bloblru: fix race condition that can compute value multiple times

internal/bloblru/cache.go

@@ -114,10 +114,19 @@ func (c *Cache) GetOrCompute(id restic.ID, compute func() ([]byte, error)) ([]by
 	if isDownloading {
 		// wait for result of parallel download
 		<-waitForResult
-		blob, ok := c.Get(id)
-		if ok {
-			return blob, nil
-		}
+	}
+
+	// try again. This is necessary independent of whether isDownloading is true or not.
+	// The calls to `c.Get()` and checking/adding the entry in `c.inProgress` are not atomic,
+	// thus the item might have been computed in the meantime.
+	// The following scenario would compute() the value multiple times otherwise:
+	// Goroutine A does not find a value in the initial call to `c.Get`, then goroutine B
+	// takes over, caches the computed value and cleans up its channel in c.inProgress.
+	// Then goroutine A continues, does not detect a parallel computation and would try
+	// to call compute() again.
+	blob, ok = c.Get(id)
+	if ok {
+		return blob, nil
 	}
 
 	// download it