cmd/go: terminate TestScript commands more aggressively when the test times out

- Avoid starting subprocesses when the test is already very close to
  timing out. The overhead of starting and stopping processes may
  cause the test to exceed its deadline even if each individual
  process is signaled soon after it is started.

- If a command does not shut down quickly enough after receiving
  os.Interrupt, send it os.Kill using the same style of grace period
  as in CL 228438.

- Fail the test if a background command whose exit status is not
  ignored is left running at the end of the test. We have no reliable
  way to distinguish a failure due to the termination signal from an
  unexpected failure, and the termination signal varies across
  platforms (so may cause failure on one platform but success on
  another).

For #38797

Change-Id: I767898cf551dca45579bf01a9d1bb312e12d6193
Reviewed-on: https://go-review.googlesource.com/c/go/+/233526
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/cmd/go/testdata/script/README b/src/cmd/go/testdata/script/README
index e22ddcaf2e..c7fa7cfef5 100644
--- a/src/cmd/go/testdata/script/README
+++ b/src/cmd/go/testdata/script/README
@@ -138,8 +138,9 @@ The commands are:
   output and standard error of the previous command is cleared, but the output
   of the background process is buffered — and checking of its exit status is
   delayed — until the next call to 'wait', 'skip', or 'stop' or the end of the
-  test. At the end of the test, any remaining background processes are
-  terminated using os.Interrupt (if supported) or os.Kill.
+  test. If any background processes remain at the end of the test, they
+  are terminated using os.Interrupt (if supported) or os.Kill and the test
+  must not depend upon their exit status.
 
 - [!] exists [-readonly] [-exec] file...
   Each of the listed files or directories must (or must not) exist.