Bug 1062579 - Ensure get_symbol() cannot return an empty job symbol

In the case where 'n' wasn't found for jobs whose symbol letter was
suppressed (eg: a mochitest job that isn't split into multiple parts),
get_symbol() could have previously returned an empty symbol.
The suppression now only takes place if 'n' was found.

tests/etl/test_buildbot.py

@@ -87,6 +87,17 @@ buildernames = [
                    'os': 'linux',
                    'os_platform': 'linux32',
                    'vm': True}}),
+    ('Rev4 MacOSX Snow Leopard 10.6 mozilla-central debug test mochitest',
+    {'build_type': 'debug',
+      'job_type': 'unittest',
+      'name': {'group_name': 'Mochitest',
+               'group_symbol': 'M',
+               'name': 'Mochitest',
+               'job_symbol': 'M'},
+      'platform': {'arch': 'x86_64',
+                   'os': 'mac',
+                   'os_platform': 'osx-10-6',
+                   'vm': False}}),
     ('Rev4 MacOSX Snow Leopard 10.6 mozilla-central debug test mochitest-devtools-chrome-2',
     {'build_type': 'debug',
       'job_type': 'unittest',

treeherder/etl/buildbot.py

@@ -802,15 +802,14 @@ def get_symbol(name, bn):
 
     s = SYMBOLS.get(name, "?")
 
-    # Mochitests, Mochitest-e10s and Mochitest OOP are the only ones
-    # that display as just a number (with no letters)
-    if s in ["M", "M-e10s", "M-oop"]:
-        s = ""
-
-    n = ""
     nummatch = NUMBER_RE.match(bn)
-    if nummatch:
-        n = nummatch.group(1)
+    n = nummatch.group(1) if nummatch else ""
+
+    # For multi-part Mochitest, Mochitest-e10s and Mochitest OOP jobs
+    # display only the job part number, and not the letters.
+    if n and s in ["M", "M-e10s", "M-oop"]:
+        return n
+
     return "{0}{1}".format(s, n)