fix(inputs.chrony): Support local sources

Local sources are directly connected to the server (PPS inputs from
oscillators, GPS, etc..). They can be seen as stratum-0 sources.

The standard way of getting their name doesn't work as it is stored into
their RefID instead.

See the following links for more details:
- https://gitlab.com/chrony/chrony/-/blob/05bd4898a9a64fe1e5432a78939c4b4b82619c2f/client.c?page=3#L2046
- https://gitlab.com/chrony/chrony/-/blob/05bd4898a9a64fe1e5432a78939c4b4b82619c2f/client.c?page=3#L2152

This fixes that

Signed-off-by: Frank Villaro-Dixon <frank@villaro-dixon.eu>

plugins/inputs/chrony/chrony.go

@@ -4,6 +4,7 @@ package chrony
 import (
 	"bytes"
 	_ "embed"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"net"
@@ -291,6 +292,24 @@ func (c *Chrony) gatherServerStats(acc telegraf.Accumulator) error {
 	return nil
 }
 
+func (c *Chrony) getSourceName(ip net.IP) (string, error) {
+	sourceNameReq := fbchrony.NewNTPSourceNamePacket(ip)
+	sourceNameRaw, err := c.client.Communicate(sourceNameReq)
+	if err != nil {
+		return "", fmt.Errorf("querying name of source %q failed: %w", ip, err)
+	}
+	sourceName, ok := sourceNameRaw.(*fbchrony.ReplyNTPSourceName)
+	if !ok {
+		return "", fmt.Errorf("got unexpected response type %T while waiting for source name", sourceNameRaw)
+	}
+
+	// Cut the string at null termination
+	if termidx := bytes.Index(sourceName.Name[:], []byte{0}); termidx >= 0 {
+		return string(sourceName.Name[:termidx]), nil
+	}
+	return string(sourceName.Name[:]), nil
+}
+
 func (c *Chrony) gatherSources(acc telegraf.Accumulator) error {
 	sourcesReq := fbchrony.NewSourcesPacket()
 	sourcesRaw, err := c.client.Communicate(sourcesReq)
@@ -316,22 +335,20 @@ func (c *Chrony) gatherSources(acc telegraf.Accumulator) error {
 		}
 
 		// Trying to resolve the source name
-		sourceNameReq := fbchrony.NewNTPSourceNamePacket(sourceData.IPAddr)
-		sourceNameRaw, err := c.client.Communicate(sourceNameReq)
-		if err != nil {
-			return fmt.Errorf("querying name of source %d failed: %w", idx, err)
-		}
-		sourceName, ok := sourceNameRaw.(*fbchrony.ReplyNTPSourceName)
-		if !ok {
-			return fmt.Errorf("got unexpected response type %T while waiting for source name", sourceNameRaw)
-		}
-
-		// Cut the string at null termination
 		var peer string
-		if termidx := bytes.Index(sourceName.Name[:], []byte{0}); termidx >= 0 {
-			peer = string(sourceName.Name[:termidx])
+
+		if sourceData.Mode == fbchrony.SourceModeRef && sourceData.IPAddr.To4() != nil {
+			//https://gitlab.com/chrony/chrony/-/blob/05bd4898a9a64fe1e5432a78939c4b4b82619c2f/client.c?page=3#L2046
+			// References of local sources (PPS, etc..) are encoded in the bits of the IPv4 address,
+			// instead of the RefId. Extract the correct name for those sources.
+			ipU32 := binary.BigEndian.Uint32(sourceData.IPAddr.To4())
+
+			peer = fbchrony.RefidToString(ipU32)
 		} else {
-			peer = string(sourceName.Name[:])
+			peer, err = c.getSourceName(sourceData.IPAddr)
+			if err != nil {
+				return err
+			}
 		}
 
 		if peer == "" {
@@ -388,22 +405,19 @@ func (c *Chrony) gatherSourceStats(acc telegraf.Accumulator) error {
 		}
 
 		// Trying to resolve the source name
-		sourceNameReq := fbchrony.NewNTPSourceNamePacket(sourceStats.IPAddr)
-		sourceNameRaw, err := c.client.Communicate(sourceNameReq)
-		if err != nil {
-			return fmt.Errorf("querying name of source %d failed: %w", idx, err)
-		}
-		sourceName, ok := sourceNameRaw.(*fbchrony.ReplyNTPSourceName)
-		if !ok {
-			return fmt.Errorf("got unexpected response type %T while waiting for source name", sourceNameRaw)
-		}
-
-		// Cut the string at null termination
 		var peer string
-		if termidx := bytes.Index(sourceName.Name[:], []byte{0}); termidx >= 0 {
-			peer = string(sourceName.Name[:termidx])
+
+		if sourceStats.IPAddr.String() == "::" {
+			// `::` IPs mean a local source directly connected to the server.
+			// For example a PPS source or a local GPS receiver.
+			// Then the name of the source is encoded in its RefID
+			// https://gitlab.com/chrony/chrony/-/blob/05bd4898a9a64fe1e5432a78939c4b4b82619c2f/client.c?page=3#L2152
+			peer = fbchrony.RefidToString(sourceStats.RefID)
 		} else {
-			peer = string(sourceName.Name[:])
+			peer, err = c.getSourceName(sourceStats.IPAddr)
+			if err != nil {
+				return err
+			}
 		}
 
 		if peer == "" {