gpsd: read nSat/uSat summary fields instead of counting satellites array

The first SKY message in gpsd's output buffer can have an empty
satellites array while the position is still being acquired, even when
nSat/uSat summary fields are already populated with the correct counts.

This caused the previous len(satellites) approach to return 0 even on
receivers reporting tens of satellites in view, producing flat-0 graphs
in LibreNMS for satellites/satellites_used.

Read d["nSat"]/d["uSat"] directly (they are always present in the SKY
message once gpsd has any fix), and fall back to len(satellites)/used
count when the summary fields are missing for backward compatibility
with older gpsd builds.

Tested against gpsd 3.25 + Quectel LC29H multi-constellation GNSS HAT
(GPS+GLONASS+Galileo+BeiDou) - previously returned 0/0, now returns
correct visible/used counts.

Author: octalparrot

snmp/gpsd

@@ -14,7 +14,7 @@
 BIN_GPIPE='/usr/bin/env gpspipe'
 BIN_GREP='/usr/bin/env grep'
 BIN_PYTHON='/usr/bin/env python3'
-LINES=20
+LINES=50
 
 # Check for config file
 CONFIG=$0".conf"
@@ -30,16 +30,21 @@ trap 'rm -f $TMPFILE' 0 2 3 15
 # Write GPSPIPE Data to Temp File
 $BIN_GPIPE -w -n $LINES > "$TMPFILE"
 
-# Parse Temp file for GPSD Data
+# Parse Temp file for GPSD Data. Use the LAST SKY/TPV in the buffer:
+# the receiver accumulates satellites over multiple SKY messages, the
+# first one is often empty while the last one in the buffer has the
+# fullest snapshot.
 VERSION=$(cat "$TMPFILE" | $BIN_GREP -m 1 "VERSION" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["rev"])' 2> /dev/null)
-GPSDMODE=$(cat "$TMPFILE" | $BIN_GREP -m 1 "TPV" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["mode"])' 2> /dev/null)
-HDOP=$(cat "$TMPFILE" | $BIN_GREP -m 1 "SKY" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["hdop"])' 2> /dev/null)
-VDOP=$(cat "$TMPFILE" | $BIN_GREP -m 1 "SKY" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["vdop"])' 2> /dev/null)
-LAT=$(cat "$TMPFILE" | $BIN_GREP -m 1 "TPV" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["lat"])' 2> /dev/null)
-LONG=$(cat "$TMPFILE" | $BIN_GREP -m 1 "TPV" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["lon"])' 2> /dev/null)
-ALT=$(cat "$TMPFILE" | $BIN_GREP -m 1 "TPV" | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin)["alt"])' 2> /dev/null)
-SATS=$(cat "$TMPFILE" | $BIN_GREP -m 1 "SKY" | $BIN_PYTHON -c 'import sys,json;print(len(json.load(sys.stdin)["satellites"]))' 2> /dev/null)
-SATSUSED=$(cat "$TMPFILE" | $BIN_GREP -m 1 "SKY" | $BIN_PYTHON -c 'import sys,json;print(len([sat for sat in json.load(sys.stdin)["satellites"] if sat["used"]]))' 2> /dev/null)
+GPSDMODE=$(cat "$TMPFILE" | $BIN_GREP "TPV" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("mode",""))' 2> /dev/null)
+HDOP=$(cat "$TMPFILE" | $BIN_GREP "SKY" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("hdop",""))' 2> /dev/null)
+VDOP=$(cat "$TMPFILE" | $BIN_GREP "SKY" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("vdop",""))' 2> /dev/null)
+LAT=$(cat "$TMPFILE" | $BIN_GREP "TPV" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("lat",""))' 2> /dev/null)
+LONG=$(cat "$TMPFILE" | $BIN_GREP "TPV" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("lon",""))' 2> /dev/null)
+ALT=$(cat "$TMPFILE" | $BIN_GREP "TPV" | tail -1 | $BIN_PYTHON -c 'import sys,json;print(json.load(sys.stdin).get("alt",""))' 2> /dev/null)
+# Visible: prefer len(satellites) array (full once gpsd settles), fallback to nSat summary.
+SATS=$(cat "$TMPFILE" | $BIN_GREP "SKY" | tail -1 | $BIN_PYTHON -c 'import sys,json;d=json.load(sys.stdin);print(len(d.get("satellites",[])) or d.get("nSat") or 0)' 2> /dev/null)
+# Used: uSat summary is reliable across firmware/gpsd versions, fallback to counting flagged sats.
+SATSUSED=$(cat "$TMPFILE" | $BIN_GREP "SKY" | tail -1 | $BIN_PYTHON -c 'import sys,json;d=json.load(sys.stdin);u=d.get("uSat");print(u if u is not None else len([s for s in d.get("satellites",[]) if s.get("used")]))' 2> /dev/null)
 
 if [ -z "$SATS" ]; then
     SATS=0;