Rollup merge of rust-lang#85060 - ChrisDenton:win-file-exists, r=yaahc

Windows implementation of feature `path_try_exists`

Draft of a Windows implementation of `try_exists` (rust-lang#83186).

The first commit reorganizes the code so I would be interested to get some feedback on if this is a good idea or not. It moves the `Path::try_exists` function to `fs::exists`. leaving the former as a wrapper for the latter. This makes it easier to provide platform specific implementations and matches the `fs::metadata` function.

The other commit implements a Windows specific variant of `exists`. I'm still figuring out my approach so this is very much a first draft. Eventually this will need some more eyes from knowledgable Windows people.

library/std/src/fs.rs

@@ -2208,3 +2208,29 @@ impl AsInnerMut<fs_imp::DirBuilder> for DirBuilder {
         &mut self.inner
     }
 }
+
+/// Returns `Ok(true)` if the path points at an existing entity.
+///
+/// This function will traverse symbolic links to query information about the
+/// destination file. In case of broken symbolic links this will return `Ok(false)`.
+///
+/// As opposed to the `exists()` method, this one doesn't silently ignore errors
+/// unrelated to the path not existing. (E.g. it will return `Err(_)` in case of permission
+/// denied on some of the parent directories.)
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(path_try_exists)]
+/// use std::fs;
+///
+/// assert!(!fs::try_exists("does_not_exist.txt").expect("Can't check existence of file does_not_exist.txt"));
+/// assert!(fs::try_exists("/root/secret_file.txt").is_err());
+/// ```
+// FIXME: stabilization should modify documentation of `exists()` to recommend this method
+// instead.
+#[unstable(feature = "path_try_exists", issue = "83186")]
+#[inline]
+pub fn try_exists<P: AsRef<Path>>(path: P) -> io::Result<bool> {
+    fs_imp::try_exists(path.as_ref())
+}

library/std/src/path.rs

@@ -2507,11 +2507,7 @@ impl Path {
     #[unstable(feature = "path_try_exists", issue = "83186")]
     #[inline]
     pub fn try_exists(&self) -> io::Result<bool> {
-        match fs::metadata(self) {
-            Ok(_) => Ok(true),
-            Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(false),
-            Err(error) => Err(error),
-        }
+        fs::try_exists(self)
     }
 
     /// Returns `true` if the path exists on disk and is pointing at a regular file.

library/std/src/sys/hermit/fs.rs

@@ -12,7 +12,7 @@ use crate::sys::time::SystemTime;
 use crate::sys::unsupported;
 use crate::sys_common::os_str_bytes::OsStrExt;
 
-pub use crate::sys_common::fs::copy;
+pub use crate::sys_common::fs::{copy, try_exists};
 //pub use crate::sys_common::fs::remove_dir_all;
 
 fn cstr(path: &Path) -> io::Result<CString> {

library/std/src/sys/unix/fs.rs

@@ -48,7 +48,7 @@ use libc::{
     dirent64, fstat64, ftruncate64, lseek64, lstat64, off64_t, open64, readdir64_r, stat64,
 };
 
-pub use crate::sys_common::fs::remove_dir_all;
+pub use crate::sys_common::fs::{remove_dir_all, try_exists};
 
 pub struct File(FileDesc);
 

library/std/src/sys/unsupported/fs.rs

@@ -275,6 +275,10 @@ pub fn remove_dir_all(_path: &Path) -> io::Result<()> {
     unsupported()
 }
 
+pub fn try_exists(_path: &Path) -> io::Result<()> {
+    unsupported()
+}
+
 pub fn readlink(_p: &Path) -> io::Result<PathBuf> {
     unsupported()
 }

library/std/src/sys/wasi/fs.rs

@@ -14,7 +14,7 @@ use crate::sys::time::SystemTime;
 use crate::sys::unsupported;
 use crate::sys_common::FromInner;
 
-pub use crate::sys_common::fs::remove_dir_all;
+pub use crate::sys_common::fs::{remove_dir_all, try_exists};
 
 pub struct File {
     fd: WasiFd,

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

@@ -173,6 +173,7 @@ pub const ERROR_INVALID_HANDLE: DWORD = 6;
 pub const ERROR_NOT_ENOUGH_MEMORY: DWORD = 8;
 pub const ERROR_OUTOFMEMORY: DWORD = 14;
 pub const ERROR_NO_MORE_FILES: DWORD = 18;
+pub const ERROR_SHARING_VIOLATION: u32 = 32;
 pub const ERROR_HANDLE_EOF: DWORD = 38;
 pub const ERROR_FILE_EXISTS: DWORD = 80;
 pub const ERROR_INVALID_PARAMETER: DWORD = 87;

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

@@ -944,3 +944,32 @@ fn symlink_junction_inner(original: &Path, junction: &Path) -> io::Result<()> {
         .map(drop)
     }
 }
+
+// Try to see if a file exists but, unlike `exists`, report I/O errors.
+pub fn try_exists(path: &Path) -> io::Result<bool> {
+    // Open the file to ensure any symlinks are followed to their target.
+    let mut opts = OpenOptions::new();
+    // No read, write, etc access rights are needed.
+    opts.access_mode(0);
+    // Backup semantics enables opening directories as well as files.
+    opts.custom_flags(c::FILE_FLAG_BACKUP_SEMANTICS);
+    match File::open(path, &opts) {
+        Err(e) => match e.kind() {
+            // The file definitely does not exist
+            io::ErrorKind::NotFound => Ok(false),
+
+            // `ERROR_SHARING_VIOLATION` means that the file has been locked by
+            // another process. This is often temporary so we simply report it
+            // as the file existing.
+            io::ErrorKind::Other if e.raw_os_error() == Some(c::ERROR_SHARING_VIOLATION as i32) => {
+                Ok(true)
+            }
+            // Other errors such as `ERROR_ACCESS_DENIED` may indicate that the
+            // file exists. However, these types of errors are usually more
+            // permanent so we report them here.
+            _ => Err(e),
+        },
+        // The file was opened successfully therefore it must exist,
+        Ok(_) => Ok(true),
+    }
+}

library/std/src/sys_common/fs.rs

@@ -41,3 +41,11 @@ fn remove_dir_all_recursive(path: &Path) -> io::Result<()> {
     }
     fs::remove_dir(path)
 }
+
+pub fn try_exists(path: &Path) -> io::Result<bool> {
+    match fs::metadata(path) {
+        Ok(_) => Ok(true),
+        Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(false),
+        Err(error) => Err(error),
+    }
+}