ci: Simplify ci-os-only handling

Handle 'ci-os-only' occurrences in the .cirrus.star file instead of
.cirrus.tasks.yml file. Now, 'ci-os-only' occurrences are controlled
from one central place instead of dealing with them in each task.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/20240413021221.hg53rvqlvldqh57i%40awork3.anarazel.de
Backpatch: 15-, where CI support was added

.cirrus.star

@@ -7,7 +7,7 @@ https://github.com/bazelbuild/starlark/blob/master/spec.md
 See also .cirrus.yml and src/tools/ci/README
 """
 
-load("cirrus", "env", "fs", "yaml")
+load("cirrus", "env", "fs", "re", "yaml")
 
 
 def main():
@@ -66,6 +66,7 @@ def main():
 def compute_environment_vars():
     cenv = {}
 
+    ###
     # Some tasks are manually triggered by default because they might use too
     # many resources for users of free Cirrus credits, but they can be
     # triggered automatically by naming them in an environment variable e.g.
@@ -82,6 +83,33 @@ def compute_environment_vars():
         else:
             value = 'manual'
         cenv[name] = value
+    ###
+
+    ###
+    # Parse "ci-os-only:" tag in commit message and set
+    # CI_{$OS}_ENABLED variable for each OS
+
+    operating_systems = [
+      'freebsd',
+      'linux',
+      'macos',
+      'windows',
+    ]
+    commit_message = env.get('CIRRUS_CHANGE_MESSAGE')
+    match_re = r"(^|.*\n)ci-os-only: ([^\n]+)($|\n.*)"
+
+    # re.match() returns an array with a tuple of (matched-string, match_1, ...)
+    m = re.match(match_re, commit_message)
+    if m and len(m) > 0:
+        os_only = m[0][2]
+        os_only_list = re.split(r'[, ]+', os_only)
+    else:
+        os_only_list = operating_systems
+
+    for os in operating_systems:
+        os_enabled = os in os_only_list
+        cenv['CI_{0}_ENABLED'.format(os.upper())] = os_enabled
+    ###
 
     return cenv
 

.cirrus.tasks.yml

@@ -47,7 +47,7 @@ task:
 
   <<: *freebsd_task_template
 
-  only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*freebsd.*'
+  only_if: $CI_FREEBSD_ENABLED
 
   sysinfo_script: |
     id
@@ -153,7 +153,7 @@ task:
 
   <<: *linux_task_template
 
-  only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*linux.*'
+  only_if: $CI_LINUX_ENABLED
 
   ccache_cache:
     folder: ${CCACHE_DIR}
@@ -239,7 +239,7 @@ task:
 
   <<: *macos_task_template
 
-  only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*(macos|darwin|osx).*'
+  only_if: $CI_MACOS_ENABLED
 
   sysinfo_script: |
     id
@@ -389,7 +389,7 @@ task:
 
   <<: *windows_task_template
 
-  only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*windows.*'
+  only_if: $CI_WINDOWS_ENABLED
 
   sysinfo_script: |
     chcp
@@ -476,7 +476,7 @@ task:
 
   # task that did not run, count as a success, so we need to recheck Linux'
   # condition here ...
-  only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*linux.*'
+  only_if: $CI_LINUX_ENABLED
 
   <<: *linux_task_template