[init] adapt Context base path for file loader

Fix issue lualatex#344

The paths changed a while ago making a change necessary. We can’t just
replace the path because older versions of the tree would fail,
rendering bisection unusable. We compensate by testing the candidate
directories beforehand.

src/luaotfload-init.lua

@@ -9,6 +9,7 @@
 
 local setmetatable = setmetatable
 local kpselookup   = kpse.lookup
+local lfsisdir     = lfs.isdir
 
 --[[doc--
 
@@ -223,7 +224,7 @@ end
 
 --- below paths are relative to the texmf-context
 local ltx = "tex/generic/context/luatex"
-local ctx = "tex/context/base"
+local ctx = { "tex/context/base/mkiv", "tex/context/base" }
 
 local context_modules = {
 
@@ -285,17 +286,37 @@ local load_context_modules = function (pth)
     local sub, spec = unpack (context_modules [i])
     if sub == false then
       ignore_module (spec)
-    elseif type (sub) == "string" then
-      if pth then
+    else
+      local tsub = type (sub)
+      if not pth then
+        load_module (spec)
+      elseif tsub == "string" then
         load_module (spec, file.join (pth, sub))
+      elseif tsub == "table" then
+        local pfx
+        local nsub = #sub
+        for j = 1, nsub do
+          local full = file.join (pth, sub [j])
+          if lfsisdir (full) then --- pick the first real one
+            pfx = full
+            break
+          end
+        end
+        if pfx then
+          load_module (spec, pfx)
+        else
+          logreport ("both", 0, "init",
+                     "None of the %d search paths for module %q exist; \z
+                      falling back to default path.",
+                     nsub, tostring (spec))
+          load_module (spec) --- maybe we’ll get by after all?
+        end
       else
-        load_module (spec)
+        logreport ("both", 0, "init",
+                   "Internal error, please report. \z
+                    This is not your fault.")
+        os.exit (-1)
       end
-    else
-      logreport ("both", 0, "init",
-                 "Internal error, please report. \z
-                  This is not your fault.")
-      os.exit (-1)
     end
   end
 
@@ -535,7 +556,7 @@ local init_main = function ()
                "Loading Context modules in lookup path.")
     load_context_modules ()
 
-  elseif lfs.isdir (fontloader) then
+  elseif lfsisdir (fontloader) then
     logreport ("log", 0, "init",
                "Loading Context files under prefix “%s”.",
                fontloader)