now -> % Expired - delete it
+                  [T, RR | RRs];
+              (#dns_rr{cnt = C}, [T | RRs])
+                when C < T -> % Older than previously oldest - use this cnt
+                  [C | RRs];
+              (#dns_rr{}, T_RRs) -> % Keep - do not delete
+                  T_RRs
+          end, [Now], CacheDb),
+    _ = [ets:delete_object(CacheDb, RR) || RR <- RRsToDelete],
+    OldestT.
+
 
 %% -------------------------------------------------------------------
 %% Allocate room for a new entry in the cache.
@@ -1783,28 +1853,26 @@ delete_n_oldest(CacheDb, TM, OldestTM, N) ->
 
 %% Delete entries with latest access time older than TM.
 %% Delete max N number of entries.
-%% Returns the number of deleted entries.
+%% Return the number of deleted entries.
+%%
 delete_older(CacheDb, TM, N) ->
-    true = ets:safe_fixtable(CacheDb, true),
-    Res = delete_older(ets:first(CacheDb), CacheDb, TM, N, 0),
-    true = ets:safe_fixtable(CacheDb, false),
-    Res.
-
-delete_older('$end_of_table', _, _, _, M) ->
-    M;
-delete_older(_, _, _, M, M) ->
-    M;
-delete_older(Key, CacheDb, TM, N, M) ->
-    Next = ets:next(CacheDb, Key),
-    M1 =
-	case ets:lookup(CacheDb, Key) of
-	    [{_,_,Cnt}] when Cnt =< TM ->
-		ets:delete(CacheDb, Key),
-		M + 1;
-	    _ ->
-		M
-	  end,
-    delete_older(Next, CacheDb, TM, N, M1).
+    RRsToDelete =
+        ets:foldl(
+          fun (#dns_rr{cnt = C} = RR, RRs)
+              when C =< TM -> % Candidate for deletion
+                  [RR | RRs];
+              (#dns_rr{}, RRs) ->
+                  RRs
+          end, [], CacheDb),
+    N - delete_older_n(CacheDb, lists:keysort(#dns_rr.cnt, RRsToDelete), N).
+
+delete_older_n(_CacheDb, RRs, 0) when is_list(RRs) ->
+    0;
+delete_older_n(_CacheDb, [], N) ->
+    N;
+delete_older_n(CacheDb, [RR | RRs], N) ->
+    ets:delete_object(CacheDb, RR),
+    delete_older_n(CacheDb, RRs, N - 1).
 
 
 %% as lists:delete/2, but delete all exact matches
-- 
2.26.2

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