Supporting variables in "location".

............
Description:
............

Before this commit, the encoded URI could be calculated at
configuration time.  Now, since variables can only be resolved at
request time, we have different situations:

- "location" contains no variables:

  In this case, we still encode the URI in the conf structure, at
  configuration time, and then we just copy the resulting string
  to the ctx structure at request time.

- "location" contains variables:

  In this case, we compile the var string at configure time, then
  when we resolve it at request time, and then we encode the
  string.

In both cases, as was being done before, if the string is empty,
either before or after resolving variables, we skip the encoding.

...........
Usefulness:
...........

An example of why this feature may be useful is redirecting HTTP
to HTTPS with something like:

"action": {
    "return": 301,
    "location": "https://${host}${uri}"
}

.....
Bugs:
.....

This feature conflicts with the relevant RFCs in the following:

'$' is used for Unit variables, but '$' is a reserved character in
a URI, to be used as a sub-delimiter.  However, it's almost never
used as that, and in fact, other parts of Unit already conflict
with '$' being a reserved character for use as a sub-delimiter, so
this is at least consistent in that sense.  VBart suggested an
easy workaround if we ever need it: adding a variable '$sign'
which resolves to a literal '$'.

......
Notes:
......

An empty string is handled as if "location" wasn't specified at
all, so no Location header is sent.

This is incorrect, and the code is slightly misleading.

The Location header consists of a URI-reference[1], which might be
a relative one, which itself might consist of an empty string[2].

[1]: <https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2>
[2]: <https://stackoverflow.com/a/43338457>

Now that we have variables, it's more likely that an empty
Location header will be requested, and we should handle it
correctly.

I think in a future commit we should modify the code to allow
differentiating between an unset "location" and an empty one,
which should be treated as any other "location" string.

.................
Testing (manual):
.................

{
  "listeners": {
    "*:80": {
      "pass": "routes/str"
    },
    "*:81": {
      "pass": "routes/empty"
    },
    "*:82": {
      "pass": "routes/var"
    },
    "*:83": {
      "pass": "routes/enc-str"
    },
    "*:84": {
      "pass": "routes/enc-var"
    }
  },
  "routes": {
    "str": [
      {
        "action": {
          "return": 301,
          "location": "foo"
        }
      }
    ],
    "empty": [
      {
        "action": {
          "return": 301,
          "location": ""
        }
      }
    ],
    "var": [
      {
        "action": {
          "return": 301,
          "location": "$host"
        }
      }
    ],
    "enc-str": [
      {
        "action": {
          "return": 301,
          "location": "f%23o#o"
        }
      }
    ],
    "enc-var": [
      {
        "action": {
          "return": 301,
          "location": "f%23o${host}#o"
        }
      }
    ]
  }
}

$ curl --dump-header - localhost:80
HTTP/1.1 301 Moved Permanently
Location: foo
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:06 GMT
Content-Length: 0

$ curl --dump-header - localhost:81
HTTP/1.1 301 Moved Permanently
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:08 GMT
Content-Length: 0

$ curl --dump-header - localhost:82
HTTP/1.1 301 Moved Permanently
Location: localhost
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:15 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: bar" localhost:82
HTTP/1.1 301 Moved Permanently
Location: bar
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:23 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: " localhost:82
HTTP/1.1 301 Moved Permanently
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:29 GMT
Content-Length: 0

$ curl --dump-header - localhost:83
HTTP/1.1 301 Moved Permanently
Location: f%23o#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:23 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: " localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%23o#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:44 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: alx" localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%23oalx#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:52 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: a#l%23x" localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%2523oa#l%2523x%23o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:23:09 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: b##ar" localhost:82
HTTP/1.1 301 Moved Permanently
Location: b#%23ar
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:25:01 GMT
Content-Length: 0

1 files changed, 1 insertions, 0 deletions

diff --git a/src/nxt_conf_validation.c b/src/nxt_conf_validation.c
index 876c5bbc..1b97bd0a 100644
--- a/src/nxt_conf_validation.c
+++ b/src/nxt_conf_validation.c
@@ -635,6 +635,7 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_return_action_members[] = {
     }, {
         .name       = nxt_string("location"),
         .type       = NXT_CONF_VLDT_STRING,
+        .flags      = NXT_CONF_VLDT_VAR,
     },
 
     NXT_CONF_VLDT_END