Use the Windows AMI for launch-windows by default

bastion/ami.go

@@ -34,17 +34,32 @@ func GetAndValidateAmi(sess *session.Session, input string, instance_type string
 		return ami, nil
 	}
 
-	input, err := GetArchitecture(sess, instance_type)
+	parameter, err := amiParameterForAlias(input, func() (string, error) {
+		return GetArchitecture(sess, instance_type)
+	})
 	if err != nil {
 		return "", err
 	}
 
-	if parameter, ok := amis[input]; ok {
-		ami, err := GetAmiFromParameter(sess, parameter)
+	return GetAmiFromParameter(sess, parameter)
+}
+
+// amiParameterForAlias returns the SSM parameter path for a named AMI alias.
+// Amazon Linux aliases (and unrecognised names) are resolved to the image
+// matching the instance type's architecture via detectArch. Other named
+// aliases, such as "windows", map directly to their own parameter.
+func amiParameterForAlias(input string, detectArch func() (string, error)) (string, error) {
+	alias := input
+	if _, known := amis[input]; !known || strings.HasPrefix(input, "amazon-linux") {
+		arch, err := detectArch()
 		if err != nil {
 			return "", err
 		}
-		return ami, nil
+		alias = arch
+	}
+
+	if parameter, ok := amis[alias]; ok {
+		return parameter, nil
 	}
 	return "", errors.New("unable to find ami")
 }

bastion/ami_test.go

@@ -0,0 +1,51 @@
+package bastion
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestAmiParameterForAlias(t *testing.T) {
+	cases := []struct {
+		name       string
+		input      string
+		arch       string
+		want       string
+		wantDetect bool
+	}{
+		{"windows alias is not replaced by architecture", "windows", "amazon-linux", amis["windows"], false},
+		{"amazon-linux on x86_64", "amazon-linux", "amazon-linux", amis["amazon-linux"], true},
+		{"amazon-linux on arm64", "amazon-linux", "amazon-linux-arm64", amis["amazon-linux-arm64"], true},
+		{"amazon-linux-arm64 follows instance architecture", "amazon-linux-arm64", "amazon-linux", amis["amazon-linux"], true},
+		{"unknown alias falls back to architecture", "something-else", "amazon-linux-arm64", amis["amazon-linux-arm64"], true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			detected := false
+			got, err := amiParameterForAlias(tc.input, func() (string, error) {
+				detected = true
+				return tc.arch, nil
+			})
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != tc.want {
+				t.Errorf("got %q, want %q", got, tc.want)
+			}
+			if detected != tc.wantDetect {
+				t.Errorf("architecture detection called = %v, want %v", detected, tc.wantDetect)
+			}
+		})
+	}
+}
+
+func TestAmiParameterForAliasDetectError(t *testing.T) {
+	wantErr := errors.New("describe failed")
+	_, err := amiParameterForAlias("amazon-linux", func() (string, error) {
+		return "", wantErr
+	})
+	if !errors.Is(err, wantErr) {
+		t.Fatalf("got error %v, want %v", err, wantErr)
+	}
+}