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index: avoid unsafe buffer sharing in Log_file #358

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Merged
craigfe merged 1 commit into from
Oct 11, 2021
Merged

index: avoid unsafe buffer sharing in Log_file #358

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66 changes: 38 additions & 28 deletions src/log_file.ml
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,13 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
Entry.encoded_size)
end

module Scratch = struct
type t = { buffer : bytes (** [Bytes.length buf = Entry.encoded_size] *) }
[@@unboxed]

let create () = { buffer = Bytes.create Entry.encoded_size }
end

type t = {
io : IO.t; (** The disk file handler *)
append_io : string -> unit; (** Pre-allocated [IO.append io] closure *)
Expand All @@ -39,7 +46,6 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
two. *)
mutable bucket_count_log2 : int;
(** Invariant: equal to [log_2 (Array.length hashtbl)] *)
mutable scratch_buf : bytes;
mutable cardinal : int;
}

Expand All @@ -61,15 +67,17 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct

let flush ?no_callback ~with_fsync t = IO.flush ?no_callback ~with_fsync t.io

let key_of_offset t off =
let r = IO.read t.io ~off ~len:Key.encoded_size t.scratch_buf in
assert (r = Key.encoded_size);
fst (Entry.decode_key (Bytes.unsafe_to_string t.scratch_buf) 0)
let key_of_offset t (scratch : Scratch.t) off =
let len = Key.encoded_size in
let r = IO.read t.io ~off ~len scratch.buffer in
assert (r = len);
fst (Entry.decode_key (Bytes.unsafe_to_string scratch.buffer) 0)

let entry_of_offset t off =
let r = IO.read t.io ~off ~len:Entry.encoded_size t.scratch_buf in
let entry_of_offset t (scratch : Scratch.t) off =
let len = Entry.encoded_size in
let r = IO.read t.io ~off ~len scratch.buffer in
assert (r = Entry.encoded_size);
Entry.decode (Bytes.unsafe_to_string t.scratch_buf) 0
Entry.decode (Bytes.unsafe_to_string scratch.buffer) 0

let elt_index t key =
(* NOTE: we use the _uppermost_ bits of the key hash to index the bucket
Expand All @@ -78,7 +86,7 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
let unneeded_bits = Key.hash_size - t.bucket_count_log2 in
(Key.hash key lsr unneeded_bits) land ((1 lsl t.bucket_count_log2) - 1)

let resize t =
let resize t scratch =
(* Scale the number of hashtbl buckets. *)
t.bucket_count_log2 <- t.bucket_count_log2 + 1;
let new_bucket_count = 1 lsl t.bucket_count_log2 in
Expand All @@ -94,7 +102,7 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
let bucket_2i, bucket_2i_plus_1 =
Small_list.to_list bucket
|> List.partition (fun offset ->
let key = key_of_offset t offset in
let key = key_of_offset t scratch offset in
let new_index = elt_index t key in
assert (new_index lsr 1 = i);
new_index land 1 = 0)
Expand All @@ -105,8 +113,8 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct

(** Replace implementation that only updates in-memory state (and doesn't
write the binding to disk). *)
let replace_memory t key offset =
if t.cardinal > 2 * Array.length t.hashtbl then resize t;
let replace_memory t scratch key offset =
if t.cardinal > 2 * Array.length t.hashtbl then resize t scratch;
let elt_idx = elt_index t key in
let bucket = t.hashtbl.(elt_idx) in
let bucket =
Expand All @@ -117,7 +125,7 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
(* We ensure there's at most one binding for a given key *)
offset'
else
let key' = key_of_offset t offset' in
let key' = key_of_offset t scratch offset' in
match Key.equal key key' with
| false -> offset'
| true ->
Expand All @@ -139,10 +147,13 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
let replace t key value =
let offset = IO.offset t.io in
Entry.encode' key value t.append_io;
replace_memory t key offset
replace_memory t (Scratch.create ()) key offset

let sync_entries ~min t =
IO.iter_keys ~min (fun offset key -> replace_memory t key offset) t.io
let scratch = Scratch.create () in
IO.iter_keys ~min
(fun offset key -> replace_memory t scratch key offset)
t.io

let reload t =
clear_memory t;
Expand All @@ -160,25 +171,20 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
in
let hashtbl = Array.make bucket_count Small_list.empty in
let t =
{
io;
append_io = IO.append io;
hashtbl;
bucket_count_log2;
scratch_buf = Bytes.create Entry.encoded_size;
cardinal;
}
{ io; append_io = IO.append io; hashtbl; bucket_count_log2; cardinal }
in
IO.iter_keys (fun offset key -> replace_memory t key offset) io;
let scratch = Scratch.create () in
IO.iter_keys (fun offset key -> replace_memory t scratch key offset) io;
t

let find t key =
let elt_idx = elt_index t key in
let bucket = t.hashtbl.(elt_idx) in
let scratch = Scratch.create () in
Small_list.find_map bucket ~f:(fun offset ->
(* We expect the keys to match most of the time, so we decode the
value at the same time. *)
let entry = entry_of_offset t offset in
let entry = entry_of_offset t scratch offset in
match Key.equal key entry.key with
| false -> None
| true -> Some entry.value)
Expand All @@ -187,21 +193,25 @@ module Make (IO : Io.S) (Key : Data.Key) (Value : Data.Value) = struct
| Some x -> x

let fold t ~f ~init =
let scratch = Scratch.create () in
ArrayLabels.fold_left t.hashtbl ~init ~f:(fun acc bucket ->
Small_list.fold_left bucket ~init:acc ~f:(fun acc offset ->
let entry = entry_of_offset t offset in
let entry = entry_of_offset t scratch offset in
f acc entry))

let iter t ~f =
let scratch = Scratch.create () in
ArrayLabels.iter t.hashtbl ~f:(fun bucket ->
Small_list.iter bucket ~f:(fun offset -> f (entry_of_offset t offset)))
Small_list.iter bucket ~f:(fun offset ->
f (entry_of_offset t scratch offset)))

let to_sorted_seq t =
let scratch = Scratch.create () in
Array.to_seq t.hashtbl
|> Seq.flat_map (fun bucket ->
let arr =
Small_list.to_array bucket
|> Array.map (fun off -> entry_of_offset t off)
|> Array.map (fun off -> entry_of_offset t scratch off)
in
Array.sort Entry.compare arr;
Array.to_seq arr)
Expand Down