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https://github.com/fish-shell/fish-shell
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Refactor the named pipe uvar notifier with a state machine
This attempts to simplify the named pipe notifier by switching to a state machine model.
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ridiculousfish
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7c5b8b8556
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e0bf23ad26
1 changed files with 151 additions and 97 deletions
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@ -1317,15 +1317,42 @@ static autoclose_fd_t make_fifo(const wchar_t *test_path, const wchar_t *suffix)
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// To post a notification, write some data to the pipe, wait a little while, and then read it back.
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//
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// To receive a notification, watch for the pipe to become readable. When it does, enter a polling
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// mode until the pipe is no longer readable. To guard against the possibility of a shell exiting
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// when there is data remaining in the pipe, if the pipe is kept readable too long, clients will
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// attempt to read data out of it (to render it no longer readable).
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// mode until the pipe is no longer readable, where we poll based on the modification date of the
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// pipe. To guard against the possibility of a shell exiting when there is data remaining in the
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// pipe, if the pipe is kept readable too long, clients will attempt to read data out of it (to
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// render it no longer readable).
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class universal_notifier_named_pipe_t final : public universal_notifier_t {
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#if !defined(__CYGWIN__)
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// We operate a state machine.
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enum state_t{
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// The pipe is not yet readable. There is nothing to do in poll.
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// If the pipe becomes readable we will enter the polling state.
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waiting_for_readable,
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// The pipe is readable. In poll, check if the pipe is still readable,
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// and whether its timestamp has changed.
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polling_during_readable,
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// We have written to the pipe (so we expect it to be readable).
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// We may read back from it in poll().
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waiting_to_drain,
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};
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// The state we are currently in.
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state_t state{waiting_for_readable};
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// When we entered that state, in microseconds since epoch.
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long long state_start_usec{-1};
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// The pipe itself; this is opened read/write.
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autoclose_fd_t pipe_fd;
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file_id_t pipe_fd_id{};
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long long readback_time_usec{0};
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size_t readback_amount{0};
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// The pipe's file ID containing the last modified timestamp.
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file_id_t pipe_timestamps{};
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// If we are in waiting_to_drain state, how much we have written and therefore are responsible
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// for draining.
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size_t drain_amount{0};
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// We "flash" the pipe to make it briefly readable, for this many usec.
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static constexpr long long k_flash_duration_usec = 1e5;
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@ -1333,10 +1360,28 @@ class universal_notifier_named_pipe_t final : public universal_notifier_t {
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// If the pipe remains readable for this many usec, we drain it.
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static constexpr long long k_readable_too_long_duration_usec = 5e6;
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bool polling_due_to_readable_fd{false};
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long long drain_if_still_readable_time_usec{0};
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/// \return the name of a state.
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static const char *state_name(state_t s) {
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switch (s) {
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case waiting_for_readable:
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return "waiting";
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case polling_during_readable:
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return "polling";
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case waiting_to_drain:
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return "draining";
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}
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DIE("Unreachable");
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}
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void drain_excessive_data() const {
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// Switch to a state (may or may not be new).
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void set_state(state_t new_state) {
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FLOGF(uvar_notifier, "changing from %s to %s", state_name(state), state_name(new_state));
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state = new_state;
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state_start_usec = get_time();
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}
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// Called when the pipe has been readable for too long.
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void drain_excess() const {
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// The pipe seems to have data on it, that won't go away. Read a big chunk out of it. We
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// don't read until it's exhausted, because if someone were to pipe say /dev/null, that
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// would cause us to hang!
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@ -1345,16 +1390,26 @@ class universal_notifier_named_pipe_t final : public universal_notifier_t {
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ignore_result(read(pipe_fd.fd(), buff, sizeof buff));
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}
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// Called when we want to read back data we have written, to mark the pipe as non-readable.
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void drain_written() {
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while (this->drain_amount > 0) {
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char buff[64];
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size_t amt = std::min(this->drain_amount, sizeof buff);
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ignore_result(read(this->pipe_fd.fd(), buff, amt));
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this->drain_amount -= amt;
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}
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}
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/// Check if the pipe's file ID (aka struct stat) is different from what we have stored.
