--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/contrib/automation/hgautomation/aws.py	Fri Mar 15 11:24:08 2019 -0700
@@ -0,0 +1,879 @@
+# aws.py - Automation code for Amazon Web Services
+#
+# Copyright 2019 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+# no-check-code because Python 3 native.
+
+import contextlib
+import copy
+import hashlib
+import json
+import os
+import pathlib
+import subprocess
+import time
+
+import boto3
+import botocore.exceptions
+
+from .winrm import (
+    run_powershell,
+    wait_for_winrm,
+)
+
+
+SOURCE_ROOT = pathlib.Path(os.path.abspath(__file__)).parent.parent.parent.parent
+
+INSTALL_WINDOWS_DEPENDENCIES = (SOURCE_ROOT / 'contrib' /
+                                'install-windows-dependencies.ps1')
+
+
+KEY_PAIRS = {
+    'automation',
+}
+
+
+SECURITY_GROUPS = {
+    'windows-dev-1': {
+        'description': 'Mercurial Windows instances that perform build automation',
+        'ingress': [
+            {
+                'FromPort': 22,
+                'ToPort': 22,
+                'IpProtocol': 'tcp',
+                'IpRanges': [
+                    {
+                        'CidrIp': '0.0.0.0/0',
+                        'Description': 'SSH from entire Internet',
+                    },
+                ],
+            },
+            {
+                'FromPort': 3389,
+                'ToPort': 3389,
+                'IpProtocol': 'tcp',
+                'IpRanges': [
+                    {
+                        'CidrIp': '0.0.0.0/0',
+                        'Description': 'RDP from entire Internet',
+                    },
+                ],
+
+            },
+            {
+                'FromPort': 5985,
+                'ToPort': 5986,
+                'IpProtocol': 'tcp',
+                'IpRanges': [
+                    {
+                        'CidrIp': '0.0.0.0/0',
+                        'Description': 'PowerShell Remoting (Windows Remote Management)',
+                    },
+                ],
+            }
+        ],
+    },
+}
+
+
+IAM_ROLES = {
+    'ephemeral-ec2-role-1': {
+        'description': 'Mercurial temporary EC2 instances',
+        'policy_arns': [
+            'arn:aws:iam::aws:policy/service-role/AmazonEC2RoleforSSM',
+        ],
+    },
+}
+
+
+ASSUME_ROLE_POLICY_DOCUMENT = '''
+{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Effect": "Allow",
+      "Principal": {
+        "Service": "ec2.amazonaws.com"
+      },
+      "Action": "sts:AssumeRole"
+    }
+  ]
+}
+'''.strip()
+
+
+IAM_INSTANCE_PROFILES = {
+    'ephemeral-ec2-1': {
+        'roles': [
+            'ephemeral-ec2-role-1',
+        ],
+    }
+}
+
+
+# User Data for Windows EC2 instance. Mainly used to set the password
+# and configure WinRM.
+# Inspired by the User Data script used by Packer
+# (from https://www.packer.io/intro/getting-started/build-image.html).
+WINDOWS_USER_DATA = '''
+<powershell>
+
+# TODO enable this once we figure out what is failing.
+#$ErrorActionPreference = "stop"
+
+# Set administrator password
+net user Administrator "%s"
+wmic useraccount where "name='Administrator'" set PasswordExpires=FALSE
+
+# First, make sure WinRM can't be connected to
+netsh advfirewall firewall set rule name="Windows Remote Management (HTTP-In)" new enable=yes action=block
+
+# Delete any existing WinRM listeners
+winrm delete winrm/config/listener?Address=*+Transport=HTTP  2>$Null
+winrm delete winrm/config/listener?Address=*+Transport=HTTPS 2>$Null
+
+# Create a new WinRM listener and configure
+winrm create winrm/config/listener?Address=*+Transport=HTTP
+winrm set winrm/config/winrs '@{MaxMemoryPerShellMB="0"}'
+winrm set winrm/config '@{MaxTimeoutms="7200000"}'
+winrm set winrm/config/service '@{AllowUnencrypted="true"}'
+winrm set winrm/config/service '@{MaxConcurrentOperationsPerUser="12000"}'
+winrm set winrm/config/service/auth '@{Basic="true"}'
+winrm set winrm/config/client/auth '@{Basic="true"}'
+
+# Configure UAC to allow privilege elevation in remote shells
+$Key = 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System'
+$Setting = 'LocalAccountTokenFilterPolicy'
+Set-ItemProperty -Path $Key -Name $Setting -Value 1 -Force
+
+# Configure and restart the WinRM Service; Enable the required firewall exception
+Stop-Service -Name WinRM
+Set-Service -Name WinRM -StartupType Automatic
+netsh advfirewall firewall set rule name="Windows Remote Management (HTTP-In)" new action=allow localip=any remoteip=any
+Start-Service -Name WinRM
+
+# Disable firewall on private network interfaces so prompts don't appear.
