[#234] services/object: Support netmap epoch and lookup dead in read ops

Support processing of NetmapEpoch and NetmapLookupDepth X-headers when
processing object read operations. Placement for operations
Get/Head/GetRange/GetRangeHash/Search is built for the epoch specified in
NetmapEpoch X-header (by default latest). Also the specified operations are
processed until success is achieved for network maps from the past up to
NetmapLookupDepth value. Behavior for default values (zero or missing) left
unchanged.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object/get/container.go

@@ -12,11 +12,43 @@ func (exec *execCtx) executeOnContainer() {
 		return
 	}
 
-	exec.log.Debug("trying to execute in container...")
+	lookupDepth := exec.netmapLookupDepth()
+
+	exec.log.Debug("trying to execute in container...",
+		zap.Uint64("netmap lookup depth", lookupDepth),
+	)
+
+	// initialize epoch number
+	ok := exec.initEpoch()
+	if !ok {
+		return
+	}
+
+	for {
+		if exec.processCurrentEpoch() {
+			break
+		}
+
+		// check the maximum depth has been reached
+		if lookupDepth == 0 {
+			break
+		}
+
+		lookupDepth--
+
+		// go to the previous epoch
+		exec.curProcEpoch--
+	}
+}
+
+func (exec *execCtx) processCurrentEpoch() bool {
+	exec.log.Debug("process epoch",
+		zap.Uint64("number", exec.curProcEpoch),
+	)
 
 	traverser, ok := exec.generateTraverser(exec.address())
 	if !ok {
-		return
+		return true
 	}
 
 	ctx, cancel := context.WithCancel(exec.context())
@@ -24,12 +56,12 @@ func (exec *execCtx) executeOnContainer() {
 
 	exec.status = statusUndefined
 
-loop:
 	for {
 		addrs := traverser.Next()
 		if len(addrs) == 0 {
 			exec.log.Debug("no more nodes, abort placement iteration")
-			break
+
+			return false
 		}
 
 		for i := range addrs {
@@ -38,7 +70,8 @@ loop:
 				exec.log.Debug("interrupt placement iteration by context",
 					zap.String("error", ctx.Err().Error()),
 				)
-				break loop
+
+				return true
 			default:
 			}
 
@@ -47,7 +80,7 @@ loop:
 			//  we reach the best result - split info with linking object ID.
 			if exec.processNode(ctx, addrs[i]) {
 				exec.log.Debug("completing the operation")
-				break loop
+				return true
 			}
 		}
 	}