diff --git a/src/streams.h b/src/streams.h index 0178df1c49..84b12f65aa 100644 --- a/src/streams.h +++ b/src/streams.h @@ -612,7 +612,6 @@ private: uint64_t nRewind; //!< how many bytes we guarantee to rewind std::vector vchBuf; //!< the buffer -protected: //! read data from the source to fill the buffer bool Fill() { unsigned int pos = nSrcPos % vchBuf.size(); @@ -630,6 +629,28 @@ protected: return true; } + //! Advance the stream's read pointer (m_read_pos) by up to 'length' bytes, + //! filling the buffer from the file so that at least one byte is available. + //! Return a pointer to the available buffer data and the number of bytes + //! (which may be less than the requested length) that may be accessed + //! beginning at that pointer. + std::pair AdvanceStream(size_t length) + { + assert(m_read_pos <= nSrcPos); + if (m_read_pos + length > nReadLimit) { + throw std::ios_base::failure("Attempt to position past buffer limit"); + } + // If there are no bytes available, read from the file. + if (m_read_pos == nSrcPos && length > 0) Fill(); + + size_t buffer_offset{static_cast(m_read_pos % vchBuf.size())}; + size_t buffer_available{static_cast(vchBuf.size() - buffer_offset)}; + size_t bytes_until_source_pos{static_cast(nSrcPos - m_read_pos)}; + size_t advance{std::min({length, buffer_available, bytes_until_source_pos})}; + m_read_pos += advance; + return std::make_pair(&vchBuf[buffer_offset], advance); + } + public: CBufferedFile(FILE* fileIn, uint64_t nBufSize, uint64_t nRewindIn, int nTypeIn, int nVersionIn) : nType(nTypeIn), nVersion(nVersionIn), nSrcPos(0), m_read_pos(0), nReadLimit(std::numeric_limits::max()), nRewind(nRewindIn), vchBuf(nBufSize, std::byte{0}) @@ -667,24 +688,21 @@ public: //! read a number of bytes void read(Span dst) { - if (dst.size() + m_read_pos > nReadLimit) { - throw std::ios_base::failure("Read attempted past buffer limit"); - } while (dst.size() > 0) { - if (m_read_pos == nSrcPos) - Fill(); - unsigned int pos = m_read_pos % vchBuf.size(); - size_t nNow = dst.size(); - if (nNow + pos > vchBuf.size()) - nNow = vchBuf.size() - pos; - if (nNow + m_read_pos > nSrcPos) - nNow = nSrcPos - m_read_pos; - memcpy(dst.data(), &vchBuf[pos], nNow); - m_read_pos += nNow; - dst = dst.subspan(nNow); + auto [buffer_pointer, length]{AdvanceStream(dst.size())}; + memcpy(dst.data(), buffer_pointer, length); + dst = dst.subspan(length); } } + //! Move the read position ahead in the stream to the given position. + //! Use SetPos() to back up in the stream, not SkipTo(). + void SkipTo(const uint64_t file_pos) + { + assert(file_pos >= m_read_pos); + while (m_read_pos < file_pos) AdvanceStream(file_pos - m_read_pos); + } + //! return the current reading position uint64_t GetPos() const { return m_read_pos; diff --git a/src/test/streams_tests.cpp b/src/test/streams_tests.cpp index 0925e2e9ee..b1b262eade 100644 --- a/src/test/streams_tests.cpp +++ b/src/test/streams_tests.cpp @@ -253,7 +253,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file) BOOST_CHECK(false); } catch (const std::exception& e) { BOOST_CHECK(strstr(e.what(), - "Read attempted past buffer limit") != nullptr); + "Attempt to position past buffer limit") != nullptr); } // The default argument removes the limit completely. BOOST_CHECK(bf.SetLimit()); @@ -322,7 +322,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file) BOOST_CHECK(!bf.SetPos(0)); // But we should now be positioned at least as far back as allowed // by the rewind window (relative to our farthest read position, 40). - BOOST_CHECK(bf.GetPos() <= 30); + BOOST_CHECK(bf.GetPos() <= 30U); // We can explicitly close the file, or the destructor will do it. bf.fclose(); @@ -330,6 +330,55 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file) fs::remove(streams_test_filename); } +BOOST_AUTO_TEST_CASE(streams_buffered_file_skip) +{ + fs::path streams_test_filename = m_args.GetDataDirBase() / "streams_test_tmp"; + FILE* file = fsbridge::fopen(streams_test_filename, "w+b"); + // The value at each offset is the byte offset (e.g. byte 1 in the file has the value 0x01). + for (uint8_t j = 0; j < 40; ++j) { + fwrite(&j, 1, 1, file); + } + rewind(file); + + // The buffer is 25 bytes, allow rewinding 10 bytes. + CBufferedFile bf(file, 25, 10, 222, 333); + + uint8_t i; + // This is like bf >> (7-byte-variable), in that it will cause data + // to be read from the file into memory, but it's not copied to us. + bf.SkipTo(7); + BOOST_CHECK_EQUAL(bf.GetPos(), 7U); + bf >> i; + BOOST_CHECK_EQUAL(i, 7); + + // The bytes in the buffer up to offset 7 are valid and can be read. + BOOST_CHECK(bf.SetPos(0)); + bf >> i; + BOOST_CHECK_EQUAL(i, 0); + bf >> i; + BOOST_CHECK_EQUAL(i, 1); + + bf.SkipTo(11); + bf >> i; + BOOST_CHECK_EQUAL(i, 11); + + // SkipTo() honors the transfer limit; we can't position beyond the limit. + bf.SetLimit(13); + try { + bf.SkipTo(14); + BOOST_CHECK(false); + } catch (const std::exception& e) { + BOOST_CHECK(strstr(e.what(), "Attempt to position past buffer limit") != nullptr); + } + + // We can position exactly to the transfer limit. + bf.SkipTo(13); + BOOST_CHECK_EQUAL(bf.GetPos(), 13U); + + bf.fclose(); + fs::remove(streams_test_filename); +} + BOOST_AUTO_TEST_CASE(streams_buffered_file_rand) { // Make this test deterministic. @@ -361,7 +410,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file_rand) // sizes; the boundaries of the objects can interact arbitrarily // with the CBufferFile's internal buffer. These first three // cases simulate objects of various sizes (1, 2, 5 bytes). - switch (InsecureRandRange(5)) { + switch (InsecureRandRange(6)) { case 0: { uint8_t a[1]; if (currentPos + 1 > fileSize) @@ -399,6 +448,16 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file_rand) break; } case 3: { + // SkipTo is similar to the "read" cases above, except + // we don't receive the data. + size_t skip_length{static_cast(InsecureRandRange(5))}; + if (currentPos + skip_length > fileSize) continue; + bf.SetLimit(currentPos + skip_length); + bf.SkipTo(currentPos + skip_length); + currentPos += skip_length; + break; + } + case 4: { // Find a byte value (that is at or ahead of the current position). size_t find = currentPos + InsecureRandRange(8); if (find >= fileSize) @@ -415,7 +474,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file_rand) currentPos++; break; } - case 4: { + case 5: { size_t requestPos = InsecureRandRange(maxPos + 4); bool okay = bf.SetPos(requestPos); // The new position may differ from the requested position