POC: New beresp.error VCL variable

[dridi]

Following several VDD discussions, this patch series shows how getting a backend error could look like in VCL, and how it could be articulated in the core code.

This should check the boxes from the last consensus:

• a plain text error message
• a new VCL variable to get the error (beresp.error)
• VCL constants for safe comparisons (error.*)

The first two commits are unrelated, I found a bug in libvcc while I was working on this. I will submit a pull request once I'm done with that bug (only partly fixed here) so the relevant commits to review are the ones starting with "POC".

If we still have consensus on this approach, I will find someone™ to submit something comprehensive.

[dridi]

Bugwash summary:

• Turn errors into a struct vrt_error { const char *type, *msg };
  • @nigoroll wanted to categorize errors (example: "backend timeout")
  • I suggested an (ERROR).type VCL attribute
  • It coud also be a list of tags (not brought up during bugwash)
• Turn beresp.error into a proper ERROR
• Teach $Error to vmods (add vmod_debug coverage)
• Error messages from the documentation should be logged (I meant to initially and forgot)

Then we reevaluate the error reporting.

[dridi]

I wanted to revisit this ticket this morning and noticed that I forgot about the libvcc bug. I pushed 0c8448e...b77b709 to fix it.

[nigoroll]

bugwash label removed because this ticket needs an update

[dridi]

New proposal:

struct vrt_error {
        const char *msg;
        const char *tags; /* comma-separated list of VCL identifiers */
};

Can be produced by a VMOD and put in context, to be consumed from vcl_backend_error (and probably vcl_synth too). The error can be null, like a header.

VCL defines new variables:

bereq.error

	Type: ERROR

	Readable from: vcl_backend_error

	TODO: description (has a tostring yielding `msg` or `NULL`).

bereq.error.*

	Type: BOOL

	Readable from: vcl_backend_error

	TODO: description, tells whether the error has an arbitrary tag.

We gradually start defining ERROR constants in VCL, as we increase our bereq.error coverage for at least faults happening in core code:

error.connection_refused

	Type: ERROR

	Error message: Connection refused by the backend

	Tags: backend, connect

	TODO: description (maybe mention ECONNREFUSED etc)

The error.* symbols are implicitly read-only and unrestricted in scope, in other words global constants.

In VCL you could do something like this:

sub vcl_backend_error {
        if (!bereq.error.backend || bereq.error == error.connection_refused) {
                return (retry);
        }
}

Likewise, VMODs learn to define errors in their VCC file:

$Error name, "msg", tag, tags... # XXX: implicit tag with the name of the VMOD?

And you can use them also:

import foo;

sub vcl_backend_error {
        if (bereq.error.foo && bereq.error != foo.bar) {
                return (abandon);
        }
}

edit: changed all beresp references to bereq to bind the error to the task (and only [be]req is available for the whole task, so it could be extended beyond backend_error/synth).