Add a new run_pipeline() function to the subprocess module that enablesrunning multiple commands connected via pipes, similar to shell pipelines.New API:- run_pipeline(*commands, ...) - Run a pipeline of commands- PipelineResult - Return type with commands, returncodes, stdout, stderr- PipelineError - Raised when check=True and any command failsFeatures:- Supports arbitrary number of commands (minimum 2)- capture_output, input, timeout, and check parameters like run()- stdin= connects to first process, stdout= connects to last process- Text mode support via text=True, encoding, errors- All processes share a single stderr pipe for simplicity- "pipefail" semantics: check=True fails if any command failsUnlike run(), this function does not accept universal_newlines.Use text=True instead.Example:    result = subprocess.run_pipeline(        ['cat', 'file.txt'],        ['grep', 'pattern'],        ['wc', '-l'],        capture_output=True, text=True    )Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Document the new run_pipeline() function, PipelineResult class, andPipelineError exception in the subprocess module documentation.Includes:- Function signature with stdin, stdout, stderr, capture_output, etc.- Note about shared stderr pipe and text mode caveat for interleaved  multi-byte character sequences- Note that universal_newlines is not supported (use text=True)- Explanation that stdin connects to first process, stdout to last- Usage examples showing basic pipelines, multi-command pipelines,  input handling, and error handling with check=True- PipelineResult attributes: commands, returncodes, returncode,  stdout, stderr, and check_returncodes() method- PipelineError attributes: commands, returncodes, stdout, stderr,  and failed listCo-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Add _communicate_streams() helper function that properly multiplexesread/write operations to prevent pipe buffer deadlocks. The helperuses selectors on POSIX and threads on Windows, similar toPopen.communicate().This fixes potential deadlocks when large amounts of data flow throughthe pipeline and significantly improves performance.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Add three tests that verify the multiplexed I/O implementationproperly handles large data volumes that would otherwise causepipe buffer deadlocks:- test_pipeline_large_data_no_deadlock: 256KB through 2-stage pipeline- test_pipeline_large_data_three_stages: 128KB through 3-stage pipeline- test_pipeline_large_data_with_stderr: 64KB with concurrent stderrThese tests would timeout or deadlock without proper multiplexing.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Remove support for raw file descriptors in _communicate_streams(),requiring all streams to be file objects. This simplifies both theWindows and POSIX implementations by removing isinstance() checksand fd-wrapping logic.The run_pipeline() function now wraps the stderr pipe's read endwith os.fdopen() immediately after creation.This change makes _communicate_streams() more compatible withPopen.communicate() which already uses file objects, enablingpotential future refactoring to share the multiplexed I/O logic.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Update the test to write 64KB to stderr from each process (128KB total)instead of just small status messages. This better tests that themultiplexed I/O handles concurrent large data on both stdout and stderrwithout deadlocking.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

The comment suggested rewriting Popen._communicate() to usenon-blocking I/O on file objects now that Python 3's io moduleis used instead of C stdio.This is unnecessary - the current approach using select() todetect ready fds followed by os.read()/os.write() is correctand efficient. The selector already solves "when is data ready?"so non-blocking mode would add complexity with no benefit.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Extract the core selector-based I/O loop into a new _communicate_io_posix()function that is shared by both _communicate_streams_posix() (used byrun_pipeline) and Popen._communicate() (used by Popen.communicate).The new function:- Takes a pre-configured selector and output buffers- Supports resume via input_offset parameter (for Popen timeout retry)- Returns (new_offset, completed) instead of raising TimeoutExpired- Does not close streams (caller decides based on use case)This reduces code duplication and ensures both APIs use the samewell-tested I/O multiplexing logic.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Move stdin writing to a background thread in _communicate_streams_windowsto avoid blocking indefinitely when writing large input to a pipelinewhere the subprocess doesn't consume stdin quickly.This mirrors the fix made to Popen._communicate() for Windows incommit5b1862b (pythongh-87512).Add test_pipeline_timeout_large_input to verify that TimeoutExpiredis raised promptly when run_pipeline() is called with large inputand a timeout, even when the first process is slow to consume stdin.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Apply the same fixes from Popen._communicate() to _communicate_streams_posixfor run_pipeline():1. Handle non-byte memoryview input by casting to byte view (pythongh-134453):   Non-byte memoryviews (e.g., int32 arrays) had incorrect length tracking   because len() returns element count, not byte count. Now cast to "b"   view for correct progress tracking.2. Handle ValueError on stdin.flush() when stdin is closed (pythongh-74389):   Ignore ValueError from flush() if stdin is already closed, matching   the BrokenPipeError handling.Add tests for memoryview input to run_pipeline:- test_pipeline_memoryview_input: basic byte memoryview- test_pipeline_memoryview_input_nonbyte: int32 array memoryviewCo-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Extract common stdin preparation logic into shared helper functionsused by both _communicate_streams_posix() and Popen._communicate():- _flush_stdin(stdin): Flush stdin, ignoring BrokenPipeError and  ValueError (for closed files)- _make_input_view(input_data): Convert input data to a byte memoryview,  handling non-byte memoryviews by casting to "b" viewThis ensures consistent behavior and makes the fixes forpythongh-134453(memoryview) andpythongh-74389 (closed stdin) shared in one place.Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

- Factor out _translate_newlines() as a module-level function, have  Popen's method delegate to it for code sharing- Remove rejection of universal_newlines kwarg in run_pipeline(), treat  it the same as text=True (consistent with Popen behavior)- Use _translate_newlines() for text mode decoding in run_pipeline()  to properly handle \r\n and \r newline sequences- Update documentation to remove mention of universal_newlines rejection- Update test to verify universal_newlines=True works like text=TrueCo-authored-by: Claude <noreply@anthropic.com>