Tests: run self check and lint in parallel (#8189)

I've seen this speed up `runtests.py` by about 20s when multiple cores are
available.

Author: JukkaL

runtests.py

@@ -1,9 +1,9 @@
 #!/usr/bin/env python3
+import subprocess
+from subprocess import Popen
 from os import system
 from sys import argv, exit, platform, executable, version_info
 
-prog, *args = argv
-
 
 # Use the Python provided to execute the script, or fall back to a sane default
 if version_info >= (3, 5, 0):
@@ -63,23 +63,71 @@
 
 assert all(cmd in cmds for cmd in FAST_FAIL)
 
-if not set(args).issubset(cmds):
-    print("usage:", prog, " ".join('[%s]' % k for k in cmds))
-    exit(1)
-
-if not args:
-    args = list(cmds)
-
-status = 0
 
-for arg in args:
-    cmd = cmds[arg]
-    print('run %s: %s' % (arg, cmd))
+def run_cmd(name: str) -> int:
+    status = 0
+    cmd = cmds[name]
+    print('run %s: %s' % (name, cmd))
     res = (system(cmd) & 0x7F00) >> 8
     if res:
-        print('\nFAILED: %s' % arg)
+        print('\nFAILED: %s' % name)
         status = res
-        if arg in FAST_FAIL:
+        if name in FAST_FAIL:
             exit(status)
+    return status
+
+
+def start_background_cmd(name: str) -> Popen:
+    cmd = cmds[name]
+    proc = subprocess.Popen(cmd,
+                            shell=True,
+                            stderr=subprocess.STDOUT,
+                            stdout=subprocess.PIPE)
+    return proc
+
+
+def wait_background_cmd(name: str, proc: Popen) -> int:
+    output = proc.communicate()[0]
+    status = proc.returncode
+    print('run %s: %s' % (name, cmds[name]))
+    if status:
+        print(output.decode().rstrip())
+        print('\nFAILED: %s' % name)
+        if name in FAST_FAIL:
+            exit(status)
+    return status
+
+
+def main() -> None:
+    prog, *args = argv
+
+    if not set(args).issubset(cmds):
+        print("usage:", prog, " ".join('[%s]' % k for k in cmds))
+        exit(1)
+
+    if not args:
+        args = list(cmds)
+
+    status = 0
+
+    if 'self' in args and 'lint' in args:
+        # Perform lint and self check in parallel as it's faster.
+        proc = start_background_cmd('lint')
+        cmd_status = run_cmd('self')
+        if cmd_status:
+            status = cmd_status
+        cmd_status = wait_background_cmd('lint', proc)
+        if cmd_status:
+            status = cmd_status
+        args = [arg for arg in args if arg not in ('self', 'lint')]
+
+    for arg in args:
+        cmd_status = run_cmd(arg)
+        if cmd_status:
+            status = cmd_status
+
+    exit(status)
+
 
-exit(status)
+if __name__ == '__main__':
+    main()