+Set-NetFirewallProfile -Name private -Enabled false
+</powershell>
+'''.lstrip()
+
+
+WINDOWS_BOOTSTRAP_POWERSHELL = '''
+Write-Output "installing PowerShell dependencies"
+Install-PackageProvider -Name NuGet -MinimumVersion 2.8.5.201 -Force
+Set-PSRepository -Name PSGallery -InstallationPolicy Trusted
+Install-Module -Name OpenSSHUtils -RequiredVersion 0.0.2.0
+
+Write-Output "installing OpenSSL server"
+Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0
+# Various tools will attempt to use older versions of .NET. So we enable
+# the feature that provides them so it doesn't have to be auto-enabled
+# later.
+Write-Output "enabling .NET Framework feature"
+Install-WindowsFeature -Name Net-Framework-Core
+'''
+
+
+class AWSConnection:
+    """Manages the state of a connection with AWS."""
+
+    def __init__(self, automation, region: str):
+        self.automation = automation
+        self.local_state_path = automation.state_path
+
+        self.prefix = 'hg-'
+
+        self.session = boto3.session.Session(region_name=region)
+        self.ec2client = self.session.client('ec2')
+        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 = 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."""
+        return self.local_state_path / 'keys' / ('keypair-%s' % name)
+
+    def key_pair_path_public(self, name):
+        return self.local_state_path / 'keys' / ('keypair-%s.pub' % name)
+
+
+def rsa_key_fingerprint(p: pathlib.Path):
+    """Compute the fingerprint of an RSA private key."""
+
+    # TODO use rsa package.
+    res = subprocess.run(
+        ['openssl', 'pkcs8', '-in', str(p), '-nocrypt', '-topk8',
+         '-outform', 'DER'],
+        capture_output=True,
+        check=True)
+
+    sha1 = hashlib.sha1(res.stdout).hexdigest()
+    return ':'.join(a + b for a, b in zip(sha1[::2], sha1[1::2]))
+
+
+def ensure_key_pairs(state_path: pathlib.Path, ec2resource, prefix='hg-'):
+    remote_existing = {}
+
+    for kpi in ec2resource.key_pairs.all():
+        if kpi.name.startswith(prefix):
+            remote_existing[kpi.name[len(prefix):]] = kpi.key_fingerprint
+
+    # Validate that we have these keys locally.
