From a24137c066f18817322ced65c997a8c1194e692a Mon Sep 17 00:00:00 2001
From: gagliardetto <gagliardetto@users.noreply.github.com>
Date: Wed, 23 Aug 2023 14:11:05 +0200
Subject: [PATCH] Remove one roundtrip when using remote CAR files

---
 epoch.go               |  1 +
 http-range.go          |  9 ++++++---
 multiepoch-getBlock.go | 24 ++++++++++++++++--------
 range-cache.go         | 17 +++++++++++++----
 4 files changed, 36 insertions(+), 15 deletions(-)

diff --git a/epoch.go b/epoch.go
index 15945f1f..435548ca 100644
--- a/epoch.go
+++ b/epoch.go
@@ -365,6 +365,7 @@ func (ser *Epoch) FindOffsetFromCid(ctx context.Context, cid cid.Cid) (uint64, e
 	if err != nil {
 		return 0, err
 	}
+	klog.Infof("found offset for CID %s: %d", cid, found)
 	ser.putCidToOffsetInCache(cid, found)
 	return found, nil
 }
diff --git a/http-range.go b/http-range.go
index 1da06d99..c66e6a0b 100644
--- a/http-range.go
+++ b/http-range.go
@@ -72,9 +72,12 @@ func remoteHTTPFileAsIoReaderAt(ctx context.Context, url string) (ReaderAtCloser
 		client:        newHTTPClient(),
 	}
 
-	rc := NewRangeCache(contentLength, func(p []byte, off int64) (n int, err error) {
-		return remoteReadAt(rr.client, rr.url, p, off)
-	})
+	rc := NewRangeCache(
+		contentLength,
+		filepath.Base(url),
+		func(p []byte, off int64) (n int, err error) {
+			return remoteReadAt(rr.client, rr.url, p, off)
+		})
 	rc.StartCacheGC(ctx, 1*time.Minute)
 	rr.ca = rc
 
diff --git a/multiepoch-getBlock.go b/multiepoch-getBlock.go
index 70d1a832..33a08ef8 100644
--- a/multiepoch-getBlock.go
+++ b/multiepoch-getBlock.go
@@ -88,6 +88,7 @@ func (multi *MultiEpoch) handleGetBlock(ctx context.Context, conn *requestContex
 			if err != nil {
 				return err
 			}
+			klog.Infof("%s -> %s", parentCid, blockCid)
 			{
 				var blockOffset, parentOffset uint64
 				wg := new(errgroup.Group)
@@ -117,18 +118,25 @@ func (multi *MultiEpoch) handleGetBlock(ctx context.Context, conn *requestContex
 				}
 
 				length := blockOffset - parentOffset
-				// cap the length to 1GB
-				GiB := uint64(1024 * 1024 * 1024)
-				if length > GiB {
-					length = GiB
+				MiB := uint64(1024 * 1024)
+				maxSize := MiB * 100
+				if length > maxSize {
+					length = maxSize
 				}
-				klog.Infof("prefetching %d bytes from %d", length, parentOffset)
-				carSection, err := epochHandler.ReadAtFromCar(ctx, parentOffset, length)
+
+				idealEntrySize := uint64(36190)
+				maybeOffsetOfLastEntry := parentOffset - idealEntrySize
+				length += idealEntrySize
+
+				klog.Infof("prefetching CAR: start=%d length=%d (parent_offset=%d)", maybeOffsetOfLastEntry, length, parentOffset)
+				carSection, err := epochHandler.ReadAtFromCar(ctx, maybeOffsetOfLastEntry, length)
 				if err != nil {
 					return err
 				}
 				dr := bytes.NewReader(carSection)
-
+				{
+					dr.Seek(int64(idealEntrySize), io.SeekStart)
+				}
 				br := bufio.NewReader(dr)
 
 				gotCid, data, err := util.ReadNode(br)
@@ -393,7 +401,7 @@ func (multi *MultiEpoch) handleGetBlock(ctx context.Context, conn *requestContex
 				blockResp.PreviousBlockhash = &parentEntryHash
 			}
 		} else {
-			klog.Infof("parent slot is in a different epoch, not implemented yet")
+			klog.Infof("parent slot is in a different epoch, not implemented yet (can't get previousBlockhash)")
 		}
 	}
 	tim.time("get parent block")
diff --git a/range-cache.go b/range-cache.go
index b3bdc87b..5e3bee2d 100644
--- a/range-cache.go
+++ b/range-cache.go
@@ -12,6 +12,7 @@ type RangeCache struct {
 	mu sync.RWMutex
 	// the size of the file.
 	size int64
+	name string
 
 	occupiedSpace uint64
 	remoteFetcher func(p []byte, off int64) (n int, err error)
@@ -40,12 +41,17 @@ func (r Range) isValidFor(size int64) bool {
 }
 
 // NewRangeCache creates a new RangeCache.
-func NewRangeCache(size int64, fetcher func(p []byte, off int64) (n int, err error)) *RangeCache {
+func NewRangeCache(
+	size int64,
+	name string,
+	fetcher func(p []byte, off int64) (n int, err error),
+) *RangeCache {
 	if fetcher == nil {
 		panic("fetcher must not be nil")
 	}
 	return &RangeCache{
 		size:          size,
+		name:          name,
 		cache:         make(map[Range]RangeCacheEntry),
 		remoteFetcher: fetcher,
 	}
@@ -143,7 +149,8 @@ func (rc *RangeCache) GetRange(ctx context.Context, start, ln int64) ([]byte, er
 	got, err := rc.getRange(ctx, start, end, func() ([]byte, error) {
 		v := make([]byte, end-start)
 		debugf(
-			orange("[cache-MISS] going to read from original reader: start=%d end=%d len=%d\n"),
+			orange("[cache-MISS] reading from source %s: start=%d end=%d len=%d\n"),
+			rc.name,
 			start,
 			end,
 			end-start,
@@ -215,7 +222,8 @@ func (rc *RangeCache) getRangeFromCache(ctx context.Context, start, end int64) (
 	}
 	if v, ok := rc.cache[Range{start, end}]; ok {
 		debugf(
-			lime("exact cache HIT: start=%d end=%d len=%d\n"),
+			lime("[exact-cache-HIT] for %s: start=%d end=%d len=%d\n"),
+			rc.name,
 			start,
 			end,
 			end-start,
@@ -230,7 +238,8 @@ func (rc *RangeCache) getRangeFromCache(ctx context.Context, start, end int64) (
 			}
 			if r.contains(Range{start, end}) {
 				debugf(
-					lime("[cache-HIT] for a superset of this range: start=%d end=%d len=%d\n"),
+					lime("[cache-HIT] range superset in %s: start=%d end=%d len=%d\n"),
+					rc.name,
 					start,
 					end,
 					end-start,