fix(dedupe): use a common blobs dir to simplify

Currently, our dedupe scheme is very complicated since master copy is
kept in one of the repo dirs and if that image is deleted, the owning
repo is changed, requiring multiple repo locks causing unnecessary
contention.

Signed-off-by: Ramkumar Chinchani <rchincha.dev@gmail.com>

Author: rchincha

pkg/storage/constants/constants.go

@@ -26,4 +26,5 @@ const (
 	DefaultGCInterval       = 1 * time.Hour
 	S3StorageDriverName     = "s3"
 	LocalStorageDriverName  = "local"
+	GlobalBlobsDir          = "_blobs"
 )

pkg/storage/imagestore/imagestore.go

@@ -29,6 +29,7 @@ import (
 	zreg "zotregistry.dev/zot/pkg/regexp"
 	"zotregistry.dev/zot/pkg/scheduler"
 	common "zotregistry.dev/zot/pkg/storage/common"
+	"zotregistry.dev/zot/pkg/storage/constants"
 	storageConstants "zotregistry.dev/zot/pkg/storage/constants"
 	storageTypes "zotregistry.dev/zot/pkg/storage/types"
 	"zotregistry.dev/zot/pkg/test/inject"
@@ -1146,22 +1147,35 @@ retry:
 	}
 
 	if dstRecord == "" {
+		// cache record doesn't exist, so make a master copy in the hidden global blobs dir
+		bdir := path.Join(is.rootDir, constants.GlobalBlobsDir, dstDigest.Algorithm().String())
+		_ = is.storeDriver.EnsureDir(bdir)
+
+		bdst := path.Join(bdir, dstDigest.Encoded())
+
 		// cache record doesn't exist, so first disk and cache entry for this digest
-		if err := is.cache.PutBlob(dstDigest, dst); err != nil {
+		if err := is.cache.PutBlob(dstDigest, bdst); err != nil {
 			is.log.Error().Err(err).Str("blobPath", dst).Str("component", "dedupe").
 				Msg("failed to insert blob record")
 
 			return err
 		}
 
 		// move the blob from uploads to final dest
-		if err := is.storeDriver.Move(src, dst); err != nil {
+		if err := is.storeDriver.Move(src, bdst); err != nil {
 			is.log.Error().Err(err).Str("src", src).Str("dst", dst).Str("component", "dedupe").
 				Msg("failed to rename blob")
 
 			return err
 		}
 
+		if err := is.storeDriver.Link(bdst, dst); err != nil {
+			is.log.Error().Err(err).Str("blobPath", dstRecord).Str("component", "dedupe").
+				Msg("failed to link blobs")
+
+			return err
+		}
+
 		is.log.Debug().Str("src", src).Str("dst", dst).Str("component", "dedupe").Msg("rename")
 	} else {
 		// cache record exists, but due to GC and upgrades from older versions,