Name the offending path in the NUL character error and add a test

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011BxKvJgr98DbCiLT3aKnmu

Author: chencheng mao

cli/filelister.cpp

@@ -168,8 +168,9 @@ std::string FileLister::addFiles(std::list<FileWithDetails> &files, const std::s
     if (path.empty())
         return "no path specified";
 
-    if (path.find('\0') != std::string::npos)
-        return "path contains a NUL character";
+    const std::string::size_type nulPos = path.find('\0');
+    if (nulPos != std::string::npos)
+        return "path '" + path.substr(0, nulPos) + "' contains a NUL character";
 
     std::list<FileWithDetails> filesSorted;
 
@@ -288,8 +289,9 @@ std::string FileLister::addFiles(std::list<FileWithDetails> &files, const std::s
     if (path.empty())
         return "no path specified";
 
-    if (path.find('\0') != std::string::npos)
-        return "path contains a NUL character";
+    const std::string::size_type nulPos = path.find('\0');
+    if (nulPos != std::string::npos)
+        return "path '" + path.substr(0, nulPos) + "' contains a NUL character";
 
     std::string corrected_path = path;
     if (endsWith(corrected_path, '/'))

test/testfilelister.cpp

@@ -36,6 +36,7 @@ class TestFileLister : public TestFixture {
     void run() override {
         TEST_CASE(recursiveAddFiles);
         TEST_CASE(recursiveAddFilesEmptyPath);
+        TEST_CASE(recursiveAddFilesNulInPath);
         TEST_CASE(excludeFile1);
         TEST_CASE(excludeFile2);
         TEST_CASE(excludeDir);
@@ -110,6 +111,13 @@ class TestFileLister : public TestFixture {
         ASSERT_EQUALS("no path specified", err);
     }
 
+    void recursiveAddFilesNulInPath() const {
+        std::list<FileWithDetails> files;
+        const std::string err = FileLister::recursiveAddFiles(files, std::string("lib\0", 4), {}, PathMatch());
+        ASSERT_EQUALS("path 'lib' contains a NUL character", err);
+        ASSERT(files.empty());
+    }
+
     void excludeFile1() const {
         const std::string basedir = findBaseDir();