support systems where 'model name' doesn't exist in /proc/cpuinfo

--------
kirr: I looked around a bit and I see that RISC-V indeed misses many
things, including model name in its /proc/cpuinfo but has isa there:

https://www.reddit.com/r/RISCV/comments/wfz050/samples_of_riscv_proccpuinfo/
https://lore.kernel.org/lkml/1538507707-22299-14-git-send-email-atish.patra@wdc.com/

However I also see that model name is x86-specific and isa is also not
present on other architectures, e.g. /proc/cpuinfo for ARM misses both
model name and isa.

So I adjusted the patch a bit to support environment where both `model
name` and `isa` are not present in /proc/cpuinfo - for example ARM.

In the future we might change to rely on `lscpu` instead completely to
avoid dealing with all these arch-specific peculiarities.

/reviewed-by @kirr
/reviewed-on !18

nxdtest/__init__.py

@@ -440,10 +440,16 @@ def system_info():
     whoami = pwd.getpwuid(os.getuid()).pw_name
     emit('xnode:\t%s@%s' % (whoami, socket.getfqdn()))
     emit('uname:\t%s' % ' '.join(os.uname()))
-    emit('cpu:\t%s' % get1('/proc/cpuinfo', 'model name'))
+    cpu = get1('/proc/cpuinfo', 'model name', None) # e.g. no 'model name' on riscv
+    isa = get1('/proc/cpuinfo', 'isa', None)        # present on riscv, but not on x86
+    if cpu:
+        emit('cpu:\t%s' % cpu)
+    if isa:
+        emit('isa:\t%s' % isa)
 
 # get1 returns first entry from file @path prefixed with ^<field>\s*:
-def get1(path, field, default=None):
+_nodefault = object()
+def get1(path, field, default=_nodefault):
     with open(path, 'r') as f:
         data = f.read()
     rex = re.compile(r'^%s\s*:\s*(.*)$' % field)
@@ -451,7 +457,7 @@ def get1(path, field, default=None):
         m = rex.match(l)
         if m is not None:
             return m.group(1)
-    if default is not None:
+    if default is not _nodefault:
         return default
     raise KeyError('%s does not have field %r' % (path, field))