HTTP: added basic URI rewrite.

This commit introduced the basic URI rewrite. It allows users to change request URI. Note the "rewrite" option ignores the contained query if any and the query from the request is preserverd.
An example:
"routes": [
    {
        "match": {
            "uri": "/v1/test"
        },
        "action": {
            "return": 200
        }
    },
    {
        "action": {
            "rewrite": "/v1$uri",
            "pass": "routes"
        }
    }
]

Reviewed-by: Alejandro Colomar <alx@nginx.com>

src/nxt_http_rewrite.c

@@ -0,0 +1,104 @@
+
+/*
+ * Copyright (C) Zhidao HONG
+ * Copyright (C) NGINX, Inc.
+ */
+
+#include <nxt_router.h>
+#include <nxt_http.h>
+
+
+nxt_int_t
+nxt_http_rewrite_init(nxt_router_conf_t *rtcf, nxt_http_action_t *action,
+     nxt_http_action_conf_t *acf)
+ {
+    nxt_str_t  str;
+
+    nxt_conf_get_string(acf->rewrite, &str);
+
+    action->rewrite = nxt_tstr_compile(rtcf->tstr_state, &str, 0);
+    if (nxt_slow_path(action->rewrite == NULL)) {
+        return NXT_ERROR;
+    }
+
+    return NXT_OK;
+}
+
+
+nxt_int_t
+nxt_http_rewrite(nxt_task_t *task, nxt_http_request_t *r,
+    nxt_http_action_t *action)
+{
+    u_char                    *p;
+    nxt_int_t                 ret;
+    nxt_str_t                 str, encoded_path, target;
+    nxt_router_conf_t         *rtcf;
+    nxt_http_request_parse_t  rp;
+
+    if (nxt_tstr_is_const(action->rewrite)) {
+        nxt_tstr_str(action->rewrite, &str);
+
+    } else {
+        rtcf = r->conf->socket_conf->router_conf;
+
+        ret = nxt_tstr_query_init(&r->tstr_query, rtcf->tstr_state,
+                                  &r->tstr_cache, r, r->mem_pool);
+        if (nxt_slow_path(ret != NXT_OK)) {
+            return NXT_ERROR;
+        }
+
+        nxt_tstr_query(task, r->tstr_query, action->rewrite, &str);
+
+        if (nxt_slow_path(nxt_tstr_query_failed(r->tstr_query))) {
+            return NXT_ERROR;
+        }
+    }
+
+    nxt_memzero(&rp, sizeof(nxt_http_request_parse_t));
+
+    rp.mem_pool = r->mem_pool;
+
+    rp.target_start = str.start;
+    rp.target_end = str.start + str.length;
+
+    ret = nxt_http_parse_complex_target(&rp);
+    if (nxt_slow_path(ret != NXT_OK)) {
+        return NXT_ERROR;
+    }
+
+    p = (rp.args.length > 0) ? rp.args.start - 1 : rp.target_end;
+
+    encoded_path.start = rp.target_start;
+    encoded_path.length = p - encoded_path.start;
+
+    if (r->args->length == 0) {
+        r->target = encoded_path;
+
+    } else {
+        target.length = encoded_path.length + 1 + r->args->length;
+
+        target.start = nxt_mp_alloc(r->mem_pool, target.length);
+        if (target.start == NULL) {
+            return NXT_ERROR;
+        }
+
+        p = nxt_cpymem(target.start, encoded_path.start, encoded_path.length);
+        *p++ = '?';
+        nxt_memcpy(p, r->args->start, r->args->length);
+
+        r->target = target;
+    }
+
+    r->path = nxt_mp_alloc(r->mem_pool, sizeof(nxt_str_t));
+    if (nxt_slow_path(r->path == NULL)) {
+        return NXT_ERROR;
+    }
+
+    *r->path = rp.path;
+
+    if (nxt_slow_path(r->log_route)) {
+        nxt_log(task, NXT_LOG_NOTICE, "URI rewritten to \"%V\"", &r->target);
+    }
+
+    return NXT_OK;
+}