debuginfo: improve robustness of code slightly (#57989)

I hit this iswindows assert locally while frobbing stuff, so I wanted to
remove it and clean up the code slightly to be more careful here about
verifying what it got from the file system.

Author: vtjnash

src/debuginfo.cpp

@@ -568,7 +568,7 @@ void JITDebugInfoRegistry::libc_frames_t::libc_deregister_frame(const char *Entr
 }
 #endif
 
-static bool getObjUUID(llvm::object::MachOObjectFile *obj, uint8_t uuid[16]) JL_NOTSAFEPOINT
+static bool getObjUUID(const object::MachOObjectFile *obj, uint8_t uuid[16]) JL_NOTSAFEPOINT
 {
     for (auto Load : obj->load_commands())
     {
@@ -830,9 +830,6 @@ static objfileentry_t find_object_file(uint64_t fbase, StringRef fname) JL_NOTSA
     std::string debuginfopath;
     uint8_t uuid[16], uuid2[16];
     if (isdarwin) {
-        // Hide Darwin symbols (e.g. CoreFoundation) from non-Darwin systems.
-#ifdef _OS_DARWIN_
-
         size_t msize = (size_t)(((uint64_t)-1) - fbase);
         std::unique_ptr<MemoryBuffer> membuf = MemoryBuffer::getMemBuffer(
                 StringRef((const char *)fbase, msize), "", false);
@@ -843,14 +840,18 @@ static objfileentry_t find_object_file(uint64_t fbase, StringRef fname) JL_NOTSA
             return entry;
         }
 
-        llvm::object::MachOObjectFile *morigobj = (llvm::object::MachOObjectFile*)
-            origerrorobj.get().get();
+        const object::MachOObjectFile *morigobj = dyn_cast<const object::MachOObjectFile>(
+            origerrorobj.get().get());
 
         // First find the uuid of the object file (we'll use this to make sure we find the
         // correct debug symbol file).
-        if (!getObjUUID(morigobj, uuid))
+        if (!morigobj || !getObjUUID(morigobj, uuid))
             return entry;
 
+        // Hide Darwin symbols (e.g. CoreFoundation) from non-Darwin systems.
+#ifndef _OS_DARWIN_
+        return entry;
+#else
         // On macOS, debug symbols are not contained in the dynamic library.
         // Use DBGCopyFullDSYMURLForUUID from the private DebugSymbols framework
         // to make use of spotlight to find the dSYM file. If that fails, lookup
@@ -906,6 +907,7 @@ static objfileentry_t find_object_file(uint64_t fbase, StringRef fname) JL_NOTSA
         if (dsfmwkbundle) {
             CFRelease(dsfmwkbundle);
         }
+#endif
 
         if (objpath.empty()) {
             // Fall back to simple path relative to the dynamic library.
@@ -915,7 +917,6 @@ static objfileentry_t find_object_file(uint64_t fbase, StringRef fname) JL_NOTSA
             debuginfopath += fname.substr(sep + 1);
             objpath = debuginfopath;
         }
-#endif
     }
     else {
         // On Linux systems we need to mmap another copy because of the permissions on the mmap'ed shared library.
@@ -974,15 +975,17 @@ static objfileentry_t find_object_file(uint64_t fbase, StringRef fname) JL_NOTSA
 
         if (isdarwin) {
             // verify the UUID matches
-            if (!getObjUUID((llvm::object::MachOObjectFile*)debugobj, uuid2) ||
-                    memcmp(uuid, uuid2, sizeof(uuid)) != 0) {
+            if (!isa<const object::MachOObjectFile>(debugobj) ||
+                !getObjUUID(cast<const object::MachOObjectFile>(debugobj), uuid2) ||
+                memcmp(uuid, uuid2, sizeof(uuid)) != 0) {
                 return entry;
             }
         }
 
         int64_t slide = 0;
         if (auto *OF = dyn_cast<const object::COFFObjectFile>(debugobj)) {
-            assert(iswindows);
+            if (!iswindows) // the COFF parser accepts some garbage inputs (like empty files) that the other parsers correctly reject, so we can end up here even when we should not
+                return entry;
             slide = OF->getImageBase() - fbase;
         }
         else {