use PySys_WriteStderr instead of eprintln

newsfragments/6261.changed.md

@@ -0,0 +1 @@
+During rust panic, all error output is printed to `sys.stderr`. Previously, some was printed to rust's stderr.

src/buffer.rs

@@ -729,7 +729,16 @@ impl Drop for PyUntypedBuffer {
         if Python::try_attach(|_| unsafe { self.0.release() }).is_none()
             && crate::internal::state::is_in_gc_traversal()
         {
-            eprintln!("Warning: PyBuffer dropped while in GC traversal, this is a bug and will leak memory.");
+            // NOTE: write using libc because we can't call python APIs during gc traversal
+            let message = "Warning: PyBuffer dropped while in GC traversal, this is a bug and will leak memory.";
+            unsafe {
+                libc::write(
+                    2,
+                    message.as_ptr().cast(),
+                    #[allow(clippy::useless_conversion, reason = "platform specific")]
+                    message.len().try_into().unwrap(),
+                );
+            }
         }
         // If `try_attach` failed and `is_in_gc_traversal()` is false, then probably the interpreter has
         // already finalized and we can just assume that the underlying memory has already been freed.

src/conversions/jiff.rs

@@ -781,10 +781,6 @@ mod tests {
                 |name: &'static str, year, month, day, hour, minute, second, ms, py_ms| {
                     let offset = Offset::from_seconds(3600).unwrap();
                     let datetime = DateTime::new(year, month, day, hour, minute, second, ms * 1000)
-                        .map_err(|e| {
-                            eprintln!("{name}: {e}");
-                            e
-                        })
                         .unwrap()
                         .to_zoned(offset.to_time_zone())
                         .unwrap();

src/err/mod.rs

@@ -8,6 +8,7 @@ use crate::inspect::PyStaticExpr;
 use crate::instance::Bound;
 #[cfg(Py_3_11)]
 use crate::intern;
+use crate::internal::stdio::write_py_stderr;
 use crate::panic::PanicException;
 use crate::platform::prelude::*;
 use crate::py_result_ext::PyResultExt;
@@ -302,8 +303,11 @@ impl PyErr {
             .map(|py_str| py_str.to_string_lossy().into())
             .unwrap_or_else(|_| String::from("Unwrapped panic from Python code"));
 
-        eprintln!("--- PyO3 is resuming a panic after fetching a PanicException from Python. ---");
-        eprintln!("Python stack trace below:");
+        write_py_stderr(
+            py,
+            c"--- PyO3 is resuming a panic after fetching a PanicException from Python. ---",
+        );
+        write_py_stderr(py, c"Python stack trace below:");
 
         PyErrState::normalized(state).restore(py);
 

src/internal.rs

@@ -6,3 +6,4 @@ pub(crate) mod macros;
 pub(crate) mod get_slot;
 pub(crate) mod pyclass_init;
 pub(crate) mod state;
+pub(crate) mod stdio;

src/internal/stdio.rs

@@ -0,0 +1,9 @@
+use core::ffi::CStr;
+
+use crate::ffi::PySys_WriteStderr;
+use crate::prelude::Python;
+
+pub(crate) fn write_py_stderr(_py: Python<'_>, message: &CStr) {
+    // SAFETY: message is a valid c string
+    unsafe { PySys_WriteStderr(message.as_ptr()) };
+}

src/platform.rs

@@ -9,9 +9,6 @@ pub(crate) mod prelude {
         string::{String, ToString},
         vec::Vec,
     };
-
-    // TODO find a `no_std` replacement for eprintln
-    pub use std::eprintln;
 }
 
 #[cfg(feature = "hashbrown")]