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AK: Correctly calculate size of the last AllocatingMemoryStream chunk

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Tim Schumacher 2023-05-29 11:40:59 +02:00 committed by Jelle Raaijmakers
parent 52aab50914
commit 58b1d9c319
2 changed files with 37 additions and 1 deletions
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10
AK/MemoryStream.cpp
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@ -210,8 +210,16 @@ ErrorOr<Optional<size_t>> AllocatingMemoryStream::offset_of(ReadonlyBytes needle
search_spans[i] = m_chunks[i].span();
}
auto used_size_of_last_chunk = m_write_offset % CHUNK_SIZE;
// The case where the stored write offset is actually the used space is the only case where a result of zero
// actually is zero. In other cases (i.e. our write offset is beyond the size of a chunk) the write offset
// already points to the beginning of the next chunk, in that case a result of zero indicates "use the last chunk in full".
if (m_write_offset >= CHUNK_SIZE && used_size_of_last_chunk == 0)
used_size_of_last_chunk = CHUNK_SIZE;
// Trimming is done first to ensure that we don't unintentionally shift around if the first and last chunks are the same.
search_spans[chunk_count - 1] = search_spans[chunk_count - 1].trim(m_write_offset % CHUNK_SIZE);
search_spans[chunk_count - 1] = search_spans[chunk_count - 1].trim(used_size_of_last_chunk);
search_spans[0] = search_spans[0].slice(m_read_offset);
return AK::memmem(search_spans.begin(), search_spans.end(), needle);

28
Tests/AK/TestMemoryStream.cpp
View file

@ -123,3 +123,31 @@ TEST_CASE(allocating_memory_stream_offset_of_after_chunk_reorder)
EXPECT_EQ(offset.value(), 15ul);
}
}
TEST_CASE(allocating_memory_stream_offset_of_with_write_offset_multiple_of_chunk_size)
{
// This tests a specific edge case where we would erroneously trim the last searched block
// to size 0 if the current write offset is a multiple of the chunk size.
AllocatingMemoryStream stream;
// First, fill exactly one chunk (in groups of 16 bytes).
for (size_t i = 0; i < (AllocatingMemoryStream::CHUNK_SIZE / 16) - 1; ++i)
MUST(stream.write_until_depleted("AAAAAAAAAAAAAAAA"sv.bytes()));
MUST(stream.write_until_depleted("BCDEFGHIJKLMNOPQ"sv.bytes()));
// Read a few bytes from the beginning to ensure that we are trying to slice into the zero-sized block.
MUST(stream.discard(32));
{
auto offset = MUST(stream.offset_of("B"sv.bytes()));
EXPECT(offset.has_value());
EXPECT_EQ(offset.value(), AllocatingMemoryStream::CHUNK_SIZE - 32 - 16);
}
{
auto offset = MUST(stream.offset_of("Q"sv.bytes()));
EXPECT(offset.has_value());
EXPECT_EQ(offset.value(), AllocatingMemoryStream::CHUNK_SIZE - 32 - 1);
}
}