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/// If it has changed, it indicates that someone has written to the pipe; update our stored id.
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/// If it has changed, it indicates that someone has modified the pipe; update our stored id.
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/// \return true if changed, false if not.
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bool check_pipe_id_changed() {
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bool update_pipe_timestamps() {
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if (!pipe_fd.valid()) return false;
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file_id_t file_id = file_id_for_fd(pipe_fd.fd());
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if (file_id == this->pipe_fd_id) {
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file_id_t timestamps = file_id_for_fd(pipe_fd.fd());
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if (timestamps == this->pipe_timestamps) {
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return false;
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}
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this->pipe_fd_id = file_id;
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this->pipe_timestamps = timestamps;
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return true;
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}
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@ -1365,120 +1420,119 @@ class universal_notifier_named_pipe_t final : public universal_notifier_t {
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~universal_notifier_named_pipe_t() override = default;
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int notification_fd() const override {
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if (polling_due_to_readable_fd) {
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// We are in polling mode because we think our fd is readable. This means that, if we
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// return it to be select()'d on, we'll be called back immediately. So don't return it.
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return -1;
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if (!pipe_fd.valid()) return -1;
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// If we are waiting for the pipe to be readable, return it for select.
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// Otherwise we expect it to be readable already; return invalid.
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switch (state) {
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case waiting_for_readable:
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return pipe_fd.fd();
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case polling_during_readable:
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case waiting_to_drain:
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return -1;
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}
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// We are not in polling mode. Return the fd so it can be watched.
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return pipe_fd.fd();
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DIE("unreachable");
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}
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bool notification_fd_became_readable(int fd) override {
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// Our fd is readable. We deliberately do not read anything out of it: if we did, other
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// sessions may miss the notification. Instead, we go into "polling mode:" we do not
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// select() on our fd for a while, and sync periodically until the fd is no longer readable.
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// However, if we are the one who posted the notification, we don't sync (until we clean
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// up!)
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assert(fd == pipe_fd.fd() && "Wrong fd");
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UNUSED(fd);
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bool should_sync = false;
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if (readback_time_usec == 0) {
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FLOG(uvar_notifier, "entering polling mode");
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polling_due_to_readable_fd = true;
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drain_if_still_readable_time_usec = get_time() + k_readable_too_long_duration_usec;
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should_sync = true;
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// Update our pipe's timestamps.
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check_pipe_id_changed();
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switch (state) {
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case waiting_for_readable:
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// We are now readable.
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// Grab the timestamp and return true indicating that we received a notification.
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set_state(polling_during_readable);
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update_pipe_timestamps();
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return true;
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case polling_during_readable:
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case waiting_to_drain:
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// We did not return an fd to wait on, so should not be called.
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DIE("should not be called in this state");
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}
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return should_sync;
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DIE("unreachable");
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}
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void post_notification() override {
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if (!pipe_fd.valid()) return;
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// We need to write some data (any data) to the pipe, then wait for a while, then read
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// it back. Nobody is expected to read it except us.
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FLOG(uvar_notifier, "posting notification by writing to pipe");
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FLOGF(uvar_notifier, "writing to pipe (written %lu)", (unsigned long)drain_amount);
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char c[1] = {'\0'};
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ssize_t amt_written = write(pipe_fd.fd(), c, sizeof c);
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if (amt_written < 0 && (errno == EWOULDBLOCK || errno == EAGAIN)) {
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// Very unsual: the pipe is full!
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drain_excessive_data();
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// Very unsual: the pipe is full! Try to read some and repeat once.
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drain_excess();
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amt_written = write(pipe_fd.fd(), c, sizeof c);
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if (amt_written < 0) {
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FLOG(uvar_notifier, "pipe could not be drained, skipping notification");
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return;
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}
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FLOG(uvar_notifier, "pipe drained");
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}
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// Now schedule a read for some time in the future.
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this->readback_time_usec = get_time() + k_flash_duration_usec;
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this->readback_amount += sizeof c;
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// No need to react to our own changes.
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check_pipe_id_changed();
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assert(amt_written >= 0 && "Amount should not be negative");
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this->drain_amount += amt_written;
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// We unconditionally set our state to waiting to drain.
