From 530d473667457e7d9ff5641856b80fb97a886484 Mon Sep 17 00:00:00 2001
From: Fred Hebert <mononcqc@ferd.ca>
Date: Sat, 6 Apr 2024 19:15:32 +0000
Subject: [PATCH] do streaming file hashing on disk backing for large files

---
 apps/revault/src/revault_file_disk.erl        | 23 ++++++++++++++---
 apps/revault/test/revault_file_disk_SUITE.erl | 25 ++++++++++++++++++-
 2 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/apps/revault/src/revault_file_disk.erl b/apps/revault/src/revault_file_disk.erl
index 0be9cf5..4b20e7b 100644
--- a/apps/revault/src/revault_file_disk.erl
+++ b/apps/revault/src/revault_file_disk.erl
@@ -1,6 +1,7 @@
 -module(revault_file_disk).
 
 -include_lib("kernel/include/file.hrl").
+-compile({no_auto_import,[size/1]}).
 
 -export([hash/1, hash_bin/1,
          copy/2,
@@ -20,9 +21,18 @@
 %% it is SHA256.
 -spec hash(file:filename_all()) -> hash().
 hash(Path) ->
-    %% TODO: support large files on this too
-    {ok, Bin} = read_file(Path),
-    hash_bin(Bin).
+    {ok, Size} = size(Path),
+    case application:get_env(revault, multipart_size) of
+        {ok, Threshold} when Size > Threshold ->
+            {ok, Fd} = file:open(Path, [read, raw, binary]),
+            HashState = crypto:hash_init(sha256),
+            NewHashState = hash_fd(Fd, 0, Size, Threshold, HashState),
+            file:close(Fd),
+            crypto:hash_final(NewHashState);
+        _ ->
+            {ok, Bin} = read_file(Path),
+            hash_bin(Bin)
+    end.
 
 %% @doc takes a binary and computes a hash for it as used to track changes
 %% and validate payloads in ReVault. This hash is not guaranteed to be
@@ -252,3 +262,10 @@ detect_tmpdir() ->
 randname() ->
     float_to_list(rand:uniform()).
 
+hash_fd(_Fd, Offset, Size, _Threshold, HashState) when Offset =:= Size ->
+    HashState;
+hash_fd(Fd, Offset, Size, Threshold, HashState) when Offset < Size ->
+    Bytes = min(Size-Offset, Threshold),
+    {ok, Bin} = file:pread(Fd, Offset, Bytes),
+    hash_fd(Fd, Offset+Bytes, Size, Threshold,
+            crypto:hash_update(HashState, Bin)).
diff --git a/apps/revault/test/revault_file_disk_SUITE.erl b/apps/revault/test/revault_file_disk_SUITE.erl
index cc36f06..0832894 100644
--- a/apps/revault/test/revault_file_disk_SUITE.erl
+++ b/apps/revault/test/revault_file_disk_SUITE.erl
@@ -4,7 +4,8 @@
 -include_lib("common_test/include/ct.hrl").
 
 all() ->
-    [read_range, multipart, multipart_hash].
+    [read_range, multipart, multipart_hash,
+     hash_large_file].
 
 read_range() ->
     [{doc, "general checks on reading subsets of files"}].
@@ -69,3 +70,25 @@ multipart_hash(Config) ->
     ?assertError(invalid_hash,
                  revault_file_disk:multipart_final(State, File, Parts, Hash)),
     ok.
+
+hash_large_file() ->
+    [{doc, "hashing large files can be done in an iterative manner"}].
+hash_large_file(Config) ->
+    WidthBytes = 100,
+    WidthBits = 8*WidthBytes,
+    Parts = 11,
+    Bin = <<0:WidthBits, 1:WidthBits, 2:WidthBits, 3:WidthBits, 4:WidthBits,
+            5:WidthBits, 6:WidthBits, 7:WidthBits, 8:WidthBits, 9:WidthBits, 10>>,
+    File = filename:join([?config(priv_dir, Config), "multipart.scratch"]),
+    ok = file:write_file(File, Bin),
+    Hash = revault_file_disk:hash_bin(<<Bin/binary>>),
+    %% do the streaming hash read
+    Multipart = application:get_env(revault, multipart_size),
+    application:set_env(revault, multipart_size, WidthBytes),
+    HashDisk = revault_file_disk:hash(File),
+    case Multipart of
+        undefined -> application:unset_env(revault, multipart_size);
+        {ok, Multipart} -> application:set_env(revault, multipart_size, Multipart)
+    end,
+    ?assertEqual(HashDisk, Hash),
+    ok.