AK: Correctly calculate size of the last AllocatingMemoryStream chunk

Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/58b1d9c319
Pull-request: https://github.com/SerenityOS/serenity/pull/19122
Reviewed-by: https://github.com/alimpfard ✅
Reviewed-by: https://github.com/gmta ✅

AK/MemoryStream.cpp

@@ -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);

Tests/AK/TestMemoryStream.cpp

@@ -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);
+    }
+}