mtest: allow quickly interrupting the test run

The new behavior of interrupting the longest running test with Ctrl-C is useful
when tests hang, but not when the run is completely broken for some reason.
Psychology tells us that the user will compulsively spam Ctrl-C in this case,
so exit if three Ctrl-C's are detected within a second.

docs/markdown/snippets/meson_test_interrupt.md

@@ -0,0 +1,5 @@
+## Ctrl-C behavior in `meson test`
+
+Starting from this version, sending a `SIGINT` signal (or pressing `Ctrl-C`)
+to `meson test` will interrupt the longest running test.  Pressing `Ctrl-C`
+three times within a second will exit `meson test`.

mesonbuild/mtest.py

@@ -55,6 +55,9 @@ GNU_SKIP_RETURNCODE = 77
 # mean that the test failed even before testing what it is supposed to test.
 GNU_ERROR_RETURNCODE = 99
 
+# Exit if 3 Ctrl-C's are received within one second
+MAX_CTRLC = 3
+
 def is_windows() -> bool:
     platname = platform.system().lower()
     return platname == 'windows'
@@ -1549,6 +1552,7 @@ class TestHarness:
         futures = deque()  # type: T.Deque[asyncio.Future]
         running_tests = dict() # type: T.Dict[asyncio.Future, str]
         interrupted = False
+        ctrlc_times = deque(maxlen=MAX_CTRLC) # type: T.Deque[float]
 
         async def run_test(test: SingleTestRunner) -> None:
             async with semaphore:
@@ -1577,15 +1581,18 @@ class TestHarness:
             del running_tests[future]
             future.cancel()
 
-        def sigterm_handler() -> None:
+        def cancel_all_tests() -> None:
             nonlocal interrupted
+            interrupted = True
+            while running_tests:
+                cancel_one_test(False)
+
+        def sigterm_handler() -> None:
             if interrupted:
                 return
-            interrupted = True
             self.flush_logfiles()
             mlog.warning('Received SIGTERM, exiting')
-            while running_tests:
-                cancel_one_test(False)
+            cancel_all_tests()
 
         def sigint_handler() -> None:
             # We always pick the longest-running future that has not been cancelled
@@ -1593,7 +1600,12 @@ class TestHarness:
             nonlocal interrupted
             if interrupted:
                 return
-            if running_tests:
+            ctrlc_times.append(asyncio.get_event_loop().time())
+            if len(ctrlc_times) == MAX_CTRLC and ctrlc_times[-1] - ctrlc_times[0] < 1:
+                self.flush_logfiles()
+                mlog.warning('CTRL-C detected, exiting')
+                cancel_all_tests()
+            elif running_tests:
                 cancel_one_test(True)
             else:
                 self.flush_logfiles()