prng: support getrandom & getentropy

If the C library supports these random functions, use them directly
instead of reading the /dev/urandom file.  This makes life easier on
platforms that don't have /dev/urandom (like WASM).

configure.ac

@@ -272,6 +272,7 @@ AC_CHECK_HEADERS(m4_normalize([
   stdlib.h
   string.h
   sys/ioctl.h
+  sys/random.h
   sys/resource.h
   sys/socket.h
   sys/stat.h
@@ -304,6 +305,8 @@ AC_TYPE_UINTPTR_T
 
 # Checks for library functions.
 AC_CHECK_FUNCS(m4_normalize([
+  getentropy
+  getrandom
   gettimeofday
   posix_memalign
   cfmakeraw

src/crypto/prng.h

@@ -33,6 +33,20 @@
 #ifndef PRNG_HPP
 #define PRNG_HPP
 
+#include "config.h"
+
+#if !defined( HAVE_GETENTROPY ) && !defined( HAVE_GETRANDOM )
+#define HAVE_URANDOM 1
+#else
+#undef HAVE_URANDOM
+#endif
+
+#include <unistd.h>
+#ifdef HAVE_SYS_RANDOM_H
+#include <sys/random.h>
+#endif
+
+#include <algorithm>
 #include <cstdint>
 #include <fstream>
 #include <string>
@@ -43,21 +57,29 @@
 
    We rely on stdio buffering for efficiency. */
 
+#ifdef HAVE_URANDOM
 static const char rdev[] = "/dev/urandom";
+#endif
 
 using namespace Crypto;
 
 class PRNG
 {
 private:
+#ifdef HAVE_URANDOM
   std::ifstream randfile;
+#endif
 
   /* unimplemented to satisfy -Weffc++ */
   PRNG( const PRNG& );
   PRNG& operator=( const PRNG& );
 
 public:
-  PRNG() : randfile( rdev, std::ifstream::in | std::ifstream::binary ) {}
+  PRNG()
+#ifdef HAVE_URANDOM
+    : randfile( rdev, std::ifstream::in | std::ifstream::binary )
+#endif
+  {}
 
   void fill( void* dest, size_t size )
   {
@@ -65,10 +87,27 @@ public:
       return;
     }
 
+#if defined( HAVE_GETRANDOM )
+    if ( getrandom( dest, size, 0 ) != static_cast<ssize_t>( size ) ) {
+      throw CryptoException( "getrandom fell short" );
+    }
+#elif defined( HAVE_GETENTROPY )
+    // getentropy() can only read up to 256 bytes at a time :(.
+    const size_t max_read = 256;
+    while ( size ) {
+      size_t this_size = std::min( max_read, size );
+      if ( getentropy( dest, this_size ) ) {
+        throw CryptoException( "getentropy fell short" );
+      }
+      size -= this_size;
+      dest = static_cast<char*>( dest ) + this_size;
+    }
+#else
     randfile.read( static_cast<char*>( dest ), size );
     if ( !randfile ) {
       throw CryptoException( "Could not read from " + std::string( rdev ) );
     }
+#endif
   }
 
   uint8_t uint8()