Now that we keep the last seen cell digests on the Exit side on the circuit
object, use that to match the SENDME v1 transforming this whole process into a
real authenticated SENDME mechanism.

Part of #26841

Signed-off-by: David Goulet <dgoulet@torproject.org>

This makes tor remember the last seen digest of a cell if that cell is the
last one before a SENDME on the Exit side.

Closes #26839

Signed-off-by: David Goulet <dgoulet@torproject.org>

This commit makes tor able to parse and handle a SENDME version 1. It will
look at the consensus parameter "sendme_accept_min_version" to know what is
the minimum version it should look at.

IMPORTANT: At this commit, the validation of the cell is not fully
implemented. For this, we need #26839 to be completed that is to match the
SENDME digest with the last cell digest.

Closes #26841

Signed-off-by: David Goulet <dgoulet@torproject.org>

This code will obey the consensus parameter "sendme_emit_min_version" to know
which SENDME version it should send. For now, the default is 0 and the
parameter is not yet used in the consensus.

This commit adds the support to send version 1 SENDMEs but aren't sent on the
wire at this commit.

Closes #26840

Signed-off-by: David Goulet <dgoulet@torproject.org>

In order to be able to deploy the authenticated SENDMEs, these two consensus
parameters are needed to control the minimum version that we can emit and
accept.

See section 4 in prop289 for more details.

Note that at this commit, the functions that return the values aren't used so
compilation fails if warnings are set to errors.

Closes #26842

Signed-off-by: David Goulet <dgoulet@torproject.org>

When we are about to send a DATA cell, we have to decrement the package window
for both the circuit and stream level.

This commit adds helper functions to handle the package window decrement.

Part of #26288

Signed-off-by: David Goulet <dgoulet@torproject.org>

When we get a relay DATA cell delivered, we have to decrement the deliver
window on both the circuit and stream level.

This commit adds helper functions to handle the deliver window decrement.

Part of #26840

Signed-off-by: David Goulet <dgoulet@torproject.org>

This is a bit of a complicated commit. It moves code but also refactors part
of it. No behavior change, the idea is to split things up so we can better
handle and understand how SENDME cells are processed where ultimately it will
be easier to handle authenticated SENDMEs (prop289) using the intermediate
functions added in this commit.

The entry point for the cell arriving at the edge (Client or Exit), is
connection_edge_process_relay_cell() for which we look if it is a circuit or
stream level SENDME. This commit refactors that part where two new functions
are introduced to process each of the SENDME types.

The sendme_process_circuit_level() has basically two code paths. If we are a
Client (the circuit is origin) or we are an Exit. Depending on which, the
package window is updated accordingly. Then finally, we resume the reading on
every edge streams on the circuit.

The sendme_process_stream_level() applies on the edge connection which will
update the package window if needed and then will try to empty the inbuf if
need be because we can now deliver more cells.

Again, no behavior change but in order to split that code properly into their
own functions and outside the relay.c file, code modification was needed.

Part of #26840.

Signed-off-by: David Goulet <dgoulet@torproject.org>

Signed-off-by: David Goulet <dgoulet@torproject.org>

Take apart the SENDME cell specific code and put it in sendme.{c|h}. This is
part of prop289 that implements authenticated SENDMEs.

Creating those new files allow for the already huge relay.c to not grow in LOC
and makes it easier to handle and test the SENDME cells in an isolated way.

This commit only moves code. No behavior change.

Signed-off-by: David Goulet <dgoulet@torproject.org>