[PATCH 2/4] xkcd: enable more efficient random number generator

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

XKCD comic 221 provides a very efficient algorithm for
a random number generator, which we can utilize in
libvirt. This is enabled simply

   LIBVIRT_XKCD=221 /usr/sbin/libvirt

The effect can be seen when defining new guests without
a MAC address

  # virsh define demo.xml
  # virsh dumpxml demo | grep mac
      <mac address='52:54:00:04:04:04'/>

Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>
---
 src/util/virrandom.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/src/util/virrandom.c b/src/util/virrandom.c
index 67a8bd0..31d2a8f 100644
--- a/src/util/virrandom.c
+++ b/src/util/virrandom.c
@@ -34,6 +34,7 @@
 #include "virerror.h"
 #include "virlog.h"
 #include "virstring.h"
+#include "virxkcd.h"
 
 #define VIR_FROM_THIS VIR_FROM_NONE
 
@@ -107,19 +108,24 @@ uint64_t virRandomBits(int nbits)
         return 0;
     }
 
-    virMutexLock(&randomLock);
+    if (virXKCDIsEnabled(221)) {
+        return 4; /* chosen by fair dice roll.
+                   * guaranteed to be random. */
+    } else {
+        virMutexLock(&randomLock);
 
-    while (nbits > RANDOM_BITS_PER_ITER) {
-        random_r(&randomData, &bits);
-        ret = (ret << RANDOM_BITS_PER_ITER) | (bits & RANDOM_BITS_MASK);
-        nbits -= RANDOM_BITS_PER_ITER;
-    }
+        while (nbits > RANDOM_BITS_PER_ITER) {
+            random_r(&randomData, &bits);
+            ret = (ret << RANDOM_BITS_PER_ITER) | (bits & RANDOM_BITS_MASK);
+            nbits -= RANDOM_BITS_PER_ITER;
+        }
 
-    random_r(&randomData, &bits);
-    ret = (ret << nbits) | (bits & ((1 << nbits) - 1));
+        random_r(&randomData, &bits);
+        ret = (ret << nbits) | (bits & ((1 << nbits) - 1));
 
-    virMutexUnlock(&randomLock);
-    return ret;
+        virMutexUnlock(&randomLock);
+        return ret;
+    }
 }
 
 
-- 
2.5.5

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