Hardcode FPComplete mirror in addition to Hackage

Stackage.hs

@@ -14,6 +14,7 @@ import qualified Crypto.Hash.SHA256 as SHA256
 import Control.Monad (forM_, unless)
 import Data.Aeson.Types
 import Data.Bifunctor
+import Data.List (intercalate)
 import Data.Yaml
 import Distribution.Package
 import Distribution.PackageDescription (FlagName, mkFlagName, unFlagName)
@@ -31,6 +32,7 @@ import qualified Data.ByteString as B
 import qualified Data.ByteString.Lazy as L
 import qualified Data.ByteString.Base16 as Base16
 import qualified Data.ByteString.Char8 as BC
+import qualified Data.List.NonEmpty as Nel
 import qualified Data.Map.Strict as Map
 import qualified Data.Text as T
 import qualified Distribution.Text as Cabal
@@ -46,15 +48,17 @@ main = do
             "https://raw.githubusercontent.com/fpco/lts-haskell/master/" ++ resolver ++ ".yaml"
           _ ->
             "https://raw.githubusercontent.com/fpco/stackage-nightly/master/" ++ resolver ++ ".yaml"
-    ltsYaml <- downloadUrl manager ltsUrl
+    ltsYaml <- downloadUrl manager (ltsUrl Nel.:| [])
     plan <- case decodeEither' $ L.toStrict ltsYaml of
         Left err -> throw err
         Right (x :: BuildPlan) -> pure x
     shas <- flip Map.traverseWithKey (planPackages plan) $ \n p -> do
-        let hackageUrl = "https://hackage.haskell.org/package/"
-                            ++ Cabal.display n ++ "-" ++ Cabal.display (planPackageVersion p)
-                            ++ ".tar.gz"
-        tar <- downloadUrl manager hackageUrl
+        let
+          tarball = Cabal.display n ++ "-" ++ Cabal.display (planPackageVersion p) ++ ".tar.gz"
+          fpcoUrl = "http://hackage.fpcomplete.com/package/"
+          hackageUrl = "http://hackage.haskell.org/package/"
+          urls = fmap (++tarball) (fpcoUrl Nel.:| [hackageUrl])
+        tar <- downloadUrl manager urls
         putStrLn (Cabal.display n)
         return $! SHA256.hashlazy tar
     writeFile out $ show $
@@ -88,16 +92,32 @@ flagsExpr :: Flags -> Expr
 flagsExpr m = ExprDict $
   bimap (ExprString . unFlagName) ExprBool <$> Map.toList m
 
-downloadUrl :: Manager -> String -> IO L.ByteString
-downloadUrl manager url = do
-    req <- parseRequest url
-    resp <- httpLbs req manager
-    let status = responseStatus resp
-    unless (statusIsSuccessful status)
-        $ error $ "Unable to download " ++ show url
-                ++ "\nStatus: " ++ show (statusCode status)
-                ++ " " ++ BC.unpack (statusMessage status)
-    return $ responseBody resp
+downloadUrl :: Manager -> Nel.NonEmpty String -> IO L.ByteString
+downloadUrl manager urls = do
+  resp <- tryUrls urls
+  let
+    allUrls = intercalate ", " $ Nel.toList urls
+    status = responseStatus resp
+  unless (statusIsSuccessful status)
+    $ error $ "Unable to download any of " ++ allUrls
+            ++ "\n Last status: " ++ show (statusCode status)
+            ++ " " ++ BC.unpack (statusMessage status)
+  return $ responseBody resp
+  where
+    tryUrl u = do
+      req <- parseRequest u
+      httpLbs req manager
+
+    tryUrls (u Nel.:| []) = do
+      resp <- tryUrl u
+      return resp
+
+    tryUrls (u Nel.:|(u':us)) = do
+      resp <- tryUrl u
+      let status = responseStatus resp
+      if (statusIsSuccessful status)
+        then return resp
+        else tryUrls $ u' Nel.:| us
 
 --------------------------------------------------------------------------------
 -- JSON data types and instances for parsing the LTS yaml file

hazel.bzl

@@ -11,22 +11,28 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl",
 load("//tools:ghc.bzl", "get_ghc_workspace", "default_ghc_workspaces")
 load("//tools:mangling.bzl", "hazel_binary", "hazel_library", "hazel_workspace")
 
+def _mirrors(pkg):
+  return [package_url_template.format(pkg) for package_url_template in [
+    "http://hackage.fpcomplete.com/package/{}.tar.gz",
+    "http://hackage.haskell.org/package/{}.tar.gz",
+  ]]
+
 def _cabal_haskell_repository_impl(ctx):
   ghc_workspace = get_ghc_workspace(ctx.attr.ghc_workspaces, ctx)
 
   pkg = "{}-{}".format(ctx.attr.package_name, ctx.attr.package_version)
-  url = "https://hackage.haskell.org/package/{}.tar.gz".format(pkg)
+  urls = _mirrors(pkg)
   # If the SHA is wrong, the error message is very unhelpful:
   # https://github.com/bazelbuild/bazel/issues/3709
   # As a workaround, we compute it manually if it's not set (and then fail
   # this rule).
   if not ctx.attr.sha256:
-    ctx.download(url=url, output="tar")
+    ctx.download(url=urls, output="tar")
     res = ctx.execute(["openssl", "sha", "-sha256", "tar"])
     fail("Missing expected attribute \"sha256\" for {}; computed {}".format(pkg, res.stdout + res.stderr))
 
   ctx.download_and_extract(
-      url=url,
+      url=urls,
       stripPrefix=ctx.attr.package_name + "-" + ctx.attr.package_version,
       sha256=ctx.attr.sha256,
       output="")
@@ -216,10 +222,7 @@ def hazel_custom_package_hackage(
       build_file and build_file_content are mutually exclusive.
   """
   package_id = package_name + "-" + version
-  url = "https://hackage.haskell.org/package/{0}/{1}.tar.gz".format(
-    package_id,
-    package_id,
-  )
+  urls = _mirrors(package_id)
   if not build_file and not build_file_content:
     build_file = "//third_party/haskell:BUILD.{0}".format(package_name)
   http_archive(
@@ -228,7 +231,7 @@ def hazel_custom_package_hackage(
     build_file_content = build_file_content,
     sha256 = sha256,
     strip_prefix = package_id,
-    urls = [url],
+    urls = urls,
   )
 
 def hazel_custom_package_github(