+    key_path = state_path / 'keys'
+    key_path.mkdir(exist_ok=True, mode=0o700)
+
+    def remove_remote(name):
+        print('deleting key pair %s' % name)
+        key = ec2resource.KeyPair(name)
+        key.delete()
+
+    def remove_local(name):
+        pub_full = key_path / ('keypair-%s.pub' % name)
+        priv_full = key_path / ('keypair-%s' % name)
+
+        print('removing %s' % pub_full)
+        pub_full.unlink()
+        print('removing %s' % priv_full)
+        priv_full.unlink()
+
+    local_existing = {}
+
+    for f in sorted(os.listdir(key_path)):
+        if not f.startswith('keypair-') or not f.endswith('.pub'):
+            continue
+
+        name = f[len('keypair-'):-len('.pub')]
+
+        pub_full = key_path / f
+        priv_full = key_path / ('keypair-%s' % name)
+
+        with open(pub_full, 'r', encoding='ascii') as fh:
+            data = fh.read()
+
+        if not data.startswith('ssh-rsa '):
+            print('unexpected format for key pair file: %s; removing' %
+                  pub_full)
+            pub_full.unlink()
+            priv_full.unlink()
+            continue
+
+        local_existing[name] = rsa_key_fingerprint(priv_full)
+
+    for name in sorted(set(remote_existing) | set(local_existing)):
+        if name not in local_existing:
+            actual = '%s%s' % (prefix, name)
+            print('remote key %s does not exist locally' % name)
+            remove_remote(actual)
+            del remote_existing[name]
+
+        elif name not in remote_existing:
+            print('local key %s does not exist remotely' % name)
+            remove_local(name)
+            del local_existing[name]
+
+        elif remote_existing[name] != local_existing[name]:
+            print('key fingerprint mismatch for %s; '
+                  'removing from local and remote' % name)
+            remove_local(name)
+            remove_remote('%s%s' % (prefix, name))
+            del local_existing[name]
+            del remote_existing[name]
+
+    missing = KEY_PAIRS - set(remote_existing)
+
+    for name in sorted(missing):
+        actual = '%s%s' % (prefix, name)
+        print('creating key pair %s' % actual)
+
+        priv_full = key_path / ('keypair-%s' % name)
+        pub_full = key_path / ('keypair-%s.pub' % name)
+
+        kp = ec2resource.create_key_pair(KeyName=actual)
+
+        with priv_full.open('w', encoding='ascii') as fh:
+            fh.write(kp.key_material)
+            fh.write('\n')
+
+        priv_full.chmod(0o0600)
+
+        # SSH public key can be extracted via `ssh-keygen`.
+        with pub_full.open('w', encoding='ascii') as fh:
+            subprocess.run(
+                ['ssh-keygen', '-y', '-f', str(priv_full)],
+                stdout=fh,
+                check=True)
+
+        pub_full.chmod(0o0600)
+
+
+def delete_instance_profile(profile):
+    for role in profile.roles:
+        print('removing role %s from instance profile %s' % (role.name,
+                                                             profile.name))
+        profile.remove_role(RoleName=role.name)
+
+    print('deleting instance profile %s' % profile.name)
+    profile.delete()
+
+
+def ensure_iam_state(iamresource, prefix='hg-'):
+    """Ensure IAM state is in sync with our canonical definition."""
+
+    remote_profiles = {}
+
+    for profile in iamresource.instance_profiles.all():
+        if profile.name.startswith(prefix):
+            remote_profiles[profile.name[len(prefix):]] = profile
+
+    for name in sorted(set(remote_profiles) - set(IAM_INSTANCE_PROFILES)):
+        delete_instance_profile(remote_profiles[name])
+        del remote_profiles[name]
+
+    remote_roles = {}
+
+    for role in iamresource.roles.all():
+        if role.name.startswith(prefix):
+            remote_roles[role.name[len(prefix):]] = role
+
+    for name in sorted(set(remote_roles) - set(IAM_ROLES)):
+        role = remote_roles[name]
+
+        print('removing role %s' % role.name)
+        role.delete()
+        del remote_roles[name]
+
+    # We've purged remote state that doesn't belong. Create missing
+    # instance profiles and roles.
+    for name in sorted(set(IAM_INSTANCE_PROFILES) - set(remote_profiles)):
+        actual = '%s%s' % (prefix, name)
+        print('creating IAM instance profile %s' % actual)
+
+        profile = iamresource.create_instance_profile(
+            InstanceProfileName=actual)
+        remote_profiles[name] = profile
+
+    for name in sorted(set(IAM_ROLES) - set(remote_roles)):
+        entry = IAM_ROLES[name]
+
+        actual = '%s%s' % (prefix, name)
+        print('creating IAM role %s' % actual)
+
+        role = iamresource.create_role(
+            RoleName=actual,
+            Description=entry['description'],
+            AssumeRolePolicyDocument=ASSUME_ROLE_POLICY_DOCUMENT,
+        )
+
+        remote_roles[name] = role
+
+        for arn in entry['policy_arns']:
+            print('attaching policy %s to %s' % (arn, role.name))
+            role.attach_policy(PolicyArn=arn)
+
+    # Now reconcile state of profiles.
