ci: master only runs coverage and only with go1.17 (+ fix flaky test) (#452)

* fix: disable debianrepo build on master branch

This just mitigates https://github.com/ooni/probe/issues/1741 and does
not fully address it, but I'd rather avoid delving into this problem until
I open a release/v3.11.0 branch and have to really fix this issue.

* fix: only run coverage using go1.17

This is the version of Go with which we are going to bless v3.11.0
therefore it's the only version of Go that matters.

Reference issue: https://github.com/ooni/probe/issues/1738.

* fix(ptx/obfs4_test.go): avoid context-vs-normal-code race

We want to test whether we get the context failure if the error
generated inside normal code happens _after_ the context cancellation.

The best way to do that is to write code that is not racy. To this
end, we just need to pause normal code until we know that the context
has returned to the caller. We also need to ensure we do not leak
a goroutine, hence we use a WaitGroup to check that.

Fixes https://github.com/ooni/probe/issues/1750

.github/workflows/coverage.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        go: [ "1.16", "1.17" ]
+        go: [ "1.17" ]
     steps:
       - uses: actions/setup-go@v1
         with:

.github/workflows/debianrepo.yml

@@ -3,8 +3,7 @@ name: linux
 on:
   push:
     branches:
-      - "master"
-      - "issue/1484"
+      - "release/**"
 
 jobs:
   test_386:

internal/ptx/obfs4_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/ooni/probe-cli/v3/internal/atomicx"
@@ -65,14 +66,17 @@ func TestOBFS4DialerFailsWithConnectionErrorAndNoContextExpiration(t *testing.T)
 func TestOBFS4DialerFailsWithConnectionErrorAndContextExpiration(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	expected := errors.New("mocked error")
+	unexpected := errors.New("mocked error")
 	o4d := DefaultTestingOBFS4Bridge()
+	sigch := make(chan interface{})
+	wg := &sync.WaitGroup{}
+	wg.Add(1)
 	o4d.UnderlyingDialer = &netxmocks.Dialer{
 		MockDialContext: func(ctx context.Context, network string, address string) (net.Conn, error) {
-			// We cancel the context before returning the error, which makes
-			// the context cancellation happen before us returning.
 			cancel()
-			return nil, expected
+			<-sigch
+			wg.Done()
+			return nil, unexpected
 		},
 	}
 	conn, err := o4d.DialContext(ctx)
@@ -82,6 +86,8 @@ func TestOBFS4DialerFailsWithConnectionErrorAndContextExpiration(t *testing.T) {
 	if conn != nil {
 		t.Fatal("expected nil conn here")
 	}
+	close(sigch)
+	wg.Wait()
 }
 
 // obfs4connwrapper allows us to observe that Close has been called