ChildProcess: account for a system error when launching a process

When a non-existing command is attempted to run, both status and
signal will be null, which leads to ChildProcess module crashing.
Obviously, this is incorrect behavior.

The problem is more general than that though: any system error that
would result in failing to run a process would result in the module
crash.

Fix that by checking for such case.

Fixes: #65

Author: Hi-Angel

CHANGELOG.md

@@ -10,6 +10,8 @@ New features:
 
 Bugfixes:
 
+- Account for child creation to possibly fail with system error (#66 by @Hi-Angel)
+
 Other improvements:
 
 ## [v11.1.0](https://github.com/purescript-node/purescript-node-child-process/releases/tag/v11.1.0) - 2023-11-14
@@ -76,7 +78,7 @@ Breaking changes:
   - Moved from `Node.ChildProces` to `Node.ChildProces.Types`
   - Changed the `BySignal`'s constructor's arg type from `Signal` to `String`
 - Breaking changes made to the `Handle` type (#46 by @JordanMartinez)
-  
+
   - Moved from `Node.ChildProces` to `Node.ChildProces.Types`
 - Converted `defaultOptions { override = Just 1}` pattern to `(_ { override = Just 1})`  (#46 by @JordanMartinez)
 

src/Node/ChildProcess.purs

@@ -21,7 +21,7 @@
 -- | defined in this library that doesn't exist in the Node docs.
 -- | It exists to allow the end-user to append additional values to the `safeStdio` value
 -- | used here. For example,
--- | 
+-- |
 -- | ```
 -- | spawn' file args (_ { appendStdio = Just [ fileDescriptor8, pipe, pipe ]})
 -- | ```
@@ -86,7 +86,7 @@ module Node.ChildProcess
 
 import Prelude
 
-import Data.Maybe (Maybe(..), fromMaybe)
+import Data.Maybe (Maybe(..), fromMaybe, isJust)
 import Data.Nullable (Nullable, toMaybe, toNullable)
 import Data.Posix (Pid, Gid, Uid)
 import Data.Posix.Signal (Signal)
@@ -237,7 +237,9 @@ spawnSync command args = (UnsafeCP.spawnSync command args) <#> \r ->
   , exitStatus: case toMaybe r.status, toMaybe r.signal of
       Just c, _ -> Normally c
       _, Just s -> BySignal s
-      _, _ -> unsafeCrashWith $ "Impossible: `spawnSync` child process neither exited nor was killed."
+      _, _ -> if isJust $ toMaybe r.error
+              then BySysError
+              else unsafeCrashWith $ "Impossible: `spawnSync` child process neither exited nor was killed."
   , error: toMaybe r.error
   }
 
@@ -282,7 +284,9 @@ spawnSync' command args buildOpts = (UnsafeCP.spawnSync' command args opts) <#>
   , exitStatus: case toMaybe r.status, toMaybe r.signal of
       Just c, _ -> Normally c
       _, Just s -> BySignal s
-      _, _ -> unsafeCrashWith $ "Impossible: `spawnSync` child process neither exited nor was killed."
+      _, _ -> if isJust $ toMaybe r.error
+              then BySysError
+              else unsafeCrashWith $ "Impossible: `spawnSync` child process neither exited nor was killed."
   , error: toMaybe r.error
   }
   where

src/Node/ChildProcess/Types.purs

@@ -52,7 +52,7 @@ foreign import data StdIO :: Type
 -- | Note: when used with `stdin`, piping the parent stdin to this stream
 -- | will not cause the child process to terminate when that parent stdin stream
 -- | ends via `Ctrl+D` user input. Rather, the child process will hang
--- | until the parent process calls `Stream.end` on the child process' 
+-- | until the parent process calls `Stream.end` on the child process'
 -- | `stdin` stream. Since it's impossible to know when the user
 -- | inputs `Ctrl+D`, `inherit` should be used instead.
 pipe :: StdIO
@@ -140,12 +140,14 @@ customShell = unsafeCoerce
 -- | what options were used.
 foreign import data StringOrBuffer :: Type
 
--- | Specifies how a child process exited; normally (with an exit code), or
--- | due to a signal.
+-- | Specifies how a child process exited; normally (with an exit code), due to
+-- | a signal or if it failed to even launch (e.g. if a command doesn't exist).
 data Exit
   = Normally Int
   | BySignal KillSignal
+  | BySysError
 
 instance showExit :: Show Exit where
   show (Normally x) = "Normally " <> show x
   show (BySignal sig) = "BySignal " <> (either show show $ fromKillSignal sig)
+  show BySysError = "BySysError"