fs.writeFile / fs.writeFileSync fails to write to hidden file

[sryze]

Version

18-pre (from master branch)

Platform

Windows

Subsystem

fs

What steps will reproduce the bug?

1. Create a hidden file: touch file.txt && attrib +h file.txt
2. Run this script:

const fs = require('fs');

fs.writeFileSync('file.txt', 'test', 'utf-8');

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior?

Expected the write call to succeed

What do you see instead?

$ node writeHiddenFile.js
node:internal/fs/utils:344
    throw err;
    ^

Error: EPERM: operation not permitted, open 'file.txt'
    at Object.openSync (node:fs:585:3)
    at Object.writeFileSync (node:fs:2153:35)
    at Object.<anonymous> (E:\Projects\Issues\nodejs-hidden-file-bug\writeHiddenFile.js:3:4)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47 {
  errno: -4048,
  syscall: 'open',
  code: 'EPERM',
  path: 'file.txt'
}

Additional information

When the file is opened via fs.open, this call to CreateFileW fails:

node/deps/uv/src/win/fs.c

Line 612 in b323cec

file = CreateFileW(req->file.pathw,

I did some debugging and noticed that the issue occurs because the dwCreationDisposition argument passed to CreateFileW() is set to CREATE_ALWAYS. If the file mode flag is changed to something that causes writeFileSync to use OPEN_EXISTING or OPEN_ALWAYS instead, it works, e.g.:

// Writes to an existing file or fails if it doesn't exist; same as OPEN_EXISTING
fs.writeFileSync('file.txt', 'test', {
    encoding: 'utf-8',
    flag: fs.constants.O_EXCL | fs.constants.O_WRONLY
});

// Writes to an existing file and creates it if it doesn't exist, same as OPEN_ALWAYS
fs.writeFileSync('file.txt', 'test', {
    encoding: 'utf-8',
    flag: fs.constants.O_CREAT | fs.constants.O_WRONLY
});

So basically one can use this ^^^ workaround to support writing to hidden files on Windows systems.

Documentation page for CreateFile mentions the following:

If CREATE_ALWAYS and FILE_ATTRIBUTE_NORMAL are specified, CreateFile fails and sets the last error to ERROR_ACCESS_DENIED if the file exists and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To avoid the error, specify the same attributes as the existing file.

I think to fix this inside Node.js, one needs to query the file attributes first before opening a file, and then apply the hidden attribute accordingly when opening it. I'm not sure if this should be handled inside Node or UV. I can try to create a pull request if this sounds like a good solution.

[sryze]

Here is a patch that fixed it for me:

diff --git a/deps/uv/src/win/fs.c b/deps/uv/src/win/fs.c
index 674070400b..9afc8bba1d 100644
--- a/deps/uv/src/win/fs.c
+++ b/deps/uv/src/win/fs.c
@@ -606,6 +606,15 @@ void fs__open(uv_fs_t* req) {
     goto einval;
   }

+
+  /* Copy existing attributes to prevent an "Access is denied" error for hidden
+   * and system files. */
+  DWORD existing_attributes = GetFileAttributesW(req->file.pathw);
+  if (existing_attributes != INVALID_FILE_ATTRIBUTES) {
+    attributes |= (existing_attributes
+      & (FILE_ATTRIBUTE_HIDDEN | FILE_ATTRIBUTE_SYSTEM));
+  }
+
   /* Setting this flag makes it possible to open a directory. */
   attributes |= FILE_FLAG_BACKUP_SEMANTICS;

[lpinca]

Feel free to open a PR in the libuv repository.

[sryze]

libuv/libuv#3380