Expose entropy on POSIX platforms

This provides a common definition for many POSIX platforms. It
can be disabled.

src/pal/pal_haiku.h

@@ -18,8 +18,11 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      *
+     * Disable the POSIX default implementation of Entropy as not supported on
+     * Haiku.
      */
-    static constexpr uint64_t pal_features = PALPOSIX::pal_features;
+    static constexpr uint64_t pal_features =
+      PALPOSIX::pal_features & ~(Entropy);
 
     /**
      * Haiku requires an explicit no-reserve flag in `mmap` to guarantee lazy

src/pal/pal_posix.h

@@ -12,6 +12,12 @@
 #include <sys/mman.h>
 #include <unistd.h>
 #include <utility>
+#if __has_include(<sys/random.h>)
+#  include <sys/random.h>
+#endif
+#if __has_include(<unistd.h>)
+#  include <unistd.h>
+#endif
 
 extern "C" int puts(const char* str);
 
@@ -113,7 +119,7 @@ namespace snmalloc
      *
      * POSIX systems are assumed to support lazy commit.
      */
-    static constexpr uint64_t pal_features = LazyCommit;
+    static constexpr uint64_t pal_features = LazyCommit | Entropy;
 
     static constexpr size_t page_size = Aal::smallest_page_size;
 
@@ -264,5 +270,31 @@ namespace snmalloc
 
       OS::error("Out of memory");
     }
+
+    /**
+     * Source of Entropy
+     *
+     * This is a default that works on many POSIX platforms.
+     */
+    static uint64_t get_entropy64()
+    {
+      if constexpr (!pal_supports<Entropy, OS>)
+      {
+        // Derived Pal does not provide entropy.
+        return 0;
+      }
+      else if constexpr (OS::get_entropy64 != get_entropy64)
+      {
+        // Derived Pal has provided a custom definition.
+        return OS::get_entropy64();
+      }
+      else
+      {
+        uint64_t result;
+        if (getentropy(&result, sizeof(result)) != 0)
+          error("Failed to get system randomness");
+        return result;
+      }
+    }
   };
 } // namespace snmalloc