net_echo.eigs

# net_echo — raw TCP sockets on the trace tape (#414).
# Requires a `make net` build (the net_* builtins are in no default build).
#
# One process plays both ends of a TCP connection over loopback: the
# dial completes against the listen backlog, so no threads are needed.
# Every nondeterministic outcome — the kernel's ephemeral port, the
# accepted connection, the received bytes, the bytes-sent counts —
# enters through the trace tape, which makes this session replayable
# with no network at all:
#
#   $ EIGS_TRACE=echo.tape  eigenscript net_echo.eigs   > first.out
#   $ EIGS_REPLAY=echo.tape eigenscript net_echo.eigs   > second.out
#   $ diff first.out second.out                         # identical
#
# The replay run performs zero socket syscalls — the tape pins what the
# network said, so the network is no longer required to say it.

# Ask the kernel for an ephemeral port (0), then read back its pick.
server is net_listen of 0
port is net_port of server
print of "listening"

# Dial ourselves. On loopback the connect lands in the listen backlog
# immediately, before we ever call accept.
client is net_dial of ["127.0.0.1", port]
conn is net_accept of [server, 2000]

# Client -> server. net_recv returns a buffer of byte values (binary-
# safe); str_from_bytes decodes text. The 2000 is a timeout in ms —
# a timeout returns null, a closed peer returns an empty buffer.
sent is net_send of [client, "ping over the tape"]
print of sent
data is net_recv of [conn, 256, 2000]
msg is str_from_bytes of (data)
print of msg

# Server echoes it back.
echoed is net_send of [conn, msg]
print of echoed
back is net_recv of [client, 256, 2000]
print of str_from_bytes of (back)

net_close of client
net_close of conn
net_close of server
print of "echo complete"