Fix stalling in Pipeline command

When a command (other than the first) in a pipeline wrote more than
64k to the stderr, and the output was consumed with the iter_lines
function, the whole pipeline stalled. Fixed by reading the output
of all commands where either stdout or stderr was set to PIPE.

plumbum/commands/base.py

@@ -398,8 +398,6 @@ def popen(self, args=(), **kwargs):
         dstproc = self.dstcmd.popen(**kwargs)
         # allow p1 to receive a SIGPIPE if p2 exits
         srcproc.stdout.close()
-        if srcproc.stderr is not None:
-            dstproc.stderr = srcproc.stderr
         if srcproc.stdin and src_kwargs.get("stdin") != PIPE:
             srcproc.stdin.close()
         dstproc.srcproc = srcproc

plumbum/commands/processes.py

@@ -18,14 +18,42 @@ def _check_process(proc, retcode, timeout, stdout, stderr):
     return proc.returncode, stdout, stderr
 
 
+def get_piped_streams(proc):
+    """Get a list of all valid standard streams for proc that were opened with PIPE option.
+
+    If proc was started from a Pipeline command, this function assumes it will have a
+    "srcproc" member pointing to the previous command in the pipeline. That link will
+    be used to traverse all started processes started from the pipeline, the list will
+    include stdout/stderr streams opened as PIPE for all commands in the pipeline.
+    If that was not the case, some processes could write to pipes no one reads from
+    which would result in process stalling after the pipe's buffer is filled.
+
+    Streams that were closed (because they were redirected to the input of a subsequent command)
+    are not included in the result
+    """
+    streams = []
+
+    def add_stream(type_, stream):
+        if stream is None or stream.closed:
+            return
+        streams.append((type_, stream))
+
+    while proc:
+        add_stream(1, proc.stderr)
+        add_stream(0, proc.stdout)
+        proc = getattr(proc, "srcproc", None)
+
+    return streams
+
+
 def _iter_lines_posix(proc, decode, linesize, line_timeout=None):
     from selectors import EVENT_READ, DefaultSelector
-
+    streams = get_piped_streams(proc)
     # Python 3.4+ implementation
     def selector():
         sel = DefaultSelector()
-        sel.register(proc.stdout, EVENT_READ, 0)
-        sel.register(proc.stderr, EVENT_READ, 1)
+        for stream_type, stream in streams:
+            sel.register(stream, EVENT_READ, stream_type)
         while True:
             ready = sel.select(line_timeout)
             if not ready and line_timeout:
@@ -41,10 +69,9 @@ def selector():
         yield ret
         if proc.poll() is not None:
             break
-    for line in proc.stdout:
-        yield 0, decode(line)
-    for line in proc.stderr:
-        yield 1, decode(line)
+    for stream_type, stream in streams:
+        for line in stream:
+            yield stream_type, decode(line)
 
 
 def _iter_lines_win32(proc, decode, linesize, line_timeout=None):

tests/test_pipelines.py

@@ -0,0 +1,70 @@
+from typing import List, Tuple
+
+import pytest
+
+import plumbum
+from plumbum.commands import BaseCommand
+
+
+@pytest.mark.timeout(1)
+def test_draining_stderr(generate_cmd, process_cmd):
+    stdout, stderr = get_output_with_iter_lines(
+        generate_cmd | process_cmd | process_cmd
+    )
+    assert len(stderr) == 15000
+    assert len(stdout) == 5000
+
+
+@pytest.mark.timeout(1)
+def test_draining_stderr_with_stderr_redirect(tmp_path, generate_cmd, process_cmd):
+    stdout, stderr = get_output_with_iter_lines(
+        generate_cmd | (process_cmd >= str(tmp_path / "output.txt")) | process_cmd
+    )
+    assert len(stderr) == 10000
+    assert len(stdout) == 5000
+
+
+@pytest.mark.timeout(1)
+def test_draining_stderr_with_stdout_redirect(tmp_path, generate_cmd, process_cmd):
+    stdout, stderr = get_output_with_iter_lines(
+        generate_cmd | process_cmd | process_cmd > str(tmp_path / "output.txt")
+    )
+    assert len(stderr) == 15000
+    assert len(stdout) == 0
+
+
+@pytest.fixture()
+def generate_cmd(tmp_path):
+    generate = tmp_path / "generate.py"
+    generate.write_text("""\
+import sys
+for i in range(5000):
+    print("generated", i, file=sys.stderr)
+    print(i)
+""")
+    return plumbum.local["python"][generate]
+
+
+@pytest.fixture()
+def process_cmd(tmp_path):
+    process = tmp_path / "process.py"
+    process.write_text("""\
+import sys
+for line in sys.stdin:
+    i = line.strip()
+    print("consumed", i, file=sys.stderr)
+    print(i)
+""")
+    return plumbum.local["python"][process]
+
+
+def get_output_with_iter_lines(cmd: BaseCommand) -> Tuple[List[str], List[str]]:
+    stderr, stdout = [], []
+    proc = cmd.popen()
+    for stdout_line, stderr_line in proc.iter_lines(retcode=[0, None]):
+        if stderr_line is not None:
+            stderr.append(stderr_line)
+        if stdout_line is not None:
+            stdout.append(stdout_line)
+    proc.wait()
+    return stdout, stderr