Try to avoid crashes in llvm::identify_magic (#11174)

The overload taking a path opens the file and then mmap its contents.
This can cause bus errors when another process truncates the file
while we are trying to read it. Instead just read the first 1024 bytes,
which should be enough for identify_magic to do its work.

lib/Interpreter/DynamicLibraryManager.cpp

@@ -18,6 +18,7 @@
 #include "llvm/Support/DynamicLibrary.h"
 #include "llvm/Support/Path.h"
 
+#include <fstream>
 #include <system_error>
 #include <sys/stat.h>
 
@@ -459,12 +460,26 @@ namespace cling {
       return false;
     }
 
-    file_magic Magic;
-    const std::error_code Error = identify_magic(libFullPath, Magic);
-    if (exists)
-      *exists = !Error;
+    // Do not use the identify_magic overload taking a path: It will open the
+    // file and then mmap its contents, possibly causing bus errors when another
+    // process truncates the file while we are trying to read it. Instead just
+    // read the first 1024 bytes, which should be enough for identify_magic to
+    // do its work.
+    // TODO: Fix the code upstream and consider going back to calling the
+    // convenience function after a future LLVM upgrade.
+    std::ifstream in(libFullPath.str(), std::ios::binary);
+    char header[1024] = {0};
+    in.read(header, sizeof(header));
+    if (in.fail()) {
+      if (exists)
+        *exists = false;
+      return false;
+    }
 
-    bool result = !Error &&
+    StringRef headerStr(header, in.gcount());
+    file_magic Magic = identify_magic(headerStr);
+
+    bool result =
 #ifdef __APPLE__
       (Magic == file_magic::macho_fixed_virtual_memory_shared_lib
        || Magic == file_magic::macho_dynamically_linked_shared_lib