fixup: use ThreadPool in flux-pstree.py

src/cmd/flux-pstree.py

@@ -11,6 +11,7 @@
 import sys
 import logging
 import argparse
+import concurrent.futures
 
 import flux
 import flux.uri
@@ -110,6 +111,30 @@ def format_prefix(self, job):
         return ""
 
 
+def process_entry(entry, formatter, filters, level, max_level, combine):
+
+    job = JobInfo(entry).get_instance_info()
+
+    # pylint: disable=comparison-with-callable
+    parent = job.uri and job.state_single == "R"
+
+    label = formatter.format(job, parent)
+    prefix = formatter.format_prefix(job)
+
+    if not parent:
+        return Tree(label, prefix)
+    return load_tree(
+        label,
+        formatter,
+        prefix=prefix,
+        uri=str(job.uri),
+        filters=filters,
+        level=level + 1,
+        max_level=max_level,
+        combine_children=combine,
+    )
+
+
 # pylint: disable=too-many-locals
 def load_tree(
     label,
@@ -147,29 +172,29 @@ def load_tree(
     except (OSError, FileNotFoundError):
         return tree
 
+    #  Since the executor cannot be used recursively, start one per
+    #   loop iteration. This is very wasteful but greatly speeds up
+    #   execution with even a moderate number of jobs using the ssh:
+    #   connector. At some point this should be replaced by a global
+    #   thread pool that can work with recursive execution.
+    #
+    executor = concurrent.futures.ThreadPoolExecutor()
+    futures = []
     for entry in jobs:
-        job = JobInfo(entry).get_instance_info()
-
-        # pylint: disable=comparison-with-callable
-        parent = job.uri and job.state_single == "R"
-
-        label = formatter.format(job, parent)
-        prefix = formatter.format_prefix(job)
-
-        if parent:
-            subtree = load_tree(
-                label,
+        futures.append(
+            executor.submit(
+                process_entry,
+                entry,
                 formatter,
-                prefix=prefix,
-                uri=str(job.uri),
-                filters=orig_filters,
-                level=level + 1,
-                max_level=max_level,
-                combine_children=combine_children,
+                orig_filters,
+                level,
+                max_level,
+                combine_children,
             )
-            tree.append_tree(subtree)
-        else:
-            tree.append(label, prefix)
+        )
+
+    for future in futures:
+        tree.append_tree(future.result())
 
     return tree