[Driver] Do a PATH lookup when using only program name with -no-canonical-prefixes

[phosek]

Summary

When -no-canonical-prefixes option is used and argv0 contains only
a program name, we need to do a PATH lookup to get an executable path,
otherwise the return value won't be a valid path and any subsequent
uses of it (e.g. invoking -cc1) will fail with an error.

This patch fixes PR9576.

[hans]

Thanks!

Please add to the description that this is for PR9576.

tools/driver/driver.cpp

58

What if "clang" is in the current directory? Presumably that should then be preferred over PATH.. will we get that right with this patch?

Another way would be to check access() on Argv0, which is what we do when trying to execute anyway. Is there any downside to that?

[phosek]

tools/driver/driver.cpp

58

I don't think there should be any downside to that approach.

[hans]

lgtm

tools/driver/driver.cpp

 #include <memory>
 #include <set>
 #include <system_error>
 using namespace clang;
 using namespace clang::driver;
 using namespace llvm::opt;
 
 std::string GetExecutablePath(const char *Argv0, bool CanonicalPrefixes) {
-  if (!CanonicalPrefixes)
-    return Argv0;
+  if (!CanonicalPrefixes) {
+    SmallString<128> ExecutablePath(Argv0);
+    // Do a PATH lookup, if there are no directory components.
+    if (llvm::sys::path::filename(ExecutablePath) == ExecutablePath)
+      if (llvm::ErrorOr<std::string> P =
+              llvm::sys::findProgramByName(ExecutablePath))
+        ExecutablePath = *P;
+    return ExecutablePath.str();
+  }
 
   // This just needs to be some symbol in the binary; C++ doesn't
   // allow taking the address of ::main however.
   void *P = (void*) (intptr_t) GetExecutablePath;
   return llvm::sys::fs::getMainExecutable(Argv0, P);
 }
 
 static const char *GetStableCStr(std::set<std::string> &SavedStrings,