gh-47798: run_pipeline: identify the failing command in error output

Annotate spawn-time OSError with an exception note naming the
PipelineCommand and its index, so a FileNotFoundError mid-pipeline
tells the caller which command failed; the exception type is unchanged.

Reformat PipelineError.__str__ as "argv (commands[i]) detail" so the
argv leads and the index is unambiguously a Python list subscript.

Author: gpshead

Doc/library/subprocess.rst

@@ -391,7 +391,7 @@ underlying :class:`Popen` interface can be used directly.
       ... )
       Traceback (most recent call last):
         ...
-      subprocess.PipelineError: Pipeline failed: command 1 ['false'] returned 1
+      subprocess.PipelineError: Pipeline failed: ['false'] (commands[1]) returned 1
 
    .. versionadded:: next
 

Lib/subprocess.py

@@ -241,8 +241,8 @@ def __str__(self):
                 cmd_display = cmd.args
             else:
                 cmd_display = cmd
-            parts.append(f"command {i} {cmd_display!r} {detail}")
-        return f"Pipeline failed: {', '.join(parts)}"
+            parts.append(f"{cmd_display!r} (commands[{i}]) {detail}")
+        return f"Pipeline failed: {'; '.join(parts)}"
 
 
 if _mswindows:
@@ -1199,8 +1199,14 @@ def run_pipeline(*commands, input=None, capture_output=False, timeout=None,
                 if cmd.shell:
                     cmd_kwargs['shell'] = True
 
-            proc = Popen(cmd.args, stdin=proc_stdin, stdout=proc_stdout,
-                         stderr=proc_stderr, **cmd_kwargs)
+            try:
+                proc = Popen(cmd.args, stdin=proc_stdin, stdout=proc_stdout,
+                             stderr=proc_stderr, **cmd_kwargs)
+            except OSError as e:
+                e.add_note(
+                    f'raised while starting {cmd!r} '
+                    f'(run_pipeline commands[{i}])')
+                raise
             processes.append(proc)
 
             # Close the parent's copy of the previous process's stdout

Lib/test/test_subprocess.py

@@ -2360,7 +2360,7 @@ def test_pipeline_spawn_failure_cleans_up(self):
         promptly rather than hanging until command 0's sleep finishes.
         """
         start = time.monotonic()
-        with self.assertRaises(NONEXISTING_ERRORS):
+        with self.assertRaises(NONEXISTING_ERRORS) as cm:
             subprocess.run_pipeline(
                 [sys.executable, '-c', 'import time; time.sleep(60)'],
                 NONEXISTING_CMD,
@@ -2370,6 +2370,10 @@ def test_pipeline_spawn_failure_cleans_up(self):
         self.assertLess(elapsed, 30,
             "run_pipeline did not promptly clean up the running first "
             "command after the second command failed to spawn")
+        notes = getattr(cm.exception, '__notes__', [])
+        self.assertTrue(
+            any('commands[1]' in n for n in notes),
+            f'expected a which-command note on the OSError; got {notes!r}')
 
     def test_pipeline_explicit_stdout_pipe(self):
         """Test pipeline with explicit stdout=PIPE"""