[llvm-windres] Resolve the --preprocessor executable in $PATH (#75390)

The llvm::sys::ExecuteAndWait function doesn't resolve the file to be
executed from $PATH - i.e. it is similar to execv(), not execvp().

Due to this, specifying a --preprocessor argument to llvm-windres only
worked if it specified an absolute path to the preprocessor executable.
This was observed as one of the issues in
https://github.com/msys2/MINGW-packages/pull/19157.

Before d2fa6b694c2052cef1ddd507f6569bc84e3bbe35, this usage of
--preprocessor seemed to work, because the first argument of Args[] was
ignored and llvm-windres just executed the autodetected clang executable
regardless.

Also improve the error messages printed if preprocessing failed. (If the
preprocessor executable was started but itself returned an error, we
don't get any error string.)

1 files changed, 15 insertions, 2 deletions

diff --git a/llvm/tools/llvm-rc/llvm-rc.cpp b/llvm/tools/llvm-rc/llvm-rc.cpp
index 4375697..27fb030 100644
--- a/llvm/tools/llvm-rc/llvm-rc.cpp
+++ b/llvm/tools/llvm-rc/llvm-rc.cpp
@@ -248,10 +248,17 @@ void preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
   SmallVector<StringRef, 8> Args = {
       Clang, "--driver-mode=gcc", "-target", Opts.Triple, "-E",
       "-xc", "-DRC_INVOKED"};
+  std::string PreprocessorExecutable;
   if (!Opts.PreprocessCmd.empty()) {
     Args.clear();
     for (const auto &S : Opts.PreprocessCmd)
       Args.push_back(S);
+    if (!sys::fs::can_execute(Args[0])) {
+      if (auto P = sys::findProgramByName(Args[0])) {
+        PreprocessorExecutable = *P;
+        Args[0] = PreprocessorExecutable;
+      }
+    }
   }
   for (const auto &S : Opts.PreprocessArgs)
     Args.push_back(S);
@@ -269,9 +276,15 @@ void preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
   }
   // The llvm Support classes don't handle reading from stdout of a child
   // process; otherwise we could avoid using a temp file.
-  int Res = sys::ExecuteAndWait(Args[0], Args);
+  std::string ErrMsg;
+  int Res =
+      sys::ExecuteAndWait(Args[0], Args, /*Env=*/std::nullopt, /*Redirects=*/{},
+                          /*SecondsToWait=*/0, /*MemoryLimit=*/0, &ErrMsg);
   if (Res) {
-    fatalError("llvm-rc: Preprocessing failed.");
+    if (!ErrMsg.empty())
+      fatalError("llvm-rc: Preprocessing failed: " + ErrMsg);
+    else
+      fatalError("llvm-rc: Preprocessing failed.");
   }
 }