From aef523ea1254e8bb9e4143ad8f5994ca89ea9d2d Mon Sep 17 00:00:00 2001
From: Moritz Angermann <moritz.angermann@gmail.com>
Date: Thu, 14 May 2020 08:18:31 +0800
Subject: Disable DLL loading if without system linker

Some platforms (musl, aarch64) do not have a working dynamic linker
implemented in the libc, even though we might see dlopen.  It will
ultimately just return that this is not supported.  Hence we'll add
a flag to the compiler to flat our disable loading dlls.  This is
needed as we will otherwise try to load the shared library even
if this will subsequently fail.  At that point we have given up
looking for static options though.
---
 compiler/GHC/Runtime/Linker.hs | 18 +++++++++++++++---
 compiler/ghc.cabal.in          |  9 +++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/compiler/GHC/Runtime/Linker.hs b/compiler/GHC/Runtime/Linker.hs
index d6b916ff39..740e3a7a43 100644
--- a/compiler/GHC/Runtime/Linker.hs
+++ b/compiler/GHC/Runtime/Linker.hs
@@ -1328,6 +1328,7 @@ linkPackage hsc_env pkg
             ("Loading package " ++ unitPackageIdString pkg ++ " ... ")
 
         -- See comments with partOfGHCi
+#if defined(CAN_LOAD_DLL)
         when (unitPackageName pkg `notElem` partOfGHCi) $ do
             loadFrameworks hsc_env platform pkg
             -- See Note [Crash early load_dyn and locateLib]
@@ -1336,7 +1337,7 @@ linkPackage hsc_env pkg
             -- For remaining `dlls` crash early only when there is surely
             -- no package's DLL around ... (not is_dyn)
             mapM_ (load_dyn hsc_env (not is_dyn) . mkSOName platform) dlls
-
+#endif
         -- After loading all the DLLs, we can load the static objects.
         -- Ordering isn't important here, because we do one final link
         -- step to resolve everything.
@@ -1471,10 +1472,15 @@ locateLib hsc_env is_hs lib_dirs gcc_dirs lib
     --   O(n). Loading an import library is also O(n) so in general we prefer
     --   shared libraries because they are simpler and faster.
     --
-  = findDll   user `orElse`
+  =
+#if defined(CAN_LOAD_DLL)
+    findDll   user `orElse`
+#endif
     tryImpLib user `orElse`
+#if defined(CAN_LOAD_DLL)
     findDll   gcc  `orElse`
     findSysDll     `orElse`
+#endif
     tryImpLib gcc  `orElse`
     findArchive    `orElse`
     tryGcc         `orElse`
@@ -1539,7 +1545,13 @@ locateLib hsc_env is_hs lib_dirs gcc_dirs lib
                          full     = dllpath $ search lib_so_name lib_dirs
                          gcc name = liftM (fmap Archive) $ search name lib_dirs
                          files    = import_libs ++ arch_files
-                     in apply $ short : full : map gcc files
+                         dlls     = [short, full]
+                         archives = map gcc files
+                     in apply $
+#if defined(CAN_LOAD_DLL)
+                          dlls ++
+#endif
+                          archives
      tryImpLib re = case os of
                        OSMinGW32 ->
                         let dirs' = if re == user then lib_dirs else gcc_dirs
diff --git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index 1024f0c98f..718047f777 100644
--- a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ -55,6 +55,11 @@ Flag integer-gmp
     Manual: True
     Default: False
 
+Flag dynamic-system-linker
+    Description: The system can load dynamic code. This is not the case for musl.
+    Default: True
+    Manual: False
+
 Library
     Default-Language: Haskell2010
     Exposed: False
@@ -108,6 +113,10 @@ Library
         CPP-Options: -DINTEGER_SIMPLE
         build-depends: integer-simple >= 0.1.1.1
 
+    -- if no dynamic system linker is available, don't try DLLs.
+    if flag(dynamic-system-linker)
+        CPP-Options: -DCAN_LOAD_DLL
+
     Other-Extensions:
         BangPatterns
         CPP
-- 
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