[llvm-windres] Resolve the --preprocessor executable in $PATH

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
msys2/MINGW-packages#19157.

Before d2fa6b6, 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.)

Author: mstorsjo

llvm/test/tools/llvm-rc/windres-preproc.test

@@ -7,4 +7,18 @@
 ; RUN: llvm-windres -### --include-dir %p/incdir1 --include %p/incdir2 "-DFOO1=\"foo bar\"" -UFOO2 -D FOO3 --preprocessor-arg "-DFOO4=\"baz baz\"" "-DFOO5=bar" %p/Inputs/empty.rc %t.res --use-temp-file | FileCheck %s --check-prefix=CHECK1
 ; CHECK1: {{^}} "clang" "--driver-mode=gcc" "-target" "{{.*}}-{{.*}}{{mingw32|windows-gnu}}" "-E" "-xc" "-DRC_INVOKED" "-I" "{{.*}}incdir1" "-I" "{{.*}}incdir2" "-D" "FOO1=\"foo bar\"" "-U" "FOO2" "-D" "FOO3" "-DFOO4=\"baz baz\"" "-D" "FOO5=bar" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc"{{$}}
 ; RUN: llvm-windres -### --preprocessor "i686-w64-mingw32-gcc -E -DFOO=\\\"foo\\ bar\\\"" %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK2
-; CHECK2: {{^}} "i686-w64-mingw32-gcc" "-E" "-DFOO=\"foo bar\"" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc"{{$}}
+; CHECK2: {{^}} "{{.*}}i686-w64-mingw32-gcc" "-E" "-DFOO=\"foo bar\"" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc"{{$}}
+
+;; Test resolving the --preprocessor executable from PATH
+
+; RUN: rm -rf %t-bin/testbin
+; RUN: mkdir -p %t-bin/testbin
+; RUN: ln -s llvm-windres %t-bin/testbin/i686-w64-mingw32-gcc
+; RUN: env PATH=%t-bin/testbin llvm-windres -### --preprocessor i686-w64-mingw32-gcc --preprocessor-arg -E --preprocessor-arg -xc -DRC_INVOKED %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK3
+; CHECK3: {{^}} "{{.*}}/testbin/i686-w64-mingw32-gcc" "-E" "-xc" "-D" "RC_INVOKED" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc"{{$}}
+
+
+;; Test error messages when unable to execute the preprocessor.
+
+; RUN: not llvm-windres --preprocessor intentionally-missing-executable %p/Inputs/empty.rc %t.res 2>&1 | FileCheck %s --check-prefix=CHECK4
+; CHECK4: llvm-rc: Preprocessing failed: Executable "intentionally-missing-executable" doesn't exist!

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.");
   }
 }