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set_state(waiting_to_drain);
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update_pipe_timestamps();
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}
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unsigned long usec_delay_between_polls() const override {
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unsigned long readback_delay = ULONG_MAX;
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if (this->readback_time_usec > 0) {
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// How long until the readback?
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long long now = get_time();
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if (now >= this->readback_time_usec) {
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// Oops, it already passed! Return something tiny.
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readback_delay = 1000;
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} else {
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readback_delay = static_cast<unsigned long>(this->readback_time_usec - now);
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}
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}
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if (!pipe_fd.valid()) return 0;
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switch (state) {
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case waiting_for_readable:
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// No polling necessary until it becomes readable.
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return 0;
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unsigned long polling_delay = ULONG_MAX;
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if (polling_due_to_readable_fd) {
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// We're in polling mode. Don't return a value less than our polling interval.
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polling_delay = k_flash_duration_usec;
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case polling_during_readable:
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case waiting_to_drain:
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return k_flash_duration_usec;
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}
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// Now return the smaller of the two values. If we get ULONG_MAX, it means there's no more
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// need to poll; in that case return 0.
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unsigned long result = std::min(readback_delay, polling_delay);
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if (result == ULONG_MAX) {
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result = 0;
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}
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return result;
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DIE("Unreachable");
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}
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bool poll() override {
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if (!pipe_fd.valid()) return false;
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switch (state) {
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case waiting_for_readable:
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// Nothing to do until the fd is readable.
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return false;
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// Check if we are past the readback time.
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if (this->readback_time_usec > 0 && get_time() >= this->readback_time_usec) {
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FLOGF(uvar_notifier, "reading back %lu bytes we wrote to pipe", this->readback_amount);
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// Read back what we wrote. We do nothing with the value.
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while (this->readback_amount > 0) {
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char buff[64];
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size_t amt_to_read = std::min(this->readback_amount, sizeof(buff));
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ignore_result(read(this->pipe_fd.fd(), buff, amt_to_read));
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this->readback_amount -= amt_to_read;
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case polling_during_readable: {
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// If we're no longer readable, go back to wait mode.
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// Conversely, if we have been readable too long, perhaps some fish died while its
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// written data was still on the pipe; drain some.
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if (!select_wrapper_t::poll_fd_readable(pipe_fd.fd())) {
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set_state(waiting_for_readable);
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} else if (get_time() >= state_start_usec + k_readable_too_long_duration_usec) {
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drain_excess();
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}
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// Sync if the pipe's timestamp is different, meaning someone modified the pipe
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// since we last saw it.
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if (update_pipe_timestamps()) {
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FLOG(uvar_notifier, "pipe changed, will sync uvars");
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return true;
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}
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return false;
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}
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assert(this->readback_amount == 0);
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this->readback_time_usec = 0;
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}
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// Check to see if we are doing readability polling.
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if (!polling_due_to_readable_fd) {
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return false;
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}
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// We are polling, so we are definitely going to sync.
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// See if this is still readable.
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if (!select_wrapper_t::poll_fd_readable(pipe_fd.fd())) {
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// No longer readable, no longer polling.
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FLOG(uvar_notifier, "exiting polling mode");
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polling_due_to_readable_fd = false;
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drain_if_still_readable_time_usec = 0;
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} else {
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// Still readable. If it's been readable for a long time, there is probably
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// lingering data on the pipe.
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if (get_time() >= drain_if_still_readable_time_usec) {
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drain_excessive_data();
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case waiting_to_drain: {
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// We wrote data to the pipe. Maybe read it back.
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// If we are still readable, then there is still data on the pipe; maybe another
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// change occurred with ours.
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if (get_time() >= state_start_usec + k_flash_duration_usec) {
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drain_written();
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if (!select_wrapper_t::poll_fd_readable(pipe_fd.fd())) {
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set_state(waiting_for_readable);
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} else {
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set_state(polling_during_readable);
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}
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}
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return update_pipe_timestamps();
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}
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}
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return check_pipe_id_changed();
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DIE("Unreachable");
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}
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#else // this class isn't valid on this system
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public:
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universal_notifier_named_pipe_t(const wchar_t *test_path) {
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