+    for name, meta in sorted(IAM_INSTANCE_PROFILES.items()):
+        profile = remote_profiles[name]
+        wanted = {'%s%s' % (prefix, role) for role in meta['roles']}
+        have = {role.name for role in profile.roles}
+
+        for role in sorted(have - wanted):
+            print('removing role %s from %s' % (role, profile.name))
+            profile.remove_role(RoleName=role)
+
+        for role in sorted(wanted - have):
+            print('adding role %s to %s' % (role, profile.name))
+            profile.add_role(RoleName=role)
+
+
+def find_windows_server_2019_image(ec2resource):
+    """Find the Amazon published Windows Server 2019 base image."""
+
+    images = ec2resource.images.filter(
+        Filters=[
+            {
+                'Name': 'owner-alias',
+                'Values': ['amazon'],
+            },
+            {
+                'Name': 'state',
+                'Values': ['available'],
+            },
+            {
+                'Name': 'image-type',
+                'Values': ['machine'],
+            },
+            {
+                'Name': 'name',
+                'Values': ['Windows_Server-2019-English-Full-Base-2019.02.13'],
+            },
+        ])
+
+    for image in images:
+        return image
+
+    raise Exception('unable to find Windows Server 2019 image')
+
+
+def ensure_security_groups(ec2resource, prefix='hg-'):
+    """Ensure all necessary Mercurial security groups are present.
+
+    All security groups are prefixed with ``hg-`` by default. Any security
+    groups having this prefix but aren't in our list are deleted.
+    """
+    existing = {}
+
+    for group in ec2resource.security_groups.all():
+        if group.group_name.startswith(prefix):
+            existing[group.group_name[len(prefix):]] = group
+
+    purge = set(existing) - set(SECURITY_GROUPS)
+
+    for name in sorted(purge):
+        group = existing[name]
+        print('removing legacy security group: %s' % group.group_name)
+        group.delete()
+
+    security_groups = {}
+
+    for name, group in sorted(SECURITY_GROUPS.items()):
+        if name in existing:
+            security_groups[name] = existing[name]
+            continue
+
+        actual = '%s%s' % (prefix, name)
+        print('adding security group %s' % actual)
+
+        group_res = ec2resource.create_security_group(
+            Description=group['description'],
+            GroupName=actual,
+        )
+
+        group_res.authorize_ingress(
+            IpPermissions=group['ingress'],
+        )
+
+        security_groups[name] = group_res
+
+    return security_groups
+
+
+def terminate_ec2_instances(ec2resource, prefix='hg-'):
+    """Terminate all EC2 instances managed by us."""
+    waiting = []
+
+    for instance in ec2resource.instances.all():
+        if instance.state['Name'] == 'terminated':
+            continue
+
+        for tag in instance.tags or []:
+            if tag['Key'] == 'Name' and tag['Value'].startswith(prefix):
+                print('terminating %s' % instance.id)
+                instance.terminate()
+                waiting.append(instance)
+
+    for instance in waiting:
+        instance.wait_until_terminated()
+
+
+def remove_resources(c, prefix='hg-'):
+    """Purge all of our resources in this EC2 region."""
+    ec2resource = c.ec2resource
+    iamresource = c.iamresource
+
+    terminate_ec2_instances(ec2resource, prefix=prefix)
+
+    for image in ec2resource.images.all():
+        if image.name.startswith(prefix):
+            remove_ami(ec2resource, image)
+
+    for group in ec2resource.security_groups.all():
+        if group.group_name.startswith(prefix):
+            print('removing security group %s' % group.group_name)
+            group.delete()
+
+    for profile in iamresource.instance_profiles.all():
+        if profile.name.startswith(prefix):
+            delete_instance_profile(profile)
+
+    for role in iamresource.roles.all():
+        if role.name.startswith(prefix):
+            print('removing role %s' % role.name)
+            role.delete()
+
+
+def wait_for_ip_addresses(instances):
+    """Wait for the public IP addresses of an iterable of instances."""
