Initial import of libdpf.

Co-authored-by: Cursor <cursoragent@cursor.com>

thirdparty/portable-snippets/once/README.md

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+# Once
+
+This is a simple abstraction for a once which will choose from a few
+different back-ends:
+
+ * If C11 threads are available, use `call_once()`.
+ * On Windows, use
+   [InitOnceExecuteOnce()](https://msdn.microsoft.com/en-us/library/windows/desktop/ms683493(v=vs.85).aspx)
+ * If `PTHREAD_ONCE_INIT` is defined (*i.e.*, if `<pthread.h>` has
+   been included prior to including `once.h`), use `pthread_once()`.
+ * Use `[atomic.h](../atomic)`.

thirdparty/portable-snippets/once/once.h

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+/* Once (v1)
+ * Portable Snippets - https://github.com/nemequ/portable-snippets
+ * Created by Evan Nemerson <evan@nemerson.com>
+ *
+ *   To the extent possible under law, the authors have waived all
+ *   copyright and related or neighboring rights to this code.  For
+ *   details, see the Creative Commons Zero 1.0 Universal license at
+ *   https://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+#if !defined(PSNIP_ONCE__H)
+#define PSNIP_ONCE__H
+
+#define PSNIP_ONCE__BACKEND_ATOMIC  1
+#define PSNIP_ONCE__BACKEND_PTHREAD 2
+#define PSNIP_ONCE__BACKEND_NONE    3
+#define PSNIP_ONCE__BACKEND_C11     11
+#define PSNIP_ONCE__BACKEND_WIN32   32
+
+#include <limits.h>
+
+#if !defined(PSNIP_ONCE_BACKEND)
+#  if defined(__STDC_NO_THREADS__) && __STDC_NO_THREADS__
+#  elif defined(__EMSCRIPTEN__)
+#  elif defined(__has_include)
+#    if __has_include(<threads.h>)
+#      if !defined(__GLIBC__) || (defined(__GLIBC__) && defined(PSNIP_ENABLE_PTHREADS))
+#        include <threads.h>
+#        define PSNIP_ONCE_BACKEND PSNIP_ONCE__BACKEND_C11
+#      endif
+#    endif
+#  elif defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201102L) && !defined(__STDC_NO_THREADS__)
+#    if (defined(__GLIBC__) && (__GLIBC__ < 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ < 16)))
+/* stdc-predef.h didn't include __STDC_NO_THREADS__ until 2.16. */
+#    else
+#      include <threads.h>
+#      define PSNIP_ONCE_BACKEND PSNIP_ONCE__BACKEND_C11
+#    endif
+#  endif
+#endif
+
+#if !defined(PSNIP_ONCE_BACKEND) && defined(_WIN32) && (!defined(WINVER) || (defined(WINVER) && (WINVER >= 0x0600)))
+#  include <Windows.h>
+#  define PSNIP_ONCE_BACKEND PSNIP_ONCE__BACKEND_WIN32
+#endif
+
+#if !defined(PSNIP_ONCE_BACKEND) && defined(PTHREAD_ONCE_INIT)
+#  define PSNIP_ONCE_BACKEND PSNIP_ONCE__BACKEND_PTHREAD
+#endif
+
+#if !defined(PSNIP_ONCE_BACKEND)
+#  include "../atomic/atomic.h"
+#  if !defined(PSNIP_ATOMIC_NOT_FOUND)
+#    define PSNIP_ONCE_BACKEND PSNIP_ONCE__BACKEND_ATOMIC
+#  endif
+#endif
+
+#if !defined(PSNIP_ONCE_BACKEND)
+#  error No once backend found.
+#endif
+
+#if defined(__GNUC__) && (__GNUC__ >= 3)
+#  define PSNIP_ONCE__UNLIKELY(expr) __builtin_expect(!!(expr), !!0)
+#else
+#  define PSNIP_ONCE__UNLIKELY(expr) (!!(expr))
+#endif
+
+#if PSNIP_ONCE_BACKEND == PSNIP_ONCE__BACKEND_C11
+#  define PSNIP_ONCE_INIT ONCE_FLAG_INIT
+typedef once_flag psnip_once;
+#  define psnip_once_call(flag, func) call_once(flag, func)
+#elif PSNIP_ONCE_BACKEND == PSNIP_ONCE__BACKEND_PTHREAD
+#  define PSNIP_ONCE_INIT PTHREAD_ONCE_INIT
+typedef pthread_once_t psnip_once;
+#  define psnip_once_call(flag, func) pthread_once(flag, func)
+#elif PSNIP_ONCE_BACKEND == PSNIP_ONCE__BACKEND_WIN32
+#  define PSNIP_ONCE_INIT INIT_ONCE_STATIC_INIT
+typedef INIT_ONCE psnip_once;
+static BOOL CALLBACK psnip_once__callback_wrap(INIT_ONCE* InitOnce, void* Parameter, void** Context) {
+  (void) Context;
+  (void) InitOnce;
+#if defined(_MSC_VER)
+#  pragma warning(push)
+#  pragma warning(disable:4055)
+#endif
+  ((void (*)(void)) Parameter)();
+#if defined(_MSC_VER)
+#  pragma warning(pop)
+#endif
+  return !0;
+}
+#  if defined(_MSC_VER) && (_MSC_VER >= 1500)
+#    define psnip_once_call(flag, func) \
+  __pragma(warning(push)) \
+  __pragma(warning(disable:4152)) \
+  InitOnceExecuteOnce(flag, &psnip_once__callback_wrap, func, NULL) \
+  __pragma(warning(pop))
+#  else
+#    define psnip_once_call(flag, func) InitOnceExecuteOnce(flag, &psnip_once__callback_wrap, func, NULL)
+#  endif
+#elif PSNIP_ONCE_BACKEND == PSNIP_ONCE__BACKEND_ATOMIC
+#  define PSNIP_ONCE_INIT PSNIP_ATOMIC_VAR_INIT(0)
+typedef psnip_atomic_int32 psnip_once;
+static void psnip_once_call(psnip_once* flag, void (*func)(void)) {
+  psnip_int32_t state = psnip_atomic_int32_load(flag);
+  if (PSNIP_ONCE__UNLIKELY(state == 0)) {
+    if (psnip_atomic_int32_compare_exchange(flag, &state, 1)) {
+      func();
+      psnip_atomic_int32_store(flag, 2);
+    } else {
+      do {
+	/* Spin; another thread is calling the initialization
+	   function. */
+      } while (psnip_atomic_int32_load(flag) == 1);
+    }
+  }
+}
+#elif PSNIP_ONCE_BACKEND == PSNIP_ONCE__BACKEND_NONE
+#  define PSNIP_ONCE_INIT 0
+typedef int psnip_once;
+static void psnip_once_call(psnip_once* flag, void (*func)(void)) {
+  if (*flag == 0) {
+    func();
+    *flag = 1;
+  }
+}
+#endif
+
+#endif /* !defined(PSNIP_ONCE__H) */