Method SavePartFile

src/PartFile.cpp:820–1053

bool CPartFile::SavePartFile(bool Initial)
{
	switch (status) {
		case PS_WAITING_FOR_HASH:
		case PS_HASHING:
		case PS_COMPLETE:
			return false;
	}

	/* Don't write anything to disk if less than 100 KB of free space is left. */
	sint64 free = CPath::GetFreeSpaceAt(GetFilePath());
	if ((free != wxInvalidOffset) && (free < (100 * 1024))) {
		return false;
	}

	// Atomic-rename save.
	//
	// Old sequence (per partfile, every save tick):
	//   1. CPath::BackupFile(m_fullname, ".backup") -- full content copy
	//      of .part.met into .part.met.backup as an in-flight intermediate.
	//   2. CPath::RemoveFile(m_fullname).
	//   3. Write new .part.met from scratch.
	//   4. CPath::RemoveFile(".backup") on success.
	//   5. CPath::BackupFile(m_fullname, PARTMET_BAK_EXT) -- full content
	//      copy of the freshly-written .part.met into .part.met.bak as the
	//      long-term recovery backup.
	//
	// Three full-size content copies per save plus a delete/create dance on
	// the live .part.met. On a sharer with hundreds of dirty partfiles per
	// timer tick the aggregate runs the main thread continuously through
	// disk I/O (see issue #669) -- write() syscalls are microseconds each,
	// but the loop length itself is the bottleneck.
	//
	// New sequence:
	//   1. Write new content into .part.met.tmp.
	//   2. rename(.part.met, .part.met.bak) -- O(1) metadata op, promotes
	//      the previous .part.met to long-term backup.
	//   3. rename(.part.met.tmp, .part.met) -- atomic install of new
	//      content, POSIX rename guarantees the target is either fully
	//      old-content or fully new-content at every observable moment.
	//
	// One content write plus two metadata renames. ~3x reduction in
	// per-save disk work, and stronger crash safety because the live
	// .part.met is never absent or partial.
	const CPath tmpName = m_fullname.AppendExt(".tmp");
	const CPath bakName = m_fullname.AppendExt(PARTMET_BAK_EXT);

	CFile file;
	try {
		if (!m_PartPath.FileExists()) {
			throw wxString(".part file not found");
		}

		uint32 lsc = lastseencomplete;

		file.Open(tmpName, CFile::write);
		if (!file.IsOpened()) {
			throw wxString("Failed to open part.met.tmp file");