pathdb: Pathdb full write-buffer check (#497)

* pathdb: limit diff layers in write-buffer

* fork.yaml: document pathdb fix

Author: protolambda

fork.yaml

@@ -205,6 +205,12 @@ def:
           globs:
             - "eth/downloader/downloader.go"
             - "eth/downloader/receiptreference.go"
+        - title: PathDB diff-layers limit
+          description: |
+            Prevent the write-buffer to grow too large, to keep the journal optional,
+            and not restart on top of unavailable state.
+          globs:
+            - "triedb/pathdb/buffer.go"
         - title: Discv5 node discovery
           description: Fix discv5 option to allow discv5 to be an active source for node-discovery.
           globs:

triedb/pathdb/buffer.go

@@ -114,7 +114,10 @@ func (b *buffer) empty() bool {
 // full returns an indicator if the size of accumulated content exceeds the
 // configured threshold.
 func (b *buffer) full() bool {
-	return b.size() > b.limit
+	// Limit not just by data size, but also the number of layers:
+	// if we allow the write-buffer to become larger,
+	// the journal becomes critical to not roll back to an unavailable state.
+	return b.size() > b.limit || b.layers >= uint64(maxDiffLayers)
 }
 
 // size returns the approximate memory size of the held content.