Linker: ignore empty paths in addEnvPaths

Previously `splitEnv` worked like this:

    > splitEnv "foo:::bar::baz:"
    ["foo","","","bar","","baz",""]

with this patch:

    > splitEnv working_dir "foo:::bar:baz:"
    ["foo",working_dir,working_dir"bar","baz",working_dir]

This fixes #14695, where having a trailing `:` in the env variable
caused ghci to pass empty `-B` parameter to `gcc`, which in turned
caused the next parameter (`--print-file-name`) to be considered as the
argument to `-B`. As a result ghci did not work.

The `working_dir` argument is to have a similar behavior with POSIX:
according to chapter 8.3 zero-length prefix means current working
directory.

Reviewers: hvr, bgamari, AndreasK, simonmar

Reviewed By: bgamari, AndreasK, simonmar

Subscribers: AndreasK, rwbarton, thomie, carter

GHC Trac Issues: #14695

Differential Revision: https://phabricator.haskell.org/D4330

3 files changed, 18 insertions, 6 deletions

diff --git a/compiler/ghci/Linker.hs b/compiler/ghci/Linker.hs
index 34813798b2..a91df323ce 100644
--- a/compiler/ghci/Linker.hs
+++ b/compiler/ghci/Linker.hs
@@ -1547,15 +1547,22 @@ getSystemDirectories = return []
 --   given. If the variable does not exist then just return the identity.
 addEnvPaths :: String -> [String] -> IO [String]
 addEnvPaths name list
-  = do values <- lookupEnv name
+  = do -- According to POSIX (chapter 8.3) a zero-length prefix means current
+       -- working directory. Replace empty strings in the env variable with
+       -- `working_dir` (see also #14695).
+       working_dir <- getCurrentDirectory
+       values <- lookupEnv name
        case values of
          Nothing  -> return list
-         Just arr -> return $ list ++ splitEnv arr
+         Just arr -> return $ list ++ splitEnv working_dir arr
     where
-      splitEnv :: String -> [String]
-      splitEnv value = case break (== envListSep) value of
-                         (x, []    ) -> [x]
-                         (x, (_:xs)) -> x : splitEnv xs
+      splitEnv :: FilePath -> String -> [String]
+      splitEnv working_dir value =
+        case break (== envListSep) value of
+          (x, []    ) ->
+            [if null x then working_dir else x]
+          (x, (_:xs)) ->
+            (if null x then working_dir else x) : splitEnv working_dir xs
 #if defined(mingw32_HOST_OS)
       envListSep = ';'
 #else
diff --git a/testsuite/tests/rts/Makefile b/testsuite/tests/rts/Makefile
index a6d248201b..ded3be1b3b 100644
--- a/testsuite/tests/rts/Makefile
+++ b/testsuite/tests/rts/Makefile
@@ -174,3 +174,7 @@ T11788:
  .PHONY: T12497
 T12497:
 	echo main | "$(TEST_HC)" $(filter-out -rtsopts, $(TEST_HC_OPTS_INTERACTIVE)) T12497.hs
+
+.PHONY: T14695
+T14695:
+	echo ":quit" | LD_LIBRARY_PATH="foo:" "$(TEST_HC)" $(filter-out -rtsopts, $(TEST_HC_OPTS_INTERACTIVE))
diff --git a/testsuite/tests/rts/all.T b/testsuite/tests/rts/all.T
index 6377bde04f..fe86dd1146 100644
--- a/testsuite/tests/rts/all.T
+++ b/testsuite/tests/rts/all.T
@@ -382,3 +382,4 @@ test('T12903', [when(opsys('mingw32'), skip)], compile_and_run, [''])
 test('T13832', exit_code(1), compile_and_run, ['-threaded'])
 test('T13894', normal, compile_and_run, [''])
 test('T14497', normal, compile_and_run, ['-O'])
+test('T14695', normal, run_command, ['$MAKE -s --no-print-directory T14695'])