Dlopen backend 5 (#779)

Major change to the backend that allows for pluggable versions of llama.cpp/ggml. This was squashed merged from dlopen_backend_5 where the history is preserved.

gpt4all-backend/dlhandle.h

@@ -0,0 +1,101 @@
+#ifndef DLHANDLE_H
+#define DLHANDLE_H
+#ifndef _WIN32
+#include <string>
+#include <stdexcept>
+#include <utility>
+#include <dlfcn.h>
+
+
+
+class Dlhandle {
+    void *chandle;
+
+public:
+    class Exception : public std::runtime_error {
+    public:
+        using std::runtime_error::runtime_error;
+    };
+
+    Dlhandle() : chandle(nullptr) {}
+    Dlhandle(const std::string& fpath, int flags = RTLD_LAZY) {
+        chandle = dlopen(fpath.c_str(), flags);
+        if (!chandle) {
+            throw Exception("dlopen(\""+fpath+"\"): "+dlerror());
+        }
+    }
+    Dlhandle(const Dlhandle& o) = delete;
+    Dlhandle(Dlhandle&& o) : chandle(o.chandle) {
+        o.chandle = nullptr;
+    }
+    ~Dlhandle() {
+        if (chandle) dlclose(chandle);
+    }
+
+    auto operator =(Dlhandle&& o) {
+        chandle = std::exchange(o.chandle, nullptr);
+    }
+
+    bool is_valid() const {
+        return chandle != nullptr;
+    }
+    operator bool() const {
+        return is_valid();
+    }
+
+    template<typename T>
+    T* get(const std::string& fname) {
+        auto fres = reinterpret_cast<T*>(dlsym(chandle, fname.c_str()));
+        return (dlerror()==NULL)?fres:nullptr;
+    }
+    auto get_fnc(const std::string& fname) {
+        return get<void*(...)>(fname);
+    }
+};
+#else
+#include <string>
+#include <exception>
+#include <stdexcept>
+#include <windows.h>
+#include <libloaderapi.h>
+
+
+
+class Dlhandle {
+    HMODULE chandle;
+
+public:
+    class Exception : public std::runtime_error {
+    public:
+        using std::runtime_error::runtime_error;
+    };
+
+    Dlhandle() : chandle(nullptr) {}
+    Dlhandle(const std::string& fpath) {
+        chandle = LoadLibraryA(fpath.c_str());
+        if (!chandle) {
+            throw Exception("dlopen(\""+fpath+"\"): Error");
+        }
+    }
+    Dlhandle(const Dlhandle& o) = delete;
+    Dlhandle(Dlhandle&& o) : chandle(o.chandle) {
+        o.chandle = nullptr;
+    }
+    ~Dlhandle() {
+        if (chandle) FreeLibrary(chandle);
+    }
+
+    bool is_valid() const {
+        return chandle != nullptr;
+    }
+
+    template<typename T>
+    T* get(const std::string& fname) {
+        return reinterpret_cast<T*>(GetProcAddress(chandle, fname.c_str()));
+    }
+    auto get_fnc(const std::string& fname) {
+        return get<void*(...)>(fname);
+    }
+};
+#endif
+#endif // DLHANDLE_H