+    for instance in instances:
+        while True:
+            if not instance.public_ip_address:
+                time.sleep(2)
+                instance.reload()
+                continue
+
+            print('public IP address for %s: %s' % (
+                instance.id, instance.public_ip_address))
+            break
+
+
+def remove_ami(ec2resource, image):
+    """Remove an AMI and its underlying snapshots."""
+    snapshots = []
+
+    for device in image.block_device_mappings:
+        if 'Ebs' in device:
+            snapshots.append(ec2resource.Snapshot(device['Ebs']['SnapshotId']))
+
+    print('deregistering %s' % image.id)
+    image.deregister()
+
+    for snapshot in snapshots:
+        print('deleting snapshot %s' % snapshot.id)
+        snapshot.delete()
+
+
+def wait_for_ssm(ssmclient, instances):
+    """Wait for SSM to come online for an iterable of instance IDs."""
+    while True:
+        res = ssmclient.describe_instance_information(
+            Filters=[
+                {
+                    'Key': 'InstanceIds',
+                    'Values': [i.id for i in instances],
+                },
+            ],
+        )
+
+        available = len(res['InstanceInformationList'])
+        wanted = len(instances)
+
+        print('%d/%d instances available in SSM' % (available, wanted))
+
+        if available == wanted:
+            return
+
+        time.sleep(2)
+
+
+def run_ssm_command(ssmclient, instances, document_name, parameters):
+    """Run a PowerShell script on an EC2 instance."""
+
+    res = ssmclient.send_command(
+        InstanceIds=[i.id for i in instances],
+        DocumentName=document_name,
+        Parameters=parameters,
+        CloudWatchOutputConfig={
+            'CloudWatchOutputEnabled': True,
+        },
+    )
+
+    command_id = res['Command']['CommandId']
+
+    for instance in instances:
+        while True:
+            try:
+                res = ssmclient.get_command_invocation(
+                    CommandId=command_id,
+                    InstanceId=instance.id,
+                )
+            except botocore.exceptions.ClientError as e:
+                if e.response['Error']['Code'] == 'InvocationDoesNotExist':
+                    print('could not find SSM command invocation; waiting')
+                    time.sleep(1)
+                    continue
+                else:
+                    raise
+
+            if res['Status'] == 'Success':
+                break
+            elif res['Status'] in ('Pending', 'InProgress', 'Delayed'):
+                time.sleep(2)
+            else:
+                raise Exception('command failed on %s: %s' % (
+                    instance.id, res['Status']))
+
+
+@contextlib.contextmanager
+def temporary_ec2_instances(ec2resource, config):
+    """Create temporary EC2 instances.
+
+    This is a proxy to ``ec2client.run_instances(**config)`` that takes care of
+    managing the lifecycle of the instances.
+
+    When the context manager exits, the instances are terminated.
+
+    The context manager evaluates to the list of data structures
+    describing each created instance. The instances may not be available
+    for work immediately: it is up to the caller to wait for the instance
+    to start responding.
+    """
+
+    ids = None
+
+    try:
+        res = ec2resource.create_instances(**config)
+
+        ids = [i.id for i in res]
+        print('started instances: %s' % ' '.join(ids))
+
+        yield res
+    finally:
+        if ids:
+            print('terminating instances: %s' % ' '.join(ids))
+            for instance in res:
+                instance.terminate()
+            print('terminated %d instances' % len(ids))
+
+
+@contextlib.contextmanager
+def create_temp_windows_ec2_instances(c: AWSConnection, config):
+    """Create temporary Windows EC2 instances.
+
+    This is a higher-level wrapper around ``create_temp_ec2_instances()`` that
+    configures the Windows instance for Windows Remote Management. The emitted
+    instances will have a ``winrm_client`` attribute containing a
+    ``pypsrp.client.Client`` instance bound to the instance.
