M70/M72 doesn't properly restore state

[jallwine]

Here are the steps I follow to reproduce the issue:

g1_error.ngc:

%
(logappend,feed.txt)
G94 G90

G1 F60 X0
(log,Should be 60: #<_feed>)

M70

G93
(log,After G93: #<_feed>)

M72
(log,After M72: #<_feed>)
(logclose)

G1 X1

%

1. Run g1_error.ngc

This is what I expected to happen:

I expect the machine to move to X0 then to X1 at 60 inches per minute. I would also expect the following to print to feed.txt:

Should be 60: 60.000000
After G93: 60.000000
After M72: 60.000000

This is what happened instead:

The interpreter errors with Cannot do g1 with zero feed rate. feed.txt:

Should be 60: 60.000000
After G93: 60.000000
After M72: 0.000000

Information about my hardware and software:

I'm using a fork of LinuxCNC 2.8 (EMCApplication+MachineKit-HAL).

[jallwine]

Similarly, the motion mode doesn't seem to be restored:

%

(logappend,motion_mode.txt)

G94 G90 G20

G0 X0
(log,Should be 0: #<_motion_mode>)

M70

G1 F60 X1
(log,Should be 10: #<_motion_mode>)

M72
(log,Should be 0: #<_motion_mode>)
(logclose)

X0

%

Running the above results in the same Cannot do g1 with zero feed rate and the following output to motion_mode.txt:

Should be 0: 0.000000
Should be 10: 10.000000
Should be 0: 10.000000

I would expect the above to move to X0 at rapid rate, then to X1 at 60 inches per minute, then back to X0 at rapid rate.

[jallwine]

Upon closer inspection of the documentation, I discovered motion mode isn't supposed to be restored: http://linuxcnc.org/docs/2.8/html/gcode/m-code.html#mcode:m70-saved-state

[andypugh]

Sorry for the delay. I found a workaround, which seems to have only a very minor cost.