[libFuzzer] Print a correct error message when a director can't be opened

[alex]

Summary

Currently if the directory cannot be opened for a reason other than non-existence (e.g. too many open file descriptors) the error message printed is incredibly confusing.

[morehouse]

LGTM.

[alex]

@morehouse I'm not familiar with the LLVM development process, what's the next step now that this has been reviewed?

[morehouse]

If you're new to LLVM, you won't have commit access, so I'll land this patch for you.

After you have a few good patches submitted, you can request commit access: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access.

[alex]

Thanks!

lib/fuzzer/FuzzerIOPosix.cpp

 void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
                              Vector<std::string> *V, bool TopDir) {
   auto E = GetEpoch(Dir);
   if (Epoch)
     if (E && *Epoch >= E) return;
 
   DIR *D = opendir(Dir.c_str());
   if (!D) {
-    Printf("No such directory: %s; exiting\n", Dir.c_str());
+    Printf("%s: %s; exiting\n", strerror(errno), Dir.c_str());
     exit(1);
   }
   while (auto E = readdir(D)) {
     std::string Path = DirPlusFile(Dir, E->d_name);
     if (E->d_type == DT_REG || E->d_type == DT_LNK ||
         (E->d_type == DT_UNKNOWN && IsFile(Path)))
       V->push_back(Path);
     else if ((E->d_type == DT_DIR ||

test/fuzzer/fuzzer-dirs.test

 RUN: cat %t/SUB1/f256 %t/SUB1/f256 %t/SUB1/f256 %t/SUB1/f256 > %t/SUB1/f1024
 RUN: cat %t/SUB1/f1024 %t/SUB1/f1024 %t/SUB1/f1024 %t/SUB1/f1024 > %t/SUB1/f4096
 RUN: cat %t/SUB1/f4096 %t/SUB1/f4096 > %t/SUB1/f8192
 RUN: %t-SimpleTest %t/SUB1 -runs=0 2>&1 | FileCheck %s --check-prefix=LONG
 LONG: INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 8192 bytes
 RUN: rm -rf %t/SUB1
 
 RUN: not %t-SimpleTest NONEXISTENT_DIR 2>&1 | FileCheck %s --check-prefix=NONEXISTENT_DIR
-NONEXISTENT_DIR: No such directory: NONEXISTENT_DIR; exiting
+NONEXISTENT_DIR: No such file or directory: NONEXISTENT_DIR; exiting