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libstore: be more economical about fcntl on RemoteStore

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download progress reports send a STDERR_RESULT frame. many concurrent
downloads send many STDERR_RESULT frames. each of these frames has us
run the report loop once. since many frames can happen in very little
time we may receive many frames in a single read from the socket, and
that in turn means we don't have to fcntl that socket on every round.
we must still ensure that the socket is in the correct state for each
part of the loop, and this does mean we may run two unnecessary fcntl
sequences per processStderr call. that's a small price to pay though.

Change-Id: I7af607d8c759b76aff0f6016435955e2f9456923
This commit is contained in:
eldritch horrors
2025-07-01 17:06:26 +02:00
parent ce6eba531e
commit ed3c202c20
1 changed files with 12 additions and 8 deletions
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lix/libstore/remote-store.cc
+12 -8
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@@ -775,19 +775,23 @@ static Logger::Fields readFields(Source & from)
kj::Promise<Result<RemoteStore::Connection::RemoteError>>
RemoteStore::Connection::processStderr(AsyncFdIoStream & stream)
try {
// SAFETY NOTE: while we're running we own the executor, and thus the stream.
// setting these flags is unsafe if the stream is shared with another thread.
const auto oldState = makeBlocking(getFD());
KJ_DEFER(resetBlockingState(getFD(), oldState));
while (true) {
// fill the read buffer asynchronously until we have at least a message type.
// once we have this we'll continue synchronously as in the sendCommand case.
while (fromBuf->used() < sizeof(uint64_t)) {
const auto available = fromBuf->getWriteBuffer();
fromBuf->added(TRY_AWAIT(stream.read(available.data(), available.size())));
if (fromBuf->used() < sizeof(uint64_t)) {
const auto oldState = makeNonBlocking(getFD());
KJ_DEFER(resetBlockingState(getFD(), oldState));
while (fromBuf->used() < sizeof(uint64_t)) {
const auto available = fromBuf->getWriteBuffer();
fromBuf->added(TRY_AWAIT(stream.read(available.data(), available.size())));
}
}
// SAFETY NOTE: while we're running we own the executor, and thus the stream.
// setting these flags is unsafe if the stream is shared with another thread.
const auto oldState = makeBlocking(getFD());
KJ_DEFER(resetBlockingState(getFD(), oldState));
FdSource from{getFD(), fromBuf};
from.specialEndOfFileError = "Nix daemon disconnected while waiting for a response";