Process tasks can attempt to execute directories on Linux

[tobil4sk]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.70.2
• OS Version: Linux x64 5.19.2-1-MANJARO

Steps to Reproduce:

1. Install an extension with "process" tasks which run executables from PATH, e.g. this minimal extension running python
2. Create a folder in the first PATH entry with the same name as the executable, e.g. ~/.local/bin/python
3. Attempt to execute the task, and receive this error:

 *  Executing task: python

execvp(3) failed.: Permission denied

 *  The terminal process "python" failed to launch (exit code: 1). 
 *  Terminal will be reused by tasks, press any key to close it.

This is because the default function used by the findExecutable function does not check whether something is a file or a directory, but only that it exists:

vscode/src/vs/platform/terminal/node/terminalEnvironment.ts

Line 27 in 68cda09

export async function findExecutable(command: string, cwd?: string, paths?: string[], env: IProcessEnvironment = process.env as IProcessEnvironment, exists: (path: string) => Promise<boolean> = pfs.Promises.exists): Promise<string | undefined> {

This was fixed for win32.findExecutable in 52daded, where directories are ignored when searching PATH. However, as these functions are in separate files, the other one missed out on this fix.

Ideally these two functions should be merged as they perform pretty much the same job, and it would avoid this problem in the future. It might be worth considering making task behaviour consistent between platforms in terms of at which point executable paths are resolved, as that seems to be why we ended up with two implementations in the first place.

[gjsjohnmurray]

Maybe related to #158666

[tobil4sk]

#158666 is somewhat related, however, the two issues come from separate parts of the code. This problem comes from the findExecutable function.

The PR will somewhat improve the situation, because this error will be caught slightly earlier, so it will say: The terminal process failed to launch: Path to shell executable "python" is not a file or a symlink.. However, the expected behaviour is that there should be no error if a valid executable exists, regardless of any directories in PATH.