Distinguish between permanent and transient errors

This patch makes the queue and couriers distinguish between permanent and
transient errors when delivering mail to individual recipients.

Pipe delivery errors are always permanent.

Procmail delivery errors are almost always permanent, except if the command
exited with code 75, which is an indication of transient.

SMTP delivery errors are almost always transient, except if the DNS resolution
for the domain failed.

internal/courier/procmail_test.go

@@ -21,7 +21,7 @@ func TestProcmail(t *testing.T) {
 		Timeout: 1 * time.Minute,
 	}
 
-	err = p.Deliver("from@x", "to@local", []byte("data"))
+	err, _ = p.Deliver("from@x", "to@local", []byte("data"))
 	if err != nil {
 		t.Fatalf("Deliver: %v", err)
 	}
@@ -35,28 +35,59 @@ func TestProcmail(t *testing.T) {
 func TestProcmailTimeout(t *testing.T) {
 	p := Procmail{"/bin/sleep", []string{"1"}, 100 * time.Millisecond}
 
-	err := p.Deliver("from", "to@local", []byte("data"))
+	err, permanent := p.Deliver("from", "to@local", []byte("data"))
 	if err != errTimeout {
 		t.Errorf("Unexpected error: %v", err)
 	}
+	if permanent {
+		t.Errorf("expected transient, got permanent")
+	}
 }
 
 func TestProcmailBadCommandLine(t *testing.T) {
 	// Non-existent binary.
 	p := Procmail{"thisdoesnotexist", nil, 1 * time.Minute}
-	err := p.Deliver("from", "to", []byte("data"))
+	err, permanent := p.Deliver("from", "to", []byte("data"))
 	if err == nil {
 		t.Errorf("unexpected success for non-existent binary")
 	}
+	if !permanent {
+		t.Errorf("expected permanent, got transient")
+	}
 
 	// Incorrect arguments.
 	p = Procmail{"cat", []string{"--fail_unknown_option"}, 1 * time.Minute}
-	err = p.Deliver("from", "to", []byte("data"))
+	err, _ = p.Deliver("from", "to", []byte("data"))
 	if err == nil {
 		t.Errorf("unexpected success for incorrect arguments")
 	}
 }
 
+// Test that local delivery failures are considered permanent or not
+// according to the exit code.
+func TestExitCode(t *testing.T) {
+	cases := []struct {
+		cmd             string
+		args            []string
+		expectPermanent bool
+	}{
+		{"does/not/exist", nil, true},
+		{"../../test/util/exitcode", []string{"1"}, true},
+		{"../../test/util/exitcode", []string{"75"}, false},
+	}
+	for _, c := range cases {
+		p := &Procmail{c.cmd, c.args, 5 * time.Second}
+		err, permanent := p.Deliver("from", "to", []byte("data"))
+		if err == nil {
+			t.Errorf("%q: pipe delivery worked, expected failure", c.cmd)
+		}
+		if c.expectPermanent != permanent {
+			t.Errorf("%q: permanent expected=%v, got=%v",
+				c.cmd, c.expectPermanent, permanent)
+		}
+	}
+}
+
 func TestSanitize(t *testing.T) {
 	cases := []struct{ v, expected string }{
 		// These are the same.