+    """
+    if 'IamInstanceProfile' in config:
+        raise ValueError('IamInstanceProfile cannot be provided in config')
+    if 'UserData' in config:
+        raise ValueError('UserData cannot be provided in config')
+
+    password = c.automation.default_password()
+
+    config = copy.deepcopy(config)
+    config['IamInstanceProfile'] = {
+        'Name': 'hg-ephemeral-ec2-1',
+    }
+    config.setdefault('TagSpecifications', []).append({
+        'ResourceType': 'instance',
+        'Tags': [{'Key': 'Name', 'Value': 'hg-temp-windows'}],
+    })
+    config['UserData'] = WINDOWS_USER_DATA % password
+
+    with temporary_ec2_instances(c.ec2resource, config) as instances:
+        wait_for_ip_addresses(instances)
+
+        print('waiting for Windows Remote Management service...')
+
+        for instance in instances:
+            client = wait_for_winrm(instance.public_ip_address, 'Administrator', password)
+            print('established WinRM connection to %s' % instance.id)
+            instance.winrm_client = client
+
+        yield instances
+
+
+def ensure_windows_dev_ami(c: AWSConnection, prefix='hg-'):
+    """Ensure Windows Development AMI is available and up-to-date.
+
+    If necessary, a modern AMI will be built by starting a temporary EC2
+    instance and bootstrapping it.
+
+    Obsolete AMIs will be deleted so there is only a single AMI having the
+    desired name.
+
+    Returns an ``ec2.Image`` of either an existing AMI or a newly-built
+    one.
+    """
+    ec2client = c.ec2client
+    ec2resource = c.ec2resource
+    ssmclient = c.session.client('ssm')
+
+    name = '%s%s' % (prefix, 'windows-dev')
+
+    config = {
+        'BlockDeviceMappings': [
+            {
+                'DeviceName': '/dev/sda1',
+                'Ebs': {
+                    'DeleteOnTermination': True,
+                    'VolumeSize': 32,
+                    'VolumeType': 'gp2',
+                },
+            }
+        ],
+        'ImageId': find_windows_server_2019_image(ec2resource).id,
+        'InstanceInitiatedShutdownBehavior': 'stop',
+        'InstanceType': 't3.medium',
+        'KeyName': '%sautomation' % prefix,
+        'MaxCount': 1,
+        'MinCount': 1,
+        'SecurityGroupIds': [c.security_groups['windows-dev-1'].id],
+    }
+
+    commands = [
+        # Need to start the service so sshd_config is generated.
+        'Start-Service sshd',
+        'Write-Output "modifying sshd_config"',
+        r'$content = Get-Content C:\ProgramData\ssh\sshd_config',
+        '$content = $content -replace "Match Group administrators","" -replace "AuthorizedKeysFile __PROGRAMDATA__/ssh/administrators_authorized_keys",""',
+        r'$content | Set-Content C:\ProgramData\ssh\sshd_config',
+        'Import-Module OpenSSHUtils',
+        r'Repair-SshdConfigPermission C:\ProgramData\ssh\sshd_config -Confirm:$false',
+        'Restart-Service sshd',
+        'Write-Output "installing OpenSSL client"',
+        'Add-WindowsCapability -Online -Name OpenSSH.Client~~~~0.0.1.0',
+        'Set-Service -Name sshd -StartupType "Automatic"',
+        'Write-Output "OpenSSH server running"',
+    ]
+
+    with INSTALL_WINDOWS_DEPENDENCIES.open('r', encoding='utf-8') as fh:
+        commands.extend(l.rstrip() for l in fh)
+
+    # Disable Windows Defender when bootstrapping because it just slows
+    # things down.
+    commands.insert(0, 'Set-MpPreference -DisableRealtimeMonitoring $true')
+    commands.append('Set-MpPreference -DisableRealtimeMonitoring $false')
+
+    # Compute a deterministic fingerprint to determine whether image needs
+    # to be regenerated.
+    fingerprint = {
+        'instance_config': config,
+        'user_data': WINDOWS_USER_DATA,
+        'initial_bootstrap': WINDOWS_BOOTSTRAP_POWERSHELL,
+        'bootstrap_commands': commands,
+    }
+
+    fingerprint = json.dumps(fingerprint, sort_keys=True)
+    fingerprint = hashlib.sha256(fingerprint.encode('utf-8')).hexdigest()
+
+    # Find existing AMIs with this name and delete the ones that are invalid.
+    # Store a reference to a good image so it can be returned one the
+    # image state is reconciled.
