Merge pull request bitcoin#1063 from ajtowns/202102-bip8-simplify-mustsignal-check

bip 8: simplify MUST_SIGNAL check

Author: luke-jr

bip-0008.mediawiki

@@ -181,16 +181,16 @@ Blocks received while in the MUST_SIGNAL phase must be checked to ensure that th
 
     if (GetStateForBlock(block) == MUST_SIGNAL) {
         int nonsignal = 0;
-        int count = 1 + (block.nHeight % 2016);
         walk = block;
-        while (count > 0) {
-            --count;
+        while (true) {
             if ((walk.nVersion & 0xE0000000) != 0x20000000 || ((walk.nVersion >> bit) & 1) != 1) {
                 ++nonsignal;
-                if (nonsignal + threshold > 2016) {
+                if (nonsignal > 2016 - threshold) {
                     return state.Invalid(BlockValidationResult::RECENT_CONSENSUS_CHANGE, "bad-version-bip8-must-signal");
                 }
-            } else if (nonsignal == 0) {
+            }
+            if (walk.nHeight % 2016 == 0) {
+                // checked every block in this retarget period
                 break;
             }
             walk = walk.parent;