Kill insecure_random and associated global state

There are only a few uses of `insecure_random` outside the tests.
This PR replaces uses of insecure_random (and its accompanying global
state) in the core code with an FastRandomContext that is automatically
seeded on creation.

This is meant to be used for inner loops. The FastRandomContext
can be in the outer scope, or the class itself, then rand32() is used
inside the loop. Useful e.g. for pushing addresses in CNode or the fee
rounding, or randomization for coin selection.

As a context is created per purpose, thus it gets rid of
cross-thread unprotected shared usage of a single set of globals, this
should also get rid of the potential race conditions.

- I'd say TxMempool::check is not called enough to warrant using a special
  fast random context, this is switched to GetRand() (open for
  discussion...)

- The use of `insecure_rand` in ConnectThroughProxy has been replaced by
  an atomic integer counter. The only goal here is to have a different
  credentials pair for each connection to go on a different Tor circuit,
  it does not need to be random nor unpredictable.

- To avoid having a FastRandomContext on every CNode, the context is
  passed into PushAddress as appropriate.

There remains an insecure_random for test usage in `test_random.h`.

1 files changed, 16 insertions, 19 deletions

diff --git a/src/random.h b/src/random.h
index 31b80bd565..e97d2d1fb0 100644
--- a/src/random.h
+++ b/src/random.h
@@ -28,25 +28,22 @@ uint256 GetRandHash();
 void GetStrongRandBytes(unsigned char* buf, int num);
 
 /**
- * Seed insecure_rand using the random pool.
- * @param Deterministic Use a deterministic seed
+ * Fast randomness source. This is seeded once with secure random data, but
+ * is completely deterministic and insecure after that.
+ * This class is not thread-safe.
  */
-void seed_insecure_rand(bool fDeterministic = false);
-
-/**
- * MWC RNG of George Marsaglia
- * This is intended to be fast. It has a period of 2^59.3, though the
- * least significant 16 bits only have a period of about 2^30.1.
- *
- * @return random value
- */
-extern uint32_t insecure_rand_Rz;
-extern uint32_t insecure_rand_Rw;
-static inline uint32_t insecure_rand(void)
-{
-    insecure_rand_Rz = 36969 * (insecure_rand_Rz & 65535) + (insecure_rand_Rz >> 16);
-    insecure_rand_Rw = 18000 * (insecure_rand_Rw & 65535) + (insecure_rand_Rw >> 16);
-    return (insecure_rand_Rw << 16) + insecure_rand_Rz;
-}
+class FastRandomContext {
+public:
+    explicit FastRandomContext(bool fDeterministic=false);
+
+    uint32_t rand32() {
+        Rz = 36969 * (Rz & 65535) + (Rz >> 16);
+        Rw = 18000 * (Rw & 65535) + (Rw >> 16);
+        return (Rw << 16) + Rz;
+    }
+
+    uint32_t Rz;
+    uint32_t Rw;
+};
 
 #endif // BITCOIN_RANDOM_H