bugfix: server_rewrite_by_lua* used the global VM when lua_code_cache off

[xiaobiaozhao]

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.

Problem

When lua_code_cache off; is configured, server_rewrite_by_lua*
(server_rewrite_by_lua_block / server_rewrite_by_lua_file) does not
work correctly. If the handler touches ngx.ctx or yields (e.g.
ngx.sleep, cosockets), the request is aborted with a 500 error:

lua entry thread aborted: runtime error: content_by_lua(nginx.conf:44):2:
attempt to index field 'ctx' (a nil value)

Even without yielding, the per-request code cache is not honored: edited
Lua files are not reloaded across requests as lua_code_cache off
requires.

Environment

• nginx core: 1.31.6
• ngx_lua: master (ngx_http_lua_version = 10032)
• Built with --with-debug -DNGX_LUA_USE_ASSERT, OpenResty LuaJIT

Root cause

ngx_http_lua_server_rewrite_handler() fetched the Lua VM before the
request context was created:

lscf = ngx_http_get_module_srv_conf(r, ngx_http_lua_module);
llcf = ngx_http_get_module_loc_conf(r, ngx_http_lua_module);
L = ngx_http_lua_get_lua_vm(r, NULL);   /* too early */
...
if (ctx == NULL) {
    ctx = ngx_http_lua_create_ctx(r);   /* builds the per-request VM */
}

With lua_code_cache off, ngx_http_lua_create_ctx() creates a fresh
per-request Lua VM stored in ctx->vm_state. Because the VM was fetched
before create_ctx(), ngx_http_lua_get_lua_vm(r, NULL) fell back to the
persistent global VM (lmcf->lua). Consequences:

1. Code cache reset had no effect on the VM actually used, defeating
   lua_code_cache off for this phase.
2. ngx_http_lua_new_thread() saw L == lmcf->lua and reused cached
   global coroutine threads instead of per-request ones.
3. On yield/resume the entry thread and the resume path ended up on
   mismatched VMs, so ngx.ctx (stored per-request) was nil in the
   content phase, aborting the request as shown above.

The sibling rewrite phase handler (ngx_http_lua_rewriteby.c) does not
have this problem: it fetches the VM inside the inline/file handlers,
i.e. after create_ctx() has run.

Fix

• Fetch the VM after create_ctx() and pass the ctx explicitly:
  L = ngx_http_lua_get_lua_vm(r, ctx);
• Reset lscf->srv.server_rewrite_src_ref = LUA_REFNIL when
  !llcf->enable_code_cache, in both the inline and file handlers, so
  chunks are recompiled per request (mirrors rewrite_src_ref handling).

Minimal reproduction

lua_code_cache off;

server_rewrite_by_lua_block {
    ngx.sleep(0.001)
    ngx.ctx.a = "resumed"
}

location = /t {
    content_by_lua_block { ngx.say(ngx.ctx.a) }
}