feat(policy): inbound whitelist + subject-regex filter

internal/policy/inbound.go

@@ -0,0 +1,29 @@
+package policy
+
+import "regexp"
+
+// InboundRule captures one account's read-side filtering.
+type InboundRule struct {
+	WhitelistInEnabled bool
+	WhitelistIn        []string
+	SubjectRegex       *regexp.Regexp // nil = no subject filter
+}
+
+// CompileSubject compiles a subject filter; empty pattern => (nil, nil).
+func CompileSubject(pattern string) (*regexp.Regexp, error) {
+	if pattern == "" {
+		return nil, nil
+	}
+	return regexp.Compile(pattern)
+}
+
+// Allows reports whether a message with the given sender and subject is visible.
+func (r InboundRule) Allows(from, subject string) bool {
+	if r.WhitelistInEnabled && !MatchAddress(r.WhitelistIn, from) {
+		return false
+	}
+	if r.SubjectRegex != nil && !r.SubjectRegex.MatchString(subject) {
+		return false
+	}
+	return true
+}

internal/policy/inbound_test.go

@@ -0,0 +1,43 @@
+package policy
+
+import "testing"
+
+func TestInboundAllows(t *testing.T) {
+	re, err := CompileSubject("(?i)invoice")
+	if err != nil {
+		t.Fatalf("compile: %v", err)
+	}
+	rule := InboundRule{
+		WhitelistInEnabled: true,
+		WhitelistIn:        []string{"@trusted.com"},
+		SubjectRegex:       re,
+	}
+	cases := []struct {
+		from, subject string
+		want          bool
+	}{
+		{"bob@trusted.com", "Your Invoice #5", true},
+		{"bob@trusted.com", "lunch?", false},        // subject fails
+		{"eve@evil.com", "Invoice", false},          // sender fails
+		{"eve@evil.com", "lunch?", false},           // both fail
+	}
+	for _, c := range cases {
+		if got := rule.Allows(c.from, c.subject); got != c.want {
+			t.Fatalf("Allows(%q,%q)=%v want %v", c.from, c.subject, got, c.want)
+		}
+	}
+}
+
+func TestInboundNoFiltersAllowsAll(t *testing.T) {
+	rule := InboundRule{} // whitelist disabled, no regex
+	if !rule.Allows("anyone@anywhere.com", "anything") {
+		t.Fatal("empty rule must allow everything")
+	}
+}
+
+func TestCompileSubjectEmpty(t *testing.T) {
+	re, err := CompileSubject("")
+	if err != nil || re != nil {
+		t.Fatalf("empty pattern: re=%v err=%v", re, err)
+	}
+}