client-stateless: GetLastRetainedHeight method returns light client's oldest height

.changelog/6382.bugfix.md

@@ -0,0 +1,8 @@
+stateless-client: Fix incorrect GetLastRetainedHeight method
+
+Previously, this method would return one of the provider's last retained
+heights which could cause issues with runtime light history reindexing.
+
+This has been fixed, by setting the last retained height to the
+light client's last retained height, as this is the oldest height
+the stateless client can use to verify consensus data against.

go/consensus/cometbft/light/client.go

@@ -122,6 +122,18 @@ func (c *Client) LastTrustedHeight() (int64, error) {
 	return height, nil
 }
 
+// FirstTrustedHeight returns the first (oldest) trusted height.
+func (c *Client) FirstTrustedHeight() (int64, error) {
+	height, err := c.lightClient.FirstTrustedHeight()
+	if err != nil {
+		return 0, err
+	}
+	if height == -1 {
+		return 0, fmt.Errorf("no trusted headers")
+	}
+	return height, nil
+}
+
 // VerifyHeader verifies the given header.
 func (c *Client) VerifyHeader(ctx context.Context, header *cmttypes.Header) error {
 	c.mu.Lock()

go/consensus/cometbft/stateless/core.go

@@ -156,9 +156,9 @@ func (c *Core) GetGenesisDocument(context.Context) (*genesisAPI.Document, error)
 
 // GetLastRetainedHeight implements api.Backend.
 func (c *Core) GetLastRetainedHeight(ctx context.Context) (int64, error) {
-	// The last retained height cannot be verified, nor does it need to be,
-	// as it is not sensitive information.
-	return c.provider.GetLastRetainedHeight(ctx)
+	// The last retained height equals to the last retained light client height
+	// as this is the oldest height stateless client can verify the data against.
+	return c.lightClient.FirstTrustedHeight()
 }
 
 // GetLatestHeight implements api.Backend.