From d3ce313842c5837ab962b31d9dd98fdb9e1d2895 Mon Sep 17 00:00:00 2001
From: Tim Pillinger <26465611+wxtim@users.noreply.github.com>
Date: Tue, 7 Mar 2023 13:50:13 +0000
Subject: [PATCH] Stop searching platform groups after first match.

---
 cylc/flow/platforms.py                    |  4 ++++
 tests/unit/test_platforms_get_platform.py | 17 ++++++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/cylc/flow/platforms.py b/cylc/flow/platforms.py
index 35ffee6bdb5..b40360724d7 100644
--- a/cylc/flow/platforms.py
+++ b/cylc/flow/platforms.py
@@ -191,6 +191,9 @@ def platform_from_name(
         platform_name = 'localhost'
 
     platform_group = None
+    # The list is reversed to allow user-set platform groups (which are loaded
+    # later than site set platforms) to be matched first and override site
+    # defined platforms.
     for platform_name_re in reversed(list(platform_groups)):
         # Platform is member of a group.
         if re.fullmatch(platform_name_re, platform_name):
@@ -198,6 +201,7 @@ def platform_from_name(
                 platform_groups[platform_name_re], group_name=platform_name,
                 bad_hosts=bad_hosts
             )
+            break
 
     for platform_name_re in list(platforms):
         if (
diff --git a/tests/unit/test_platforms_get_platform.py b/tests/unit/test_platforms_get_platform.py
index d26af0575b2..5e6a7ff1892 100644
--- a/tests/unit/test_platforms_get_platform.py
+++ b/tests/unit/test_platforms_get_platform.py
@@ -237,18 +237,25 @@ def test_get_platform_using_platform_name_from_job_info(
 
 
 def test_get_platform_groups_basic(mock_glbl_cfg):
-    # get platform from group works.
+    """get platform from group works.
+
+    Additionally, ensure that we stop after selecting the first
+    appropriate platform.
+    """
     mock_glbl_cfg(
         'cylc.flow.platforms.glbl_cfg',
         '''
         [platforms]
-            [[aleph]]
-                hosts = aleph
-            [[bet]]
-                hosts = bet
+            [[aleph, bet, alpha, beta]]
 
         [platform groups]
             [[hebrew_letters]]
+                platforms = alpha, beta
+                [[[selection]]]
+                    method = definition order
+            [[aleph]]
+                platforms = alpha
+            [[.*_letters]]
                 platforms = aleph, bet
                 [[[selection]]]
                     method = definition order