fix(scrapers): login fast paths and re-auth hardening for httpx platforms (#357)

## Problem

On CSES, Kattis, and USACO, `:CP <platform> login` always prompted
for credentials and ran a full web login even when a valid session was
already cached. Submit also had weak stale-session detection.

## Solution

`credentials.lua` now tries cached credentials first before prompting,
delegating fast-path detection to each scraper. CSES `login()` checks
the cached API token and returns immediately if valid. USACO `login()`
and `submit()` call `_check_usaco_login()` upfront. Kattis `submit()`
emits `checking_login` consistently and also triggers re-auth on HTTP
400/403, not just on the `"Request validation failed"` text match.
The premature `Submitting...` log emitted by Lua before the scraper
started is removed — Python's own status events are sufficient.

lua/cp/credentials.lua

@@ -11,19 +11,9 @@ local STATUS_MESSAGES = {
   installing_browser = 'Installing browser...',
 }
 
----@param platform string?
+---@param platform string
-function M.login(platform)
+---@param display string
-  platform = platform or state.get_platform()
+local function prompt_and_login(platform, display)
-  if not platform then
-    logger.log(
-      'No platform specified. Usage: :CP <platform> login',
-      { level = vim.log.levels.ERROR }
-    )
-    return
-  end
-
-  local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
-
   vim.ui.input({ prompt = display .. ' username: ' }, function(username)
     if not username or username == '' then
       logger.log('Cancelled', { level = vim.log.levels.WARN })
@@ -37,15 +27,7 @@ function M.login(platform)
       return
     end
 
-    cache.load()
+    local credentials = { username = username, password = password }
-    local existing = cache.get_credentials(platform) or {}
-    local credentials = {
-      username = username,
-      password = password,
-    }
-    if existing.token then
-      credentials.token = existing.token
-    end
 
     local scraper = require('cp.scraper')
     scraper.login(platform, credentials, function(ev)
@@ -69,6 +51,47 @@ function M.login(platform)
   end)
 end
 
+---@param platform string?
+function M.login(platform)
+  platform = platform or state.get_platform()
+  if not platform then
+    logger.log(
+      'No platform specified. Usage: :CP <platform> login',
+      { level = vim.log.levels.ERROR }
+    )
+    return
+  end
+
+  local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
+
+  cache.load()
+  local existing = cache.get_credentials(platform) or {}
+
+  if existing.username and existing.password then
+    local scraper = require('cp.scraper')
+    scraper.login(platform, existing, function(ev)
+      vim.schedule(function()
+        local msg = STATUS_MESSAGES[ev.status] or ev.status
+        logger.log(display .. ': ' .. msg, { level = vim.log.levels.INFO, override = true })
+      end)
+    end, function(result)
+      vim.schedule(function()
+        if result.success then
+          logger.log(
+            display .. ' login successful',
+            { level = vim.log.levels.INFO, override = true }
+          )
+        else
+          prompt_and_login(platform, display)
+        end
+      end)
+    end)
+    return
+  end
+
+  prompt_and_login(platform, display)
+end
+
 ---@param platform string?
 function M.logout(platform)
   platform = platform or state.get_platform()

lua/cp/submit.lua

@@ -79,7 +79,6 @@ function M.submit(opts)
 
   prompt_credentials(platform, function(creds)
     vim.cmd.update()
-    logger.log('Submitting...', { level = vim.log.levels.INFO, override = true })
 
     require('cp.scraper').submit(
       platform,

scrapers/cses.py

@@ -248,7 +248,21 @@ class CSESScraper(BaseScraper):
         if not username or not password:
             return self._login_error("Missing username or password")
 
