fix(target-allocator): fix the potential issue of duplicate targets (#3617)

Author: mike9421

.chloggen/item_hash_cal.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. collector, target allocator, auto-instrumentation, opamp, github action)
+component: target allocator
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Compute hash based on the relabeled label set to maintain Prometheus compatibility and resolve duplication issues (#3617)
+
+# One or more tracking issues related to the change
+issues: []
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

cmd/otel-allocator/internal/prehook/relabel.go

@@ -4,6 +4,7 @@
 package prehook
 
 import (
+	"github.com/prometheus/common/model"
 	"slices"
 
 	"github.com/go-logr/logr"
@@ -48,6 +49,14 @@ func (tf *relabelConfigTargetFilter) Apply(targets []*target.Item) []*target.Ite
 		}
 
 		if keepTarget {
+			// Only if the key model.AddressLabel remains after relabeling is the value considered valid.
+			// For detail, see https://github.com/prometheus/prometheus/blob/e6cfa720fbe6280153fab13090a483dbd40bece3/scrape/target.go#L457
+			if address := lset.Get(model.AddressLabel); len(address) != 0 {
+				hash := lset.Hash()
+				tItem.HashFunc = func() uint64 {
+					return hash
+				}
+			}
 			targets[writeIndex] = tItem
 			writeIndex++
 		}

cmd/otel-allocator/internal/target/target.go

@@ -29,11 +29,18 @@ type Item struct {
 	Labels        labels.Labels
 	CollectorName string
 	hash          ItemHash
+	HashFunc      func() uint64
 }
 
 func (t *Item) Hash() ItemHash {
 	if t.hash == 0 {
-		t.hash = ItemHash(t.Labels.Hash())
+		hashFunc := t.Labels.Hash
+		if t.HashFunc != nil {
+			hashFunc = t.HashFunc
+			// Release the reference to avoid unintentionally retaining large objects.
+			t.HashFunc = nil
+		}
+		t.hash = ItemHash(hashFunc())
 	}
 	return t.hash
 }
@@ -55,7 +62,7 @@ func (t *Item) GetNodeName() string {
 
 // NewItem Creates a new target item.
 // INVARIANTS:
-// * Item fields must not be modified after creation.
+// * Item fields initialized in NewItem must not be modified after creation.
 func NewItem(jobName string, targetURL string, labels labels.Labels, collectorName string) *Item {
 	return &Item{
 		JobName:       jobName,