Chain ranged export: rework and address current shortcomings

This commit moderately refactors the ranged export code. It addresses several
problems:
  * Code does not finish cleanly and things hang on ctrl-c
  * Same block is read multiple times in a row (artificially increasing cached blockstore metrics to 50%)
  * It is unclear whether there are additional races (a single worker quits when reaching height 0)

The changes:

  * Use a FIFO instead of stack: this is likely going to be more memory
    friendly as the slice capacity will evolve as needed as we remove elements
    from the head (previous stack would allocate a very large amount of memory)
  * We avoid a probably not small amount of allocations by not using unnecessary pointers
  * Flow is a bit simplified so that context cancellation correctly shuts-down workers
  and drains channels.
  * We ensure all work is finished before terminating the work, something that might
  have been broken in some edge cases previous.

Initial testing shows the code is currently about 5% faster.

api/api_full.go

@@ -172,9 +172,9 @@ type FullNode interface {
 	// If oldmsgskip is set, messages from before the requested roots are also not included.
 	ChainExport(ctx context.Context, nroots abi.ChainEpoch, oldmsgskip bool, tsk types.TipSetKey) (<-chan []byte, error) //perm:read
 
-	ChainExportRange(ctx context.Context, head, tail types.TipSetKey, cfg *ChainExportConfig) (<-chan []byte, error) //perm:read
+	ChainExportRange(ctx context.Context, head, tail types.TipSetKey, cfg ChainExportConfig) (<-chan []byte, error) //perm:read
 
-	ChainExportRangeInternal(ctx context.Context, head, tail types.TipSetKey, cfg *ChainExportConfig) error //perm:read
+	ChainExportRangeInternal(ctx context.Context, head, tail types.TipSetKey, cfg ChainExportConfig) error //perm:read
 
 	// ChainPrune prunes the stored chain state and garbage collects; only supported if you
 	// are using the splitstore

api/types.go

@@ -399,9 +399,10 @@ func (m *MsgUuidMapType) UnmarshalJSON(b []byte) error {
 	return nil
 }
 
+// ChainExportConfig holds configuration for chain ranged exports.
 type ChainExportConfig struct {
 	WriteBufferSize   int
-	Workers           int64
+	NumWorkers        int
 	CacheSize         int
 	IncludeMessages   bool
 	IncludeReceipts   bool

api/v0api/full.go

@@ -161,9 +161,9 @@ type FullNode interface {
 	// If oldmsgskip is set, messages from before the requested roots are also not included.
 	ChainExport(ctx context.Context, nroots abi.ChainEpoch, oldmsgskip bool, tsk types.TipSetKey) (<-chan []byte, error) //perm:read
 
-	ChainExportRange(ctx context.Context, head, tail types.TipSetKey, cfg *api.ChainExportConfig) (<-chan []byte, error) //perm:read
+	ChainExportRange(ctx context.Context, head, tail types.TipSetKey, cfg api.ChainExportConfig) (<-chan []byte, error) //perm:read
 
-	ChainExportRangeInternal(ctx context.Context, head, tail types.TipSetKey, cfg *api.ChainExportConfig) error //perm:read
+	ChainExportRangeInternal(ctx context.Context, head, tail types.TipSetKey, cfg api.ChainExportConfig) error //perm:read
 
 	// MethodGroup: Beacon
 	// The Beacon method group contains methods for interacting with the random beacon (DRAND)