suggested by phk

7ec28f1a did not work with object
methods because we need a partial symbol table lookup to identify them,
otherwise the symbol kind we find is just SUB for the method's return
value.

Additions to the test case and vmod_debug by @slimhazard, thank you

Fixes #3521

For the next commit, we need a partial lookup without error.

Alternatively, we could also a an error mode.

Diff best viewed with -b option

- avoid the second call to VCC_SymbolGet(), unless necessary
- reorder from simple to complex

Ref 7ec28f1a

    sub vcl_synth {
        if (resp.status == 12404) {
            [...]       // this specific 404
        } else if (resp.status % 1000 == 404) {
            [...]       // all other 404's
        }
    }

that is,

	call vmod.returning_sub();

in VCL.

Background:

The existing

	call sub;

is special in that it instantiates "sub", if it does not exist
already, enabling use-before-definition semantics.

The doctrine of this change is to preserve this behaviour and to not
make existing, static calls any less efficient.

Implementation:

To support calling both literal SUBs as well as SUB expressions, we
peek into the symbol table to check if the called expression is a
function, based on the knowledge that, as of now, only functions can
have a non-literal SUB return. We also know that no other expression
can yield a SUB. So if peeking ahead tells us that a function follows,
we call into expression evaluation.

Otherwise, we expect a literal SUB as before.

Null check:

For dynamic SUB calls from vmods via VRT_call(), we require the SUB
argument be non-NULL. But we can not for call: To signal error with a
SUB return, a vmod function needs to be allowed to return NULL,
otherwise it would need to have a dummy return value available for the
VRT_fail() case. So we need to check for NULL in VGC.

Alternatives considered:

- We could make expression evaluation return SUB also for literal SUB
symbols, turning all calls into sub->func(ctx, ...) C statements. I
tried this path and it resulted in a change of behaviour with
cc_command="clang -g -O2 ...": Where, previously, sub code was
inlined, it would now generate an explicit C function call always,
despite the fact that the C function is known at compile time (because
all named VCL_SUB structs are).

So this option was dismissed for violating the doctrine.

- The null check could go to VPI, via a VPI_call(), basically identical
to VRT_call(), but checking for NULL. This option was dismissed because
it would foreclose the requirement to allow for SUB arguments in the
future.

Acknowledgements:

reza mentioned the idea of call SUB at one point, and I can not remember
if I had it before or not, so when in doubt, it was his idea.

Dridi helped with ideas on the implementation and spotted a bug.

To control the threshold used to warn.

As a recurring pitfall sometimes hard to identify, it deserves its own
notice. The accounting of variants reuses the worker's strangelove field
since it doesn't conflict where other parts of the code base use it.

and protect this pattern in the test-case.

a comment why it is not necessary.

Only call vcl_hash{} if the initial hash is still intact when we get there.

This lifts internal restrictions on the hashkey production,
but does not change how things work from a VCL perspective.

	expect	resp.http.content-length -le 1024

And use that for logging purposes when a successfully opened h2 session
ends. RX_JUNK is still the default session close reason when existing
reasons aren't accurate enough.

Fixes #3393

In the same fashion as include/tbl/params.h for legibility.

with manual polish for cstyle and un-doing some return ((x)).

Either flexelint or me, probabbly the latter, do not understand the
code.

As far as I am concerned, I would have thought that flexelint could
follow for

	AN(y);
	REPLACE(x, y);

x being guranteed to be non-NULL

This reverts commit 897e5313.
This reverts commit 6f4d0ec1.

to slim down one of my PRs with a trivial addition which, I hope, will
not be controversial.

Missing include for REPLACE() / memset() added manually.