Auto merge of #91182 - ChrisDenton:command-broken-symlink, r=m-ou-se

Maintain broken symlink behaviour for the Windows exe resolver

When the resolver was updated to remove the current directory from the search path (see #87704), care was take to avoid unintentional changes that hadn't been discussed. However, I missed the broken symlink behaviour. This PR fixes that.

**Edit** This turned out to be more important than I first realised. There are some types of application stubs that will redirect to the actual process when run using `CreateProcessW`, but due to the way they're implemented they cannot be opened normally using a `File::open` that follows reparse points. So this doesn't work with our current `exists` and `try_exists` methods.

Fixes #91177

library/std/src/sys/windows/c.rs

@@ -83,6 +83,7 @@ pub const CSTR_GREATER_THAN: c_int = 3;
 pub const FILE_ATTRIBUTE_READONLY: DWORD = 0x1;
 pub const FILE_ATTRIBUTE_DIRECTORY: DWORD = 0x10;
 pub const FILE_ATTRIBUTE_REPARSE_POINT: DWORD = 0x400;
+pub const INVALID_FILE_ATTRIBUTES: DWORD = DWORD::MAX;
 
 pub const FILE_SHARE_DELETE: DWORD = 0x4;
 pub const FILE_SHARE_READ: DWORD = 0x1;
@@ -1075,6 +1076,7 @@ extern "system" {
         lpBuffer: LPWSTR,
         lpFilePart: *mut LPWSTR,
     ) -> DWORD;
+    pub fn GetFileAttributesW(lpFileName: LPCWSTR) -> DWORD;
 }
 
 #[link(name = "ws2_32")]

library/std/src/sys/windows/process.rs

@@ -394,7 +394,7 @@ fn resolve_exe<'a>(
 
         // Append `.exe` if not already there.
         path = path::append_suffix(path, EXE_SUFFIX.as_ref());
-        if path.try_exists().unwrap_or(false) {
+        if program_exists(&path) {
             return Ok(path);
         } else {
             // It's ok to use `set_extension` here because the intent is to
@@ -415,7 +415,7 @@ fn resolve_exe<'a>(
             if !has_extension {
                 path.set_extension(EXE_EXTENSION);
             }
-            if let Ok(true) = path.try_exists() { Some(path) } else { None }
+            if program_exists(&path) { Some(path) } else { None }
         });
         if let Some(path) = result {
             return Ok(path);
@@ -485,6 +485,21 @@ where
     None
 }
 
+/// Check if a file exists without following symlinks.
+fn program_exists(path: &Path) -> bool {
+    unsafe {
+        to_u16s(path)
+            .map(|path| {
+                // Getting attributes using `GetFileAttributesW` does not follow symlinks
+                // and it will almost always be successful if the link exists.
+                // There are some exceptions for special system files (e.g. the pagefile)
+                // but these are not executable.
+                c::GetFileAttributesW(path.as_ptr()) != c::INVALID_FILE_ATTRIBUTES
+            })
+            .unwrap_or(false)
+    }
+}
+
 impl Stdio {
     fn to_handle(&self, stdio_id: c::DWORD, pipe: &mut Option<AnonPipe>) -> io::Result<Handle> {
         match *self {

library/std/src/sys/windows/process/tests.rs

@@ -135,6 +135,8 @@ fn windows_env_unicode_case() {
 fn windows_exe_resolver() {
     use super::resolve_exe;
     use crate::io;
+    use crate::sys::fs::symlink;
+    use crate::sys_common::io::test::tmpdir;
 
     let env_paths = || env::var_os("PATH");
 
@@ -178,4 +180,13 @@ fn windows_exe_resolver() {
     // The application's directory is also searched.
     let current_exe = env::current_exe().unwrap();
     assert!(resolve_exe(current_exe.file_name().unwrap().as_ref(), empty_paths, None).is_ok());
+
+    // Create a temporary path and add a broken symlink.
+    let temp = tmpdir();
+    let mut exe_path = temp.path().to_owned();
+    exe_path.push("exists.exe");
+    symlink("<DOES NOT EXIST>".as_ref(), &exe_path).unwrap();
+
+    // A broken symlink should still be resolved.
+    assert!(resolve_exe(OsStr::new("exists.exe"), empty_paths, Some(temp.path().as_ref())).is_ok());
 }