Write unchanged, excluded files to stdout when read via stdin (#8596)

## Summary

When you run Ruff via stdin, and pass `format` or `check --fix`, we
typically write the changed or unchanged contents to stdout. It turns
out we forgot to do this when the file is _excluded_, so if you run
`ruff format /path/to/excluded/file.py`, we don't write _anything_ to
`stdout`. This led to a bug in the LSP whereby we deleted file contents
for third-party files.

The right thing to do here is write back the unchanged contents, as it
should always be safe to write the output of stdout back to a file.

crates/ruff_cli/src/commands/check_stdin.rs

@@ -8,7 +8,7 @@ use ruff_workspace::resolver::{match_exclusion, python_file_at_path, PyprojectCo
 
 use crate::args::CliOverrides;
 use crate::diagnostics::{lint_stdin, Diagnostics};
-use crate::stdin::read_from_stdin;
+use crate::stdin::{parrot_stdin, read_from_stdin};
 
 /// Run the linter over a single file, read from `stdin`.
 pub(crate) fn check_stdin(
@@ -21,6 +21,9 @@ pub(crate) fn check_stdin(
     if pyproject_config.settings.file_resolver.force_exclude {
         if let Some(filename) = filename {
             if !python_file_at_path(filename, pyproject_config, overrides)? {
+                if fix_mode.is_apply() {
+                    parrot_stdin()?;
+                }
                 return Ok(Diagnostics::default());
             }
 
@@ -29,14 +32,17 @@ pub(crate) fn check_stdin(
                 .file_name()
                 .is_some_and(|name| match_exclusion(filename, name, &lint_settings.exclude))
             {
+                if fix_mode.is_apply() {
+                    parrot_stdin()?;
+                }
                 return Ok(Diagnostics::default());
             }
         }
     }
+    let stdin = read_from_stdin()?;
     let package_root = filename.and_then(Path::parent).and_then(|path| {
         packaging::detect_package_root(path, &pyproject_config.settings.linter.namespace_packages)
     });
-    let stdin = read_from_stdin()?;
     let mut diagnostics = lint_stdin(
         filename,
         package_root,

crates/ruff_cli/src/commands/format_stdin.rs

@@ -15,7 +15,7 @@ use crate::commands::format::{
     FormatResult, FormattedSource,
 };
 use crate::resolve::resolve;
-use crate::stdin::read_from_stdin;
+use crate::stdin::{parrot_stdin, read_from_stdin};
 use crate::ExitStatus;
 
 /// Run the formatter over a single file, read from `stdin`.
@@ -34,6 +34,9 @@ pub(crate) fn format_stdin(cli: &FormatArguments, overrides: &CliOverrides) -> R
     if pyproject_config.settings.file_resolver.force_exclude {
         if let Some(filename) = cli.stdin_filename.as_deref() {
             if !python_file_at_path(filename, &pyproject_config, overrides)? {
+                if mode.is_write() {
+                    parrot_stdin()?;
+                }
                 return Ok(ExitStatus::Success);
             }
 
@@ -42,6 +45,9 @@ pub(crate) fn format_stdin(cli: &FormatArguments, overrides: &CliOverrides) -> R
                 .file_name()
                 .is_some_and(|name| match_exclusion(filename, name, &format_settings.exclude))
             {
+                if mode.is_write() {
+                    parrot_stdin()?;
+                }
                 return Ok(ExitStatus::Success);
             }
         }
@@ -50,6 +56,9 @@ pub(crate) fn format_stdin(cli: &FormatArguments, overrides: &CliOverrides) -> R
     let path = cli.stdin_filename.as_deref();
 
     let SourceType::Python(source_type) = path.map(SourceType::from).unwrap_or_default() else {
+        if mode.is_write() {
+            parrot_stdin()?;
+        }
         return Ok(ExitStatus::Success);
     };
 

crates/ruff_cli/src/stdin.rs

@@ -1,5 +1,5 @@
 use std::io;
-use std::io::Read;
+use std::io::{Read, Write};
 
 /// Read a string from `stdin`.
 pub(crate) fn read_from_stdin() -> Result<String, io::Error> {
@@ -7,3 +7,11 @@ pub(crate) fn read_from_stdin() -> Result<String, io::Error> {
     io::stdin().lock().read_to_string(&mut buffer)?;
     Ok(buffer)
 }
+
+/// Read bytes from `stdin` and write them to `stdout`.
+pub(crate) fn parrot_stdin() -> Result<(), io::Error> {
+    let mut buffer = String::new();
+    io::stdin().lock().read_to_string(&mut buffer)?;
+    io::stdout().write_all(buffer.as_bytes())?;
+    Ok(())
+}

crates/ruff_cli/tests/format.rs

@@ -320,6 +320,11 @@ if __name__ == '__main__':
     exit_code: 0
     ----- stdout -----
 
+    from test import say_hy
+
+    if __name__ == '__main__':
+        say_hy("dear Ruff contributor")
+
     ----- stderr -----
     "###);
     Ok(())