allow pomerium to be embedded as a library (#3415)

ui/embed_ext.go

@@ -0,0 +1,37 @@
+//go:build embed_pomerium
+
+package ui
+
+import (
+	"fmt"
+	"io/fs"
+)
+
+var (
+	// ExtUIFS must be set to provide access to UI dist/ files
+	ExtUIFS fs.FS
+)
+
+func openFile(name string) (f fs.File, etag string, err error) {
+	if ExtUIFS == nil {
+		return nil, "", fmt.Errorf("ui package was incorrectly compiled with embed_pomerium yet no FS was provided")
+	}
+	f, err = ExtUIFS.Open(name)
+	if err != nil {
+		return nil, "", fmt.Errorf("open %s: %w", name, err)
+	}
+
+	fi, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+		return nil, "", err
+	}
+
+	modTime := fi.ModTime()
+	if modTime.IsZero() {
+		modTime = startTime
+	}
+	etag = fmt.Sprintf("%x", modTime.UnixNano())
+
+	return f, etag, nil
+}