+    images = ec2resource.images.filter(
+        Filters=[{'Name': 'name', 'Values': [name]}])
+
+    existing_image = None
+
+    for image in images:
+        if image.tags is None:
+            print('image %s for %s lacks required tags; removing' % (
+                image.id, image.name))
+            remove_ami(ec2resource, image)
+        else:
+            tags = {t['Key']: t['Value'] for t in image.tags}
+
+            if tags.get('HGIMAGEFINGERPRINT') == fingerprint:
+                existing_image = image
+            else:
+                print('image %s for %s has wrong fingerprint; removing' % (
+                      image.id, image.name))
+                remove_ami(ec2resource, image)
+
+    if existing_image:
+        return existing_image
+
+    print('no suitable Windows development image found; creating one...')
+
+    with create_temp_windows_ec2_instances(c, config) as instances:
+        assert len(instances) == 1
+        instance = instances[0]
+
+        wait_for_ssm(ssmclient, [instance])
+
+        # On first boot, install various Windows updates.
+        # We would ideally use PowerShell Remoting for this. However, there are
+        # trust issues that make it difficult to invoke Windows Update
+        # remotely. So we use SSM, which has a mechanism for running Windows
+        # Update.
+        print('installing Windows features...')
+        run_ssm_command(
+            ssmclient,
+            [instance],
+            'AWS-RunPowerShellScript',
+            {
+                'commands': WINDOWS_BOOTSTRAP_POWERSHELL.split('\n'),
+            },
+        )
+
+        # Reboot so all updates are fully applied.
+        print('rebooting instance %s' % instance.id)
+        ec2client.reboot_instances(InstanceIds=[instance.id])
+
+        time.sleep(15)
+
+        print('waiting for Windows Remote Management to come back...')
+        client = wait_for_winrm(instance.public_ip_address, 'Administrator',
+                                c.automation.default_password())
+        print('established WinRM connection to %s' % instance.id)
+        instance.winrm_client = client
+
+        print('bootstrapping instance...')
+        run_powershell(instance.winrm_client, '\n'.join(commands))
+
+        print('bootstrap completed; stopping %s to create image' % instance.id)
+        instance.stop()
+
+        ec2client.get_waiter('instance_stopped').wait(
+            InstanceIds=[instance.id],
+            WaiterConfig={
+                'Delay': 5,
+            })
+        print('%s is stopped' % instance.id)
+
+        image = instance.create_image(
+            Name=name,
+            Description='Mercurial Windows development environment',
+        )
+
+        image.create_tags(Tags=[
+            {
+                'Key': 'HGIMAGEFINGERPRINT',
+                'Value': fingerprint,
+            },
+        ])
+
+        print('waiting for image %s' % image.id)
+
+        ec2client.get_waiter('image_available').wait(
+            ImageIds=[image.id],
+        )
+
+        print('image %s available as %s' % (image.id, image.name))
+
+        return image
+
+
+@contextlib.contextmanager
+def temporary_windows_dev_instances(c: AWSConnection, image, instance_type,
+                                    prefix='hg-', disable_antivirus=False):
+    """Create a temporary Windows development EC2 instance.
+
+    Context manager resolves to the list of ``EC2.Instance`` that were created.
+    """
+    config = {
+        'BlockDeviceMappings': [
+            {
+                'DeviceName': '/dev/sda1',
+                'Ebs': {
+                    'DeleteOnTermination': True,
+                    'VolumeSize': 32,
+                    'VolumeType': 'gp2',
+                },
+            }
+        ],
+        'ImageId': image.id,
+        'InstanceInitiatedShutdownBehavior': 'stop',
+        'InstanceType': instance_type,
+        'KeyName': '%sautomation' % prefix,
+        'MaxCount': 1,
+        'MinCount': 1,
+        'SecurityGroupIds': [c.security_groups['windows-dev-1'].id],
+    }
+
+    with create_temp_windows_ec2_instances(c, config) as instances:
+        if disable_antivirus:
+            for instance in instances:
+                run_powershell(
+                    instance.winrm_client,
+                    'Set-MpPreference -DisableRealtimeMonitoring $true')
+
+        yield instances