Add missing bits of VRT_fail() handling during vcl_init {} / vcl_fini{}

This change is based on the precondition that VRT_fail() should work
wherever we have a VRT_CTX and DTRT, see also previous.

on vcl_init {}
--------------

We already handled a "direct" VRT_fail() via a vmod function
correctly.  This commit adds a test.

Yet we can also VRT_fail() via a PRIV_TASK fini callback and did not
handle that case (triggered an assertion in VRT_fail()). Notice that
this case has worked in client / backend tasks for long and a test
case has been added with 746384b2 . It
has now gained relevance with 43d9e5fb

on vcl_fini {}
--------------

By design, vcl_fini {} must not fail, which is also why the only valid
return() value is "ok" (VCL_RET_OK).

Consequently, we had

	if (method && *ctx->handling && *ctx->handling != VCL_RET_OK) {
		assert(ev == VCL_EVENT_LOAD);

in vcl_send_event().

And then came VRT_fail().

If we wanted to sustain an assertion similar to the above, we would
need to require vmod authors to never call VRT_fail() from
VCL_MET_FINI - which would complicate code, be likely overlooked and
receive little to no test coverage.

So instead we now ignore errors during vcl_fini {} and emit a
VCL_error log line.

I considered the alternative to check for VCL_MET_FINI in VRT_fail(),
but decided to handle the exception where it applies, which is in
vcl_send_event().

I am aware that this part of the change may trigger some controversy
and I am open to discussing alternatives. If anything, we now avoid
unmotivated assertion failures triggered for the new tests in
v00051.vtc for the time being.

bin/varnishd/cache/cache_vcl.c

@@ -172,6 +172,39 @@ VCL_Rel_CliCtx(struct vrt_ctx **ctx)
 
 /*--------------------------------------------------------------------*/
 
+/* VRT_fail() can be called
+ * - from the vcl sub via a vmod
+ * - via a PRIV_TASK .fini callback
+ *
+ * if this happens during init, we fail it
+ * if during fini, we ignore, because otherwise VMOD authors would be forced to
+ * handle VCL_MET_FINI specifically everywhere.
+ */
+
+static int
+vcl_event_handling(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+
+	if (*ctx->handling == 0)
+		return (0);
+
+	assert(*ctx->handling == VCL_RET_FAIL);
+
+	if (ctx->method == VCL_MET_INIT)
+		return (1);
+
+	/*
+	 * EVENT_WARM / EVENT_COLD: method == 0
+	 * must not set handling
+	 */
+	assert(ctx->method == VCL_MET_FINI);
+
+	*ctx->handling = 0;
+	VRT_fail(ctx, "VRT_fail() from vcl_fini{} has no effect");
+	return (0);
+}
+
 static int
 vcl_send_event(struct vcl *vcl, enum vcl_event_e ev, struct vsb **msg)
 {
@@ -216,13 +249,7 @@ vcl_send_event(struct vcl *vcl, enum vcl_event_e ev, struct vsb **msg)
 	VCL_TaskEnter(cli_task_privs);
 	r = ctx->vcl->conf->event_vcl(ctx, ev);
 	VCL_TaskLeave(ctx, cli_task_privs);
-
-	/* if the warm event did not get to vcl_init, vcl_fini
-	 * won't be run, so handling may be zero */
-	if (method && *ctx->handling && *ctx->handling != VCL_RET_OK) {
-		assert(ev == VCL_EVENT_LOAD);
-		r = 1;
-	}
+	r |= vcl_event_handling(ctx);
 
 	*msg = VCL_Rel_CliCtx(&ctx);
 

bin/varnishtest/tests/v00051.vtc

@@ -488,3 +488,71 @@ varnish v1 -expect vcl_fail == 14
 varnish v1 -expect sc_vcl_failure == 11
 
 logexpect l1 -wait
+
+#######################################################################
+# Fail in vcl_init with direct VRT_fail()
+
+varnish v1 -errvcl {Forced failure} {
+	import debug;
+	backend proforma none;
+	sub vcl_init {
+		debug.fail();
+	}
+}
+
+#######################################################################
+# Fail in vcl_init via fini callback
+
+varnish v1 -errvcl {thou shalt not fini} {
+	import debug;
+	backend proforma none;
+	sub vcl_init {
+		debug.fail_task_fini();
+	}
+}
+
+#######################################################################
+# Fail in vcl_fini with direct VRT_fail()
+
+varnish v1 -vcl {
+	import debug;
+	backend proforma none;
+	sub vcl_fini {
+		debug.fail();
+	}
+}
+
+varnish v1 -vcl+backend {
+}
+
+logexpect l1 -v v1 -g raw {
+	expect * 0	CLI		"^Rd vcl.state vcl16 0cold"
+	expect 3 0	VCL_Error	"^Forced failure"
+	expect 0 0	VCL_Error	{^\QVRT_fail() from vcl_fini{} has no effect\E$}
+} -start
+
+varnish v1 -cliok "vcl.discard vcl16"
+logexpect l1 -wait
+
+#######################################################################
+# Fail in vcl_fini via fini callback - ignored but logged as VMOD BUG
+
+varnish v1 -vcl {
+	import debug;
+	backend proforma none;
+	sub vcl_fini {
+		debug.fail_task_fini();
+	}
+}
+
+varnish v1 -vcl+backend {
+}
+
+logexpect l1 -v v1 -g raw {
+	expect * 0	CLI		"^Rd vcl.state vcl18 0cold"
+	expect 3 0	VCL_Error	"^thou shalt not fini"
+	expect 0 0	VCL_Error	{^\QVRT_fail() from vcl_fini{} has no effect\E$}
+} -start
+
+varnish v1 -cliok "vcl.discard vcl18"
+logexpect l1 -wait