build simply stops

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I just updated to 6.99.10/amd64, and now when I do a parallel build, the build
simply stops. The make processes remain, using no CPU. At one point it looked
as though it was gcc which stopped. Running gdb against gcc, and attaching
to the process, just got me "Attaching to process" and the prompt didn't
appear. All very odd, eg:

 UID   PID  PPID   CPU LID NLWP PRI NI    VSZ    RSS WCHAN  STAT TTY       TIME 
COMMAND
   0 11961  1119  9177   1    1  83  0  13268   1800 wait   I    ttyp1  0:00.03 
sh build.sh -j4 -x -E build
   0  1162 15565 30213   1    1  78  0  13264   1448 wait   I    ttyp1  0:00.00 
sh
   0  7109 18821 33448   1    1  77  0  13264   1448 wait   I    ttyp1  0:00.00 
sh
   0 14196 24998 33442   1    1  77  0  13264   1448 wait   I    ttyp1  0:00.00 
sh
   0 22196 11318 33373   1    1  77  0  13264   1452 wait   I    ttyp1  0:00.01 
sh
   0 26540 11024 33373   1    1  77  0  13264   1448 wait   I    ttyp1  0:00.00 
sh
   0 27309 29337 31374   1    1  78  0  13264   1452 wait   I    ttyp1  0:00.00 
sh
   0 18821 14196     0   1    1  85  0   8876   3532 select S    ttyp1  0:00.12 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ obj
   0 11024 22196     0   1    1  85  0   7852   2084 select S    ttyp1  0:00.08 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ do-x11
   0 11318 11961     0   1    1  85  0   7852   2100 select S    ttyp1  0:01.74 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake -j 4 build
   0 17164 27309     0   1    1  85  0   6872   1568 -      O    ttyp1 18:57.13 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ obj
   0 15565 26540     0   1    1  85  0   6828   1780 select S    ttyp1  0:00.08 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ build
   0 24998  1162     0   1    1  85  0   6828   1820 select S    ttyp1  0:00.09 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ obj
   0 29337  7109     0   1    1  85  0   6828   1692 select S    ttyp1  0:00.02 
/usr/src/obj.amd64/tooldir.NetBSD-6.99.10-amd64/bin/nbmake _THISDIR_ obj


and the xterm just says "--- obj ---"

Hit ctl-C to get a prompt, and everything appears to behave normally...

Thoughts?

Cheers,

Patrick