Auto merge of #85060 - ChrisDenton:win-file-exists, r=yaahc

Windows implementation of feature `path_try_exists`

Draft of a Windows implementation of `try_exists` (#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.

6 files changed, 37 insertions, 3 deletions

diff --git a/library/std/src/sys/hermit/fs.rs b/library/std/src/sys/hermit/fs.rs
index 5b3f2fa4e82..76ea70d997f 100644
--- a/library/std/src/sys/hermit/fs.rs
+++ b/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> {
diff --git a/library/std/src/sys/unix/fs.rs b/library/std/src/sys/unix/fs.rs
index 79617aa77b7..ef14865fbcd 100644
--- a/library/std/src/sys/unix/fs.rs
+++ b/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);
 
diff --git a/library/std/src/sys/unsupported/fs.rs b/library/std/src/sys/unsupported/fs.rs
index cd533761e37..6b45e29c145 100644
--- a/library/std/src/sys/unsupported/fs.rs
+++ b/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<bool> {
+    unsupported()
+}
+
 pub fn readlink(_p: &Path) -> io::Result<PathBuf> {
     unsupported()
 }
diff --git a/library/std/src/sys/wasi/fs.rs b/library/std/src/sys/wasi/fs.rs
index ed0f03e4b71..45e38f68b8c 100644
--- a/library/std/src/sys/wasi/fs.rs
+++ b/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,
diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index e91c489361e..50d6e8cf27a 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/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;
diff --git a/library/std/src/sys/windows/fs.rs b/library/std/src/sys/windows/fs.rs
index 8e6bd76f85f..2b6143de960 100644
--- a/library/std/src/sys/windows/fs.rs
+++ b/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),
+    }
+}