cabal-install: don't pass exe name to external commands

Previously the executable name of the external command was passed to external commands as the
first argument.

This behaviour was adapated from cargo which does this because of reasons that are internal
to rust that do not affect GHC Haskell, and are even orthogonal to patterns that see common use in
Haskell.

Additionally, it complicates the 'simple' case which is what we should optimize for when building
such a feature.

The previous use case (one executable that serves multiple external subcommands) is still possible
by the following means:
- using a wrapper around the executable
- using a symlink and check argv[0] in the executable

Resolves #10275

Author: MangoIV

cabal-install/src/Distribution/Client/Main.hs

@@ -372,17 +372,20 @@ mainWorker args = do
       -> [String]
       -> IO (CommandParse Action)
     delegateToExternal commands' name cmdArgs = do
+      -- we rely on cabal's implementation of findProgramOnSearchPath not following
+      -- symlinks here. If that ever happens, then the argv[0] of the called executable
+      -- will be different to the intended one and will break tools that work by reading it.
       mCommand <- findProgramOnSearchPath normal defaultProgramSearchPath ("cabal-" <> name)
       case mCommand of
-        Just (exec, _) -> return (CommandReadyToGo $ \_ -> callExternal exec name cmdArgs)
+        Just (exec, _) -> return (CommandReadyToGo $ \_ -> callExternal exec cmdArgs)
         Nothing -> defaultCommandFallback commands' name cmdArgs
 
-    callExternal :: String -> String -> [String] -> IO ()
-    callExternal exec name cmdArgs = do
+    callExternal :: String -> [String] -> IO ()
+    callExternal exec cmdArgs = do
       cur_env <- getEnvironment
       cabal_exe <- getExecutablePath
       let new_env = ("CABAL", cabal_exe) : cur_env
-      result <- try $ createProcess ((proc exec (name : cmdArgs)){env = Just new_env})
+      result <- try $ createProcess ((proc exec cmdArgs){env = Just new_env})
       case result of
         Left ex -> printErrors ["Error executing external command: " ++ show (ex :: SomeException)]
         Right (_, _, _, ph) -> waitForProcess ph >>= exitWith

cabal-testsuite/PackageTests/ExternalCommandHelp/setup-test/AAAA.hs

@@ -5,5 +5,5 @@ import System.Environment
 main = do
   args <- getArgs
   case args of
-    ["aaaa" , "--help"] -> putStrLn "I am helping with the aaaa command"
+    ["--help"] -> putStrLn "I am helping with the aaaa command"
     _ -> putStrLn "aaaa"

changelog.d/pr-11232.md

@@ -0,0 +1,19 @@
+---
+synopsis: don't pass exe name to external commands
+packages: [cabal-install]
+prs: 11232
+issues: [10275]
+significance: significant
+---
+
+Previously the executable name of the external command was passed to external commands as the first argument.
+
+This behaviour was adapated from cargo which does this because of reasons that are internal to rust that do not affect GHC Haskell, and are even orthogonal to patterns that see common use in Haskell.
+
+Additionally, it complicates the 'simple' case which is what we should optimize for when building such a feature - with this change, for any executable `cabal-foo` in your search-path, `cabal foo` will be a valid invocation of that command.
+
+The previous use case (one executable that serves multiple external subcommands) is still possible by the following means:
+
+- using a wrapper around the executable
+- using a symlink and check argv\[0\] in the executable
+