diff --git a/common/cache.go b/common/cache.go
index ade36ba..d3eda6c 100644
--- a/common/cache.go
+++ b/common/cache.go
@@ -51,20 +51,27 @@ func (c *BlockCache) HashMismatch(prevhash []byte) bool {
 	return c.latestHash != nil && !bytes.Equal(c.latestHash, prevhash)
 }
 
+// Make the block at the given height the lowest height that we don't have.
+// In other words, wipe out this height and beyond.
+// This should never increase the size of the cache, only decrease.
+// Caller should hold c.mutex.Lock().
 func (c *BlockCache) setDbFiles(height int) {
-	index := height - c.firstBlock
-	if err := c.lengthsFile.Truncate(int64(index * 4)); err != nil {
-		Log.Fatal("truncate lengths file failed: ", err)
+	if height < c.nextBlock {
+		index := height - c.firstBlock
+		if err := c.lengthsFile.Truncate(int64(index * 4)); err != nil {
+			Log.Fatal("truncate lengths file failed: ", err)
+		}
+		if err := c.blocksFile.Truncate(c.starts[index]); err != nil {
+			Log.Fatal("truncate blocks file failed: ", err)
+		}
+		c.Sync()
+		c.starts = c.starts[:index+1]
+		c.nextBlock = height
+		c.setLatestHash()
 	}
-	if err := c.blocksFile.Truncate(c.starts[index]); err != nil {
-		Log.Fatal("truncate blocks file failed: ", err)
-	}
-	c.Sync()
-	c.starts = c.starts[:index+1]
-	c.nextBlock = height
-	c.setLatestHash()
 }
 
+// Caller should hold c.mutex.Lock().
 func (c *BlockCache) recoverFromCorruption(height int) {
 	Log.Warning("CORRUPTION detected in db blocks-cache files, height ", height, " redownloading")
 	c.setDbFiles(height)
@@ -86,6 +93,7 @@ func checksum(height int, b []byte) []byte {
 	return cs.Sum(nil)
 }
 
+// Caller should hold (at least) c.mutex.RLock().
 func (c *BlockCache) readBlock(height int) *walletrpc.CompactBlock {
 	blockLen := c.blockLength(height)
 	b := make([]byte, blockLen+8)
@@ -116,6 +124,7 @@ func (c *BlockCache) readBlock(height int) *walletrpc.CompactBlock {
 	return block
 }
 
+// Caller should hold c.mutex.Lock().
 func (c *BlockCache) setLatestHash() {
 	c.latestHash = nil
 	// There is at least one block; get the last block's hash
@@ -132,6 +141,7 @@ func (c *BlockCache) setLatestHash() {
 }
 
 // NewBlockCache returns an instance of a block cache object.
+// (No locking here, we assume this is single-threaded.)
 func NewBlockCache(dbPath string, chainName string, startHeight int, redownload bool) *BlockCache {
 	c := &BlockCache{}
 	c.firstBlock = startHeight
@@ -298,7 +308,12 @@ func (c *BlockCache) Get(height int) *walletrpc.CompactBlock {
 	}
 	block := c.readBlock(height)
 	if block == nil {
-		c.recoverFromCorruption(height)
+		go func() {
+			// We hold only the read lock, need the exclusive lock.
+			c.mutex.Lock()
+			c.recoverFromCorruption(height)
+			c.mutex.Unlock()
+		}()
 		return nil
 	}
 	return block