[MachO] Fix two bugs with reexported dylibs

[comex]

Summary

When linking against a Mach-O dylib that reexports other dylibs, lld searches for the reexported dylibs in multiple places: it tries the full path referenced by LC_REEXPORT_DYLIB (with and without sysroot prepended), and also searches in -L directories for a library matching the basename of that path. This patch fixes two bugs related to that search:

1. When looking in -L paths, MachOLinkingContext::findIndirectDylib() passed the full basename to searchLibrary(), e.g. "libfoo.dylib". But searchLibrary() prepends "lib" and appends ".dylib", so it ended up looking for "liblibfoo.dylib.dylib". Fix findIndirectDylib() to strip "lib" and ".dylib".

2. If the library is not found for whatever reason, there was no error handling. entry.file would just be set to null, which would later trigger an assert "assert(dylib.file)" in a different function. To fix this, print an error if find() returns; since that (intentionally) does not abort the process and still leaves entry.file as null, change the function with the assert to tolerate that case.

[comex]

Oops, forgot to mention:

This will reproduce the issue:

echo 'int x = 2;' > sub.c
clang -dynamiclib -o libsub.dylib sub.c -install_name /usr/local/lib/libsub.dylib
clang -dynamiclib -o libreexporter.dylib -x c /dev/null -Wl,-reexport-lsub -L.
echo 'extern int x; int main() { return x; }' > client.c
clang -c -o client.o client.c
# crashes:
/path/to/ld64.lld -o client -L. client.o -lreexporter -lSystem

Also, I don't have commit access, so someone will have to commit the patch for me.

[lhames]

Is it possible to write a test case for this? Otherwise LGTM. Thanks for all the work on this! :)

lib/ReaderWriter/MachO/File.h

 //===- lib/ReaderWriter/MachO/File.h ----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_READER_WRITER_MACHO_FILE_H
 #define LLD_READER_WRITER_MACHO_FILE_H
 
 #include "Atoms.h"
 #include "DebugInfo.h"
 #include "MachONormalizedFile.h"
+#include "lld/Common/ErrorHandler.h"
 #include "lld/Core/SharedLibraryFile.h"
 #include "lld/Core/Simple.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/Format.h"
 #include <unordered_map>
 
 namespace lld {
public:
   void setCompatVersion(uint32_t version) { _compatVersion = version; }
   void setCurrentVersion(uint32_t version) { _currentVersion = version; }
 
   typedef std::function<MachODylibFile *(StringRef)> FindDylib;
 
   void loadReExportedDylibs(FindDylib find) {
     for (ReExportedDylib &entry : _reExportedDylibs) {
       entry.file = find(entry.path);
+      if (!entry.file)
+        error(Twine("could not load reexported dylib ") + entry.path);
     }
   }
 
   StringRef getDSOName() const override { return _installName; }
 
   std::error_code doParse() override {
     // Convert binary file to normalized mach-o.
     auto normFile = normalized::readBinary(_mb, _ctx->arch());
if (entry != _nameToAtom.end()) {
                                                             installName,
                                                             weakDef);
       entry->second.atom = atom;
       return atom;
     }
 
     // Next, check if symbol is implemented in some re-exported dylib.
     for (const ReExportedDylib &dylib : _reExportedDylibs) {
-      assert(dylib.file);
+      if (!dylib.file)
+        continue;
       auto atom = dylib.file->exports(name, installName);
       if (atom.get())
         return atom;
     }
 
     // Symbol not exported or re-exported by this dylib.
     return nullptr;
   }

lib/ReaderWriter/MachO/MachOLinkingContext.cpp

MachODylibFile* MachOLinkingContext::findIndirectDylib(StringRef path) {
   if (pos != _pathToDylibMap.end())
     return pos->second;
 
   // Search -L paths if of the form "libXXX.dylib"
   std::pair<StringRef, StringRef> split = path.rsplit('/');
   StringRef leafName = split.second;
   if (leafName.startswith("lib") && leafName.endswith(".dylib")) {
     // FIXME: Need to enhance searchLibrary() to only look for .dylib
-    auto libPath = searchLibrary(leafName);
+    auto libPath = searchLibrary(leafName.slice(3, leafName.size() - 6));
     if (libPath)
       return loadIndirectDylib(libPath.getValue());
   }
 
   // Try full path with sysroot.
   for (StringRef sysPath : _syslibRoots) {
     SmallString<256> fullPath;
     fullPath.assign(sysPath);