0.3.2 --- download_to_file reports the writes the disk refused - #19

Problem

HttpClient::download_to_file (0.2.9 through 0.3.1) called ofs.write(...) for each body chunk without checking the stream, counted downloaded += n from the network, and set result.bytesWritten = downloaded after ofs.close() without checking that either. On a full disk (ENOSPC) the transfer therefore "succeeded". Measured on a real download: 420,831,054 bytes received, 220,979,200 bytes left on disk, bytesWritten == 420,831,054, ok() == true. The caller (xlings) then blamed the source ("sha256 mismatch") instead of the local disk.

Change (0.3.2)

• Each chunk is flushed and the stream is checked. On the first failure the transfer stops reading, error becomes write <path>: <reason> (the reason is errno through std::generic_category, e.g. No space left on device), and the connection is dropped instead of being returned to the pool with the rest of the body still on it.
• close() is checked the same way, since buffered bytes only reach the file at flush.
• New API on DownloadToFileResult, both additive and placed after the existing fields:
  • bool writeFailed is true when the fault is the destination and not the source: a failed write, a failed close, or a file that could not be opened (that error string is unchanged: Cannot open file: <path>).
  • std::int64_t bytesReceived is the number of body bytes the connection delivered.
• bytesWritten now counts bytes the file accepted, where it used to count bytes the network delivered. The two are equal for every transfer that did not fail to write. When a write fails, the chunk that failed is not counted, though the file may hold part of it, so the value is a floor on the file's size and never above it. This is the one meaning change.
• http.cppm gains a global module fragment with <cerrno>; import std does not carry the errno macro.
• Version 0.3.1 to 0.3.2 in mcpp.toml, a CHANGELOG entry, and the README install lines.

Tests

tests/test_pool.cpp uses the existing in-process TLS server:

• ADownloadIntoAFullDeviceFailsAsALocalWriteAndCountsNothing (Linux only, skipped without /dev/full) downloads 64 KiB into /dev/full and asserts: not ok(), writeFailed, statusCode == 200, bytesWritten == 0, 0 < bytesReceived < 64 KiB (reading stopped), the error starts with write /dev/full: and carries the ENOSPC text, and a following download to a good path succeeds on a new connection (accepts() == 2), so the dirty connection was not reused.
• ADestinationThatCannotBeOpenedIsALocalFailure pins writeFailed for the open failure.

Checked by mutation: with the stream checks removed, the first test fails on ok() (true) and bytesWritten (65536).

mcpp test: all four test binaries pass locally (test_framing, test_pool 27 tests, test_download and test_resolver, which reach the network). tools/template_smoke.sh passes.