httpserver: Raise error when adjusted site addresses clash at startup

See discussion on #2015 for how this situation was discovered. For a
Caddyfile like this:

	localhost {
		...
	}
	:2015 {
		...
	}

Running Caddy like this:

	caddy -host localhost

Produces two sites both defined as `localhost:2015` because the flag
changes the default host value to be `localhost`. This should be an
error since the sites are not distinct and it is confusing. It can also
cause issues with TLS handshakes loading the wrong cert, as the linked
discussion shows.

caddy.go

@@ -612,7 +612,7 @@ func ValidateAndExecuteDirectives(cdyfile Input, inst *Instance, justValidate bo
 
 	sblocks, err = inst.context.InspectServerBlocks(cdyfile.Path(), sblocks)
 	if err != nil {
-		return err
+		return fmt.Errorf("error inspecting server blocks: %v", err)
 	}
 
 	return executeDirectives(inst, cdyfile.Path(), stype.Directives(), sblocks, justValidate)

caddyhttp/httpserver/plugin.go

@@ -117,12 +117,14 @@ func (h *httpContext) saveConfig(key string, cfg *SiteConfig) {
 // executing directives and otherwise prepares the directives to
 // be parsed and executed.
 func (h *httpContext) InspectServerBlocks(sourceFile string, serverBlocks []caddyfile.ServerBlock) ([]caddyfile.ServerBlock, error) {
+	siteAddrs := make(map[string]string)
+
 	// For each address in each server block, make a new config
 	for _, sb := range serverBlocks {
 		for _, key := range sb.Keys {
 			key = strings.ToLower(key)
 			if _, dup := h.keysToSiteConfigs[key]; dup {
-				return serverBlocks, fmt.Errorf("duplicate site address: %s", key)
+				return serverBlocks, fmt.Errorf("duplicate site key: %s", key)
 			}
 			addr, err := standardizeAddress(key)
 			if err != nil {
@@ -138,6 +140,23 @@ func (h *httpContext) InspectServerBlocks(sourceFile string, serverBlocks []cadd
 				addr.Port = Port
 			}
 
+			// Make sure the adjusted site address is distinct
+			addrCopy := addr // make copy so we don't disturb the original, carefully-parsed address struct
+			if addrCopy.Port == "" && Port == DefaultPort {
+				addrCopy.Port = Port
+			}
+			addrStr := strings.ToLower(addrCopy.String())
+			if otherSiteKey, dup := siteAddrs[addrStr]; dup {
+				err := fmt.Errorf("duplicate site address: %s", addrStr)
+				if (addrCopy.Host == Host && Host != DefaultHost) ||
+					(addrCopy.Port == Port && Port != DefaultPort) {
+					err = fmt.Errorf("site defined as %s is a duplicate of %s because of modified "+
+						"default host and/or port values (usually via -host or -port flags)", key, otherSiteKey)
+				}
+				return serverBlocks, err
+			}
+			siteAddrs[addrStr] = key
+
 			// If default HTTP or HTTPS ports have been customized,
 			// make sure the ACME challenge ports match
 			var altHTTPPort, altTLSSNIPort string

caddyhttp/httpserver/plugin_test.go

@@ -153,6 +153,23 @@ func TestInspectServerBlocksWithCustomDefaultPort(t *testing.T) {
 	}
 }
 
+// See discussion on PR #2015
+func TestInspectServerBlocksWithAdjustedAddress(t *testing.T) {
+	Port = DefaultPort
+	Host = "example.com"
+	filename := "Testfile"
+	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)
+	input := strings.NewReader("example.com {\n}\n:2015 {\n}")
+	sblocks, err := caddyfile.Parse(filename, input, nil)
+	if err != nil {
+		t.Fatalf("Expected no error setting up test, got: %v", err)
+	}
+	_, err = ctx.InspectServerBlocks(filename, sblocks)
+	if err == nil {
+		t.Fatalf("Expected an error because site definitions should overlap, got: %v", err)
+	}
+}
+
 func TestInspectServerBlocksCaseInsensitiveKey(t *testing.T) {
 	filename := "Testfile"
 	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)