+        token = credentials.get("token")
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            if token:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if await self._check_token(client, token):
+                    return LoginResult(
+                        success=True,
+                        error="",
+                        credentials={
+                            "username": username,
+                            "password": password,
+                            "token": token,
+                        },
+                    )
+
             print(json.dumps({"status": "logging_in"}), flush=True)
             token = await self._web_login(client, username, password)
             if not token:
@@ -460,7 +474,8 @@ class CSESScraper(BaseScraper):
 
             if r.status_code not in range(200, 300):
                 try:
-                    err = r.json().get("message", r.text)
+                    body = r.json()
+                    err = body.get("error") or body.get("message") or r.text
                 except Exception:
                     err = r.text
                 return self._submit_error(f"Submit request failed: {err}")

scrapers/kattis.py

@@ -329,6 +329,7 @@ class KattisScraper(BaseScraper):
             return self._submit_error("Missing credentials. Use :CP kattis login")
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            print(json.dumps({"status": "checking_login"}), flush=True)
             await _load_kattis_cookies(client)
             if not client.cookies:
                 print(json.dumps({"status": "logging_in"}), flush=True)
@@ -366,7 +367,7 @@ class KattisScraper(BaseScraper):
             except Exception as e:
                 return self._submit_error(f"Submit request failed: {e}")
 
-            if r.text == "Request validation failed":
+            if r.status_code in (400, 403) or r.text == "Request validation failed":
                 _COOKIE_PATH.unlink(missing_ok=True)
                 print(json.dumps({"status": "logging_in"}), flush=True)
                 ok = await _do_kattis_login(client, username, password)

scrapers/usaco.py

@@ -429,7 +429,19 @@ class USACOScraper(BaseScraper):
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
             await _load_usaco_cookies(client)
-            if not client.cookies:
+            if client.cookies:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if not await _check_usaco_login(client, username):
+                    client.cookies.clear()
+                    print(json.dumps({"status": "logging_in"}), flush=True)
+                    try:
+                        ok = await _do_usaco_login(client, username, password)
+                    except Exception as e:
+                        return self._submit_error(f"Login failed: {e}")
+                    if not ok:
+                        return self._submit_error("Login failed (bad credentials?)")
+                    await _save_usaco_cookies(client)
+            else:
                 print(json.dumps({"status": "logging_in"}), flush=True)
                 try:
                     ok = await _do_usaco_login(client, username, password)
@@ -470,7 +482,8 @@ class USACOScraper(BaseScraper):
                 headers=HEADERS,
                 timeout=HTTP_TIMEOUT,
             )
-            if "login" in page_r.url.path.lower() or "Login" in page_r.text[:2000]:
+            page_url = str(page_r.url)
+            if "/login" in page_url or "Login" in page_r.text[:2000]:
                 return self._submit_error("auth_failure")
             form_url, hidden_fields, lang_val = _parse_submit_form(
                 page_r.text, language_id
@@ -513,6 +526,16 @@ class USACOScraper(BaseScraper):
             return self._login_error("Missing username or password")
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            await _load_usaco_cookies(client)
+            if client.cookies:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if await _check_usaco_login(client, username):
+                    return LoginResult(
+                        success=True,
+                        error="",
+                        credentials={"username": username, "password": password},
+                    )
+
             print(json.dumps({"status": "logging_in"}), flush=True)
             try:
                 ok = await _do_usaco_login(client, username, password)

t/minimal_init.lua

@@ -1,21 +0,0 @@
-vim.opt.runtimepath:prepend(vim.fn.expand('~/dev/cp.nvim'))
-vim.opt.runtimepath:prepend(vim.fn.expand('~/dev/fzf-lua'))
-
-vim.g.cp = {
-  languages = {
-    cpp = {
-      extension = 'cc',
-      commands = {
-        build = { 'g++', '-std=c++23', '-O2', '{source}', '-o', '{binary}' },
-        run = { '{binary}' },
-      },
-    },
-  },
-  platforms = {
-    codechef = {
-      enabled_languages = { 'cpp' },
-      default_language = 'cpp',
-    },
-  },
-  ui = { picker = 'fzf-lua' },
-}