automation: don't create resources when deleting things

Otherwise running these commands can result in resources being
created. In the case of `purge-ec2-resources`, we will create
resources only to delete them immediately afterwards!

With this change, `purge-ec2-resources` now no-ops if no
resources exist.

# no-check-commit because foo_bar function name

Differential Revision: https://phab.mercurial-scm.org/D6285

contrib/automation/hgautomation/__init__.py

@@ -53,7 +53,7 class HGAutomation:
 
         return password
 
-    def aws_connection(self, region: str):
+    def aws_connection(self, region: str, ensure_ec2_state: bool=True):
         """Obtain an AWSConnection instance bound to a specific region."""
 
-        return AWSConnection(self, region)
+        return AWSConnection(self, region, ensure_ec2_state=ensure_ec2_state)

contrib/automation/hgautomation/aws.py

@@ -180,7 +180,7 Install-WindowsFeature -Name Net-Framewo
 class AWSConnection:
     """Manages the state of a connection with AWS."""
 
-    def __init__(self, automation, region: str):
+    def __init__(self, automation, region: str, ensure_ec2_state: bool=True):
         self.automation = automation
         self.local_state_path = automation.state_path
 
@@ -191,11 +191,12 class AWSConnection:
         self.ec2resource = self.session.resource('ec2')
         self.iamclient = self.session.client('iam')
         self.iamresource = self.session.resource('iam')
-
-        ensure_key_pairs(automation.state_path, self.ec2resource)
+        self.security_groups = {}
 
-        self.security_groups = ensure_security_groups(self.ec2resource)
-        ensure_iam_state(self.iamresource)
+        if ensure_ec2_state:
+            ensure_key_pairs(automation.state_path, self.ec2resource)
+            self.security_groups = ensure_security_groups(self.ec2resource)
+            ensure_iam_state(self.iamresource)
 
     def key_pair_path_private(self, name):
         """Path to a key pair private key file."""

contrib/automation/hgautomation/cli.py

@@ -95,12 +95,12 def build_all_windows_packages(hga: HGAu
 
 
 def terminate_ec2_instances(hga: HGAutomation, aws_region):
-    c = hga.aws_connection(aws_region)
+    c = hga.aws_connection(aws_region, ensure_ec2_state=False)
     aws.terminate_ec2_instances(c.ec2resource)
 
 
 def purge_ec2_resources(hga: HGAutomation, aws_region):
-    c = hga.aws_connection(aws_region)
+    c = hga.aws_connection(aws_region, ensure_ec2_state=False)
     aws.remove_resources(c)