standalone: Dont cache duplicate subsidy intervals

The subsidy cache releases its mutex while calculating the subsidy for
an interval that is not already cached and only reacquires it to store
the result. This meant that concurrent goroutines calculating the same
subsidy interval could insert the result twice.

Author: jholdstock

blockchain/standalone/subsidy.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2015-2023 The Decred developers
+// Copyright (c) 2015-2026 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -230,10 +230,22 @@ func (c *SubsidyCache) CalcBlockSubsidy(height int64) int64 {
 	// subsidy became zero when applicable.  The cached intervals are stored in
 	// a map for O(1) lookup and also tracked via a sorted array to support the
 	// binary searches for efficient sparse interval query support.
+	//
+	// Note that the mutex is not held while performing the calculation above,
+	// so another goroutine might have already cached the same interval in the
+	// interim.  Avoid inserting the interval again in that case since doing so
+	// would add duplicate entries to the sorted interval array, causing it to
+	// grow without bound and require repeated sorting.
+	//
+	// The calculation is fully deterministic so any subsidy another goroutine
+	// cached for the interval is necessarily identical to the one calculated
+	// here.
 	c.mtx.Lock()
-	c.cache[reqInterval] = subsidy
-	c.cachedIntervals = append(c.cachedIntervals, reqInterval)
-	sort.Sort((*uint64s)(&c.cachedIntervals))
+	if _, ok := c.cache[reqInterval]; !ok {
+		c.cache[reqInterval] = subsidy
+		c.cachedIntervals = append(c.cachedIntervals, reqInterval)
+		sort.Sort((*uint64s)(&c.cachedIntervals))
+	}
 	c.mtx.Unlock()
 	return subsidy
 }