Bug 1576748 - make symbolstore.py complain loudly if expected output isn't found; r=nalexander

This change surfaces errors faster, and ensures that we don't silently
end up with empty crashreporter symbols if `dump_syms` can't run for
some reason.

Differential Revision: https://phabricator.services.mozilla.com/D43520

--HG--
extra : moz-landing-system : lando

toolkit/crashreporter/tools/symbolstore.py

@@ -524,9 +524,21 @@ class Dumper:
         try:
             cmd = self.dump_syms_cmdline(file, arch, dsymbundle=dsymbundle)
             print(' '.join(cmd), file=sys.stderr)
+            # We're interested in `stderr` in the case that something goes
+            # wrong with dump_syms, but we don't want to use
+            # `stderr=subprocess.PIPE` here, as that can land us in a
+            # deadlock when we try to read only from `stdout`, below.  The
+            # Python documentation recommends using `communicate()` in such
+            # cases, but `stderr` can be rather large, and we don't want to
+            # waste time accumulating all of it in the non-error case.  So we
+            # completely ignore `stderr` here and capture it separately,
+            # below.
             proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
                                     stderr=open(os.devnull, 'wb'))
+            try:
                 module_line = proc.stdout.next()
+            except StopIteration:
+                module_line = ''
             if module_line.startswith("MODULE"):
                 # MODULE os cpu guid debug_file
                 (guid, debug_file) = (module_line.split())[3:5]
@@ -606,8 +618,22 @@ class Dumper:
                 if self.copy_debug and arch_num == 0:
                     self.CopyDebug(file, debug_file, guid,
                                    code_file, code_id)
-        except StopIteration:
-            pass
+            else:
+                # For some reason, we didn't see the MODULE line as the first
+                # line of output.  It's very possible that the interesting error
+                # message(s) are on stderr, so let's re-execute the process and
+                # capture the entirety of stderr.
+                proc = subprocess.Popen(cmd, stdout=open(os.devnull, 'wb'),
+                                        stderr=subprocess.PIPE)
+                (_, dumperr) = proc.communicate()
+                retcode = proc.returncode
+                message = [
+                    "dump_syms failed to produce the expected output",
+                    "return code: %d" % retcode,
+                    "first line of output: %s" % module_line,
+                    "stderr: %s" % dumperr
+                ]
+                raise RuntimeError('\n----------\n'.join(message))
         except Exception as e:
             print("Unexpected error: %s" % str(e), file=sys.stderr)
             raise