fix executablePath test for NetBSD

executablePath support for NetBSD was added in
a172be07e3dce758a2325104a3a37fc8b1d20c9c, but the test was not
updated.

Update the test so that it works for NetBSD.  This requires handling
some quirks:

- The result of getExecutablePath could include "./" segments.
  Therefore use System.FilePath.equalFilePath to compare paths.

- The sysctl(2) call returns the original executable name even after
  it was deleted.  Add `canQueryAfterDelete :: [FilePath]` and
  adjust expectations for the post-delete query accordingly.

Also add a note to the `executablePath` haddock to advise that
NetBSD behaves differently from other OSes when the file has been
deleted.

Also accept a decrease in memory usage for T16875.  On Windows, the
metric is -2.2% of baseline, just outside the allowed ±2%.  I don't
see how this commit could have influenced this metric, so I suppose
it's something in the CI environment.

Metric Decrease:
    T16875

2 files changed, 23 insertions, 8 deletions

diff --git a/libraries/base/System/Environment/ExecutablePath.hsc b/libraries/base/System/Environment/ExecutablePath.hsc
index 923d61883c..1bee1122e8 100644
--- a/libraries/base/System/Environment/ExecutablePath.hsc
+++ b/libraries/base/System/Environment/ExecutablePath.hsc
@@ -102,6 +102,10 @@ getExecutablePath :: IO FilePath
 -- Note that for scripts and interactive sessions, the result is the path to
 -- the interpreter (e.g. ghci.)
 --
+-- Note also that while most operating systems return @Nothing@ if the
+-- executable file was deleted/unlinked, some (including NetBSD) return the
+-- original path.
+--
 -- @since 4.17.0.0
 executablePath :: Maybe (IO (Maybe FilePath))
 
diff --git a/testsuite/tests/lib/base/executablePath.hs b/testsuite/tests/lib/base/executablePath.hs
index 180f423cc2..27758531a9 100644
--- a/testsuite/tests/lib/base/executablePath.hs
+++ b/testsuite/tests/lib/base/executablePath.hs
@@ -1,12 +1,14 @@
 import Control.Monad (unless)
 import System.Environment (executablePath, getArgs)
 import System.Directory (removeFile, getCurrentDirectory)
-import System.FilePath ((</>), dropExtension)
+import System.FilePath ((</>), dropExtension, equalFilePath)
 import System.Exit (exitSuccess, die)
 
-canQuery, canDelete :: [String]
-canQuery = ["mingw32", "freebsd", "linux", "darwin"]
-canDelete = ["freebsd", "linux", "darwin"]
+canQuery, canDelete, canQueryAfterDelete :: [String]
+canQuery = ["mingw32", "freebsd", "linux", "darwin", "netbsd"]
+canDelete = ["freebsd", "linux", "darwin", "netbsd"]
+canQueryAfterDelete = ["netbsd"]
+
 
 main :: IO ()
 main = do
@@ -33,7 +35,7 @@ main = do
     -- (e.g. ".exe" on Windows). Drop the extension when comparing.
     expected  = cwd </> "executablePath"
     actual    = dropExtension before
-  unless (actual == expected) $
+  unless (equalFilePath actual expected) $
       die $ "executablePath query returned `" <> actual <> "`; expected `" <> expected <> "`"
 
   unless (os `elem` canDelete)
@@ -44,7 +46,16 @@ main = do
   -- Remove the file
   removeFile before
 
-  -- Now query should return Nothing
+  -- Now query again, after deletion
   after <- query
-  unless (after == Nothing) $ die $
-    "executablePath expected to return Nothing, returned " <> show after
+  case after of
+    Nothing
+      | os `elem` canQueryAfterDelete
+      -> die "query failed after deletion, but expected success"
+      | otherwise
+      -> pure ()
+    Just _
+      | os `elem` canQueryAfterDelete
+      -> pure ()
+      | otherwise
+      -> die $ "query succeeded after deleted (result: " <> show after <> "), but expected failure"