[libclang] Fall back to getMainExecutable when dladdr fails

[aykevl]

Summary

musl-libc doesn't support dladdr in statically linked binaries:

Are you using static or dynamic linking? If static, dladdr is just a
stub that always fails. It could be implemented to work under some
conditions, but it would be highly dependent on what options you
compile the binary with, since by default static binaries do not
contain the bloat that would be needed to perform introspection.

Source: https://www.openwall.com/lists/musl/2013/01/15/25 (in response to a bug report).

Libclang unfortunately uses dladdr to find the ResourcesPath so will fail if it is linked statically on Alpine Linux. This patch fixes this issue by falling back to getMainExecutable if dladdr returns an error.

Reference: https://github.com/llvm/llvm-project/issues/40641#issuecomment-981011427

[MaskRay]

I think the file is linked into libclang.so.

The resource directory is relative to`libclang.so` but may not be relative to the main executable... So getMainExecutable will be incorrect.

clang/tools/libclang/CIndexer.cpp

142

Diagnostics generally are not capitalized: https://llvm.org/docs/CodingStandards.html#error-and-warning-messages

You can ignore the numerous anti-examples (I know, lots of inconsistency)...

[aykevl]

This file is also linked into libclang.a.

When linking libclang.a on Alpine Linux, dladdr is not available and the only fallback I can think of is getMainExecutable.

I'm open to other solutions. The code as-is essentially compiles to a segmentation fault, which is bad.

clang/tools/libclang/CIndexer.cpp

#else
   cygwin_conv_to_full_posix_path(w32path, path);
 #endif
 #endif
 
   LibClangPath += path;
 #elif defined(_AIX)
   getClangResourcesPathImplAIX(LibClangPath);
 #else
-  // This silly cast below avoids a C++ warning.
   Dl_info info;
-  if (dladdr((void *)(uintptr_t)clang_createTranslationUnit, &info) == 0)
-    llvm_unreachable("Call to dladdr() failed");
-
+  std::string Path;
+  // This silly cast below avoids a C++ warning.
+  if (dladdr((void *)(uintptr_t)clang_createTranslationUnit, &info) != 0) {
     // We now have the CIndex directory, locate clang relative to it.
     LibClangPath += info.dli_fname;
+  } else if (!(Path = llvm::sys::fs::getMainExecutable(nullptr, nullptr)).empty()) {
+    // If we can't get the path using dladdr, try to get the main executable
+    // path. This may be needed when we're statically linking libclang with
+    // musl libc, for example.
+    LibClangPath += Path;
+  } else {
+    // It's rather unlikely we end up here. But it could happen, so report an
+    // error instead of crashing.
+    llvm::report_fatal_error("Could not locate Clang resource path");
+  }
+
 #endif
 
   // Cache our result.
   ResourcesPath = driver::Driver::GetResourcesPath(LibClangPath);
   return ResourcesPath;
 }
 
 StringRef CIndexer::getClangToolchainPath() {