Require an explicit registry instead of defaulting to one

The shipped default named one private, internet-unreachable host, so an
unconfigured install failed against a server it could never reach rather
than saying what to set.

- drop the registry default and make the option required
- reject a non-string, empty, or non-http(s) registry with the option name
  and how to set it
- ask for a registry only when a run has to resolve one, so clean, list,
  and specs with an explicit url keep working without it

lua/arti-pack/init.lua

@@ -5,8 +5,9 @@ local spec_mod = require('arti-pack.spec')
 
 local M = {}
 
+-- registry has no default: it is site-specific, so any shipped value would point
+-- every install at a host it cannot reach. See M.setup.
 local DEFAULTS = {
-  registry = 'https://artifactapi.k8s.syd1.au.unkin.net/api/v1/remote/github',
   plugins = {},
   auto_sync = true,
   concurrency = 4,
@@ -42,6 +43,17 @@ local function context()
   }
 end
 
+-- A spec carrying its own url never touches the registry, so only demand one
+-- when something in this run would have to be resolved against it.
+local function registry_error(specs, registry)
+  for _, spec in ipairs(specs) do
+    if not spec.url then
+      local _, err = spec_mod.check_registry(registry)
+      return err
+    end
+  end
+end
+
 local function installed_dir(ctx, name, opt)
   return fs.join(ctx.pack_dir, opt and 'opt' or 'start', name)
 end
@@ -110,6 +122,11 @@ function M.sync(opts)
     return notify('everything up to date')
   end
 
+  local reg_err = registry_error(pending, ctx.registry)
+  if reg_err then
+    return notify(reg_err, vim.log.levels.ERROR)
+  end
+
   running = true
   notify(string.format('syncing %d plugin(s)', #pending))
   installer.run(ctx, pending, function(results)
@@ -213,6 +230,13 @@ function M.setup(opts)
   -- a user's plugin list; the caller's list is always authoritative.
   M.config.plugins = opts.plugins or {}
 
+  if opts.registry ~= nil then
+    local _, err = spec_mod.check_registry(opts.registry)
+    if err then
+      notify(err, vim.log.levels.ERROR)
+    end
+  end
+
   local p = paths()
   fs.mkdirp(fs.join(p.pack_dir, 'start'))
   fs.mkdirp(fs.join(p.pack_dir, 'opt'))

lua/arti-pack/spec.lua

@@ -2,6 +2,8 @@ local M = {}
 
 local SOURCE_PATTERN = '^([%w%.%_%-]+)/([%w%.%_%-]+)$'
 local EXTENSIONS = { ['zip'] = true, ['tar.gz'] = true }
+local SETUP_HINT =
+  "set it in require('arti-pack').setup{ registry = 'https://artifacts.example.com/artifactory/github-remote' }"
 
 local function trim(s)
   return (s:gsub('^%s+', ''):gsub('%s+$', ''))
@@ -65,16 +67,29 @@ function M.parse(entry)
   }
 end
 
+-- There is no default registry: the URL is site-specific, and a wrong one only
+-- surfaces as an opaque curl failure. Returns the base with trailing slashes
+-- stripped so joining it to a path never produces "//".
+function M.check_registry(registry)
+  if registry == nil then
+    return nil, 'registry is not configured; ' .. SETUP_HINT
+  end
+  if type(registry) ~= 'string' or not registry:match('^https?://[^%s/]') then
+    return nil, string.format('registry %s is not an http(s) URL; %s', vim.inspect(registry), SETUP_HINT)
+  end
+  return (registry:gsub('/+$', ''))
+end
+
 -- Tagged source archives live at the same path shape on every GitHub-alike, so
 -- the registry is just the proxy prefix in front of "<owner>/<repo>".
 function M.url(spec, registry)
   if spec.url then
     return spec.url
   end
-  if type(registry) ~= 'string' or registry == '' then
-    return nil, 'registry is not configured'
+  local base, err = M.check_registry(registry)
+  if not base then
+    return nil, err
   end
-  local base = registry:gsub('/+$', '')
   return table.concat({
     base,
     spec.owner,