log_vc_info: handle long command output (#5821)

* Log the commands run by log_vc_info (debug level).
* Handle the risk of large command output clogging up the buffer causing
  commands to hang.

changes.d/5821.fix.md

@@ -0,0 +1 @@
+Fixed issue where large uncommitted changes could cause `cylc install` to hang.

cylc/flow/install_plugins/log_vc_info.py

@@ -63,12 +63,22 @@
 from pathlib import Path
 from subprocess import Popen, DEVNULL, PIPE
 from typing import (
-    Any, Dict, Iterable, List, Optional, TYPE_CHECKING, TextIO, Union, overload
+    Any,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    TYPE_CHECKING,
+    TextIO,
+    Union,
+    overload,
 )
 
 from cylc.flow import LOG as _LOG, LoggerAdaptor
 from cylc.flow.exceptions import CylcError
 import cylc.flow.flags
+from cylc.flow.pipe_poller import pipe_poller
+from cylc.flow.util import format_cmd
 from cylc.flow.workflow_files import WorkflowFiles
 
 if TYPE_CHECKING:
@@ -171,7 +181,7 @@ def get_vc_info(path: Union[Path, str]) -> Optional[Dict[str, Any]]:
             ):
                 LOG.debug(f"Source dir {path} is not a {vcs} repository")
             elif cylc.flow.flags.verbosity > -1:
-                LOG.warning(f"$ {vcs} {' '.join(args)}\n{exc}")
+                LOG.warning(f"$ {vcs} {format_cmd(args)}\n{exc}")
             continue
 
         info['version control system'] = vcs
@@ -217,9 +227,7 @@ def _run_cmd(
         args: The args to pass to the version control command.
         cwd: Directory to run the command in.
         stdout: Where to redirect output (either PIPE or a
-            text stream/file object). Note: only use PIPE for
-            commands that will not generate a large output, otherwise
-            the pipe might get blocked.
+            text stream/file object).
 
     Returns:
         Stdout output if stdout=PIPE, else None as the output has been
@@ -231,6 +239,7 @@ def _run_cmd(
         OSError: Non-zero return code for VCS command.
     """
     cmd = [vcs, *args]
+    LOG.debug(f'$ {format_cmd(cmd)}')
     try:
         proc = Popen(  # nosec
             cmd,
@@ -245,13 +254,15 @@ def _run_cmd(
         # This will only be raised if the VCS command is not installed,
         # otherwise Popen() will succeed with a non-zero return code
         raise VCSNotInstalledError(vcs, exc)
-    ret_code = proc.wait()
-    out, err = proc.communicate()
-    if ret_code:
+    if stdout == PIPE:
+        out, err = pipe_poller(proc, proc.stdout, proc.stderr)
+    else:
+        out, err = proc.communicate()
+    if proc.returncode:
         if any(err.lower().startswith(msg) for msg in NO_BASE_ERRS[vcs]):
             # No base commit in repo
             raise VCSMissingBaseError(vcs, cwd)
-        raise OSError(ret_code, err)
+        raise OSError(proc.returncode, err)
     return out
 
 

cylc/flow/pipe_poller.py

@@ -0,0 +1,73 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Utility for preventing pipes from getting clogged up.
+
+If you're reading files from Popen (i.e. to extract command output) where the
+command output has the potential to be long-ish, then you should use this
+function to protect against the buffer filling up.
+
+Note, there is a more advanced version of this baked into the subprocpool.
+"""
+
+from select import select
+
+
+def pipe_poller(proc, *files, chunk_size=4096):
+    """Read from a process without hitting buffer issues.
+
+    Standin for subprocess.Popen.communicate.
+
+    When PIPE'ing from subprocesses, the output goes into a buffer. If the
+    buffer gets full, the subprocess will hang trying to write to it.
+
+    This function polls the process, reading output from the buffers into
+    memory to prevent them from filling up.
+
+    Args:
+        proc:
+            The process to poll.
+        files:
+            The files you want to read from, likely anything you've directed to
+            PIPE.
+        chunk_size:
+            The amount of text to read from the buffer on each pass.
+
+    Returns:
+        tuple - The text read from each of the files in the order they were
+        specified.
+
+    """
+    _files = {
+        file: b'' if 'b' in getattr(file, 'mode', 'r') else ''
+        for file in files
+    }
+
+    def _read(timeout=1.0):
+        # read any data from files
+        nonlocal chunk_size, files
+        for file in select(list(files), [], [], timeout)[0]:
+            buffer = file.read(chunk_size)
+            if len(buffer) > 0:
+                _files[file] += buffer
+
+    while proc.poll() is None:
+        # read from the buffers
+        _read()
+    # double check the buffers now that the process has finished
+    _read(timeout=0.01)
+
+    return tuple(_files.values())

tests/unit/test_pipe_poller.py

@@ -0,0 +1,33 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from subprocess import Popen, PIPE
+
+from cylc.flow.pipe_poller import pipe_poller
+
+
+def test_pipe_poller_str():
+    proc = Popen(['echo', 'Hello World!'], stdout=PIPE, text=True)
+    (stdout,) = pipe_poller(proc, proc.stdout)
+    assert proc.returncode == 0
+    assert stdout == 'Hello World!\n'
+
+
+def test_pipe_poller_bytes():
+    proc = Popen(['echo', 'Hello World!'], stdout=PIPE, text=False)
+    (stdout,) = pipe_poller(proc, proc.stdout)
+    assert proc.returncode == 0
+    assert stdout == b'Hello World!\n'