Align the lifetime of the PID file to the VUT process

For out-of-tree code that may set up multiple VUTs in a single process,
we don't want the VUT that initialized a PID file to remove it if it
finishes before other VUTs.

A global pseudo-VUT is introduced to keep track of the PID file and
the error callback in order to use them in the atexit(3) callback.

Original test case by Geoff, slightly modified.

Closes #2651

bin/varnishtest/tests/r02646.vtc

+varnishtest "#2646: VUT should fail gracefully when removing a pid file fails"
+
+varnish v1 -vcl {
+	backend be {
+		.host = "${bad_backend}";
+	}
+} -start
+
+process p1 {
+	exec varnishncsa -n ${v1_name} -P ${tmpdir}/ncsa.pid -w ${tmpdir}/ncsa.log
+} -start
+
+delay 1
+
+shell "rm -f ${tmpdir}/ncsa.pid"
+
+process p1 -expect-exit 1 -stop -wait
+
+shell -expect "Cannot remove pid file ${tmpdir}/ncsa.pid" {
+	cat ${tmpdir}/p1/stderr
+}

lib/libvarnishapi/vut.c

@@ -62,6 +62,8 @@ static int vut_options(const struct vopt_spec *);
 static struct vpf_fh	*pfh;
 static unsigned		daemonized;
 
+static struct VUT pfh_vut;
+
 static int
 vut_daemon(struct VUT *vut)
 {
@@ -74,10 +76,18 @@ vut_daemon(struct VUT *vut)
 static void
 vut_vpf_remove(void)
 {
-	if (pfh != NULL) {
-		AZ(VPF_Remove(pfh));
-		pfh = NULL;
-	}
+
+	assert(VALID_OBJ(&pfh_vut, VUT_MAGIC));
+	AN(pfh);
+	AN(pfh_vut.P_arg);
+
+	if (VPF_Remove(pfh) != 0)
+		VUT_Error(&pfh_vut, 1, "Cannot remove pid file %s: %s",
+		    pfh_vut.P_arg, strerror(errno));
+
+	free(pfh_vut.P_arg);
+	ZERO_OBJ(&pfh_vut, sizeof pfh_vut);
+	pfh = NULL;
 }
 
 static int v_matchproto_(VSLQ_dispatch_f)
@@ -306,6 +316,13 @@ VUT_Setup(struct VUT *vut)
 	if (vut->P_arg) {
 		AN(pfh);
 		AZ(VPF_Write(pfh));
+
+		/* NB: move ownership to a global pseudo-VUT. */
+		INIT_OBJ(&pfh_vut, VUT_MAGIC);
+		pfh_vut.P_arg = vut->P_arg;
+		pfh_vut.error_f = vut->error_f;
+		vut->P_arg = NULL;
+
 		AZ(atexit(vut_vpf_remove));
 	}
 }
@@ -319,13 +336,10 @@ VUT_Fini(struct VUT **vutp)
 	AN(vut->progname);
 
 	free(vut->n_arg);
-	free(vut->P_arg);
 	free(vut->q_arg);
 	free(vut->r_arg);
 	free(vut->t_arg);
-
-	vut_vpf_remove();
-	AZ(pfh);
+	AZ(vut->P_arg);
 
 	if (vut->vslq)
 		VSLQ_Delete(&vut->vslq);