fix: check if the file already starts with a UTF-8 BOM (#8551)

fix #8512

If the file is already in UTF-8 BOM format, there's no need to convert
it again. If it's converted again, it will result in garbled characters.

---------

Co-authored-by: beyondkmp <beyondkmkp@gmail.com>

.changeset/six-spoons-heal.md

@@ -0,0 +1,5 @@
+---
+"app-builder-lib": patch
+---
+
+Check if the file already starts with a UTF-8 BOM

packages/app-builder-lib/src/targets/nsis/nsisLicense.ts

@@ -1,3 +1,4 @@
+import { log } from "builder-util"
 import { lcid } from "../../util/langs"
 import { getLicenseFiles, getNotLocalizedLicenseFile } from "../../util/license"
 import * as path from "path"
@@ -9,17 +10,27 @@ import * as fs from "fs"
 
 function convertFileToUtf8WithBOMSync(filePath: string): boolean {
   try {
+    const UTF8_BOM_HEADER = Buffer.from([0xef, 0xbb, 0xbf])
     const data = fs.readFileSync(filePath)
-    // UTF-8 BOM is EF BB BF
-    const BOM = Buffer.from([0xef, 0xbb, 0xbf])
-    const dataWithBOM = Buffer.concat([BOM, data])
+
+    // Check if the file already starts with a UTF-8 BOM
+    log.debug({ file: log.filePath(filePath) }, "checking file for BOM header")
+    if (data.length >= UTF8_BOM_HEADER.length && data.subarray(0, UTF8_BOM_HEADER.length).equals(UTF8_BOM_HEADER)) {
+      log.debug({ file: log.filePath(filePath) }, "file is already in BOM format, skipping conversion.")
+      return true
+    }
+
+    // If not, add the BOM
+    const dataWithBOM = Buffer.concat([UTF8_BOM_HEADER, data])
     fs.writeFileSync(filePath, dataWithBOM)
+    log.debug({ file: log.filePath(filePath) }, "file successfully converted to UTF-8 with BOM")
     return true
-  } catch (err) {
-    console.error("Failed to convert file to UTF-8 with BOM: ", err)
+  } catch (err: any) {
+    log.error({ file: log.filePath(filePath), message: err.message ?? err.stack }, "unable to convert file to UTF-8 with BOM")
     return false
   }
 }
+
 export async function computeLicensePage(packager: WinPackager, options: NsisOptions, scriptGenerator: NsisScriptGenerator, languages: Array<string>): Promise<void> {
   const license = await getNotLocalizedLicenseFile(options.license, packager)
   if (license != null) {