protect against emtpy wmi query result sets

If an WMI query were to return an empty result set, trying to access the
first element in the result set would result in a `panic: runtime error:
index out of range` error.

add logic to explicitly check if the result set size is 0 and return an
error rather.

Fixes https://github.com/martinlindhe/wmi_exporter/issues/240

collector/ad.go

@@ -3,6 +3,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -616,6 +618,9 @@ func (c *ADCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, er
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.AddressBookOperationsTotal,

collector/cs.go

@@ -4,6 +4,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -60,6 +62,9 @@ func (c *CSCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, er
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.LogicalProcessors,

collector/dns.go

@@ -4,6 +4,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -237,6 +239,9 @@ func (c *DNSCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.ZoneTransferRequestsReceived,

collector/iis.go

@@ -7,6 +7,7 @@
 package collector
 
 import (
+	"errors"
 	"fmt"
 	"regexp"
 
@@ -1738,7 +1739,10 @@ func (c *IISCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 		return nil, err
 	}
 
-	if len(dst_cache) > 0 {
+	if len(dst_cache) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
+
 	ch <- prometheus.MustNewConstMetric(
 		c.ServiceCache_ActiveFlushedEntries,
 		prometheus.GaugeValue,
@@ -1947,7 +1951,6 @@ func (c *IISCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 		prometheus.CounterValue,
 		float64(dst_cache[0].OutputCacheTotalFlushes),
 	)
-	}
 
 	return nil, nil
 }

collector/mssql.go

@@ -1769,7 +1769,10 @@ func (c *MSSQLCollector) collectAccessMethods(ch chan<- prometheus.Metric, sqlIn
 		return nil, err
 	}
 
-	if len(dst) > 0 {
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
+
 	v := dst[0]
 	ch <- prometheus.MustNewConstMetric(
 		c.AccessMethodsAUcleanupbatches,
@@ -2071,7 +2074,6 @@ func (c *MSSQLCollector) collectAccessMethods(ch chan<- prometheus.Metric, sqlIn
 		float64(v.WorktablesFromCacheRatio),
 		sqlInstance,
 	)
-	}
 	return nil, nil
 }
 
@@ -2201,8 +2203,10 @@ func (c *MSSQLCollector) collectBufferManager(ch chan<- prometheus.Metric, sqlIn
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
-	if len(dst) > 0 {
 	v := dst[0]
 
 	ch <- prometheus.MustNewConstMetric(
@@ -2358,7 +2362,7 @@ func (c *MSSQLCollector) collectBufferManager(ch chan<- prometheus.Metric, sqlIn
 		float64(v.Targetpages),
 		sqlInstance,
 	)
-	}
+
 	return nil, nil
 }
 
@@ -2998,8 +3002,10 @@ func (c *MSSQLCollector) collectGeneralStatistics(ch chan<- prometheus.Metric, s
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
-	if len(dst) > 0 {
 	v := dst[0]
 	ch <- prometheus.MustNewConstMetric(
 		c.GenStatsActiveTempTables,
@@ -3168,7 +3174,7 @@ func (c *MSSQLCollector) collectGeneralStatistics(ch chan<- prometheus.Metric, s
 		float64(v.UserConnections),
 		sqlInstance,
 	)
-	}
+
 	return nil, nil
 }
 
@@ -3280,8 +3286,10 @@ func (c *MSSQLCollector) collectMemoryManager(ch chan<- prometheus.Metric, sqlIn
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
-	if len(dst) > 0 {
 	v := dst[0]
 
 	ch <- prometheus.MustNewConstMetric(
@@ -3423,7 +3431,7 @@ func (c *MSSQLCollector) collectMemoryManager(ch chan<- prometheus.Metric, sqlIn
 		float64(v.TotalServerMemoryKB*1024),
 		sqlInstance,
 	)
-	}
+
 	return nil, nil
 }
 
@@ -3451,7 +3459,10 @@ func (c *MSSQLCollector) collectSQLStats(ch chan<- prometheus.Metric, sqlInstanc
 		return nil, err
 	}
 
-	if len(dst) > 0 {
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
+
 	v := dst[0]
 
 	ch <- prometheus.MustNewConstMetric(
@@ -3530,6 +3541,6 @@ func (c *MSSQLCollector) collectSQLStats(ch chan<- prometheus.Metric, sqlInstanc
 		float64(v.UnsafeAutoParamsPersec),
 		sqlInstance,
 	)
-	}
+
 	return nil, nil
 }

collector/os.go

@@ -4,6 +4,7 @@
 package collector
 
 import (
+	"errors"
 	"time"
 
 	"github.com/StackExchange/wmi"
@@ -142,6 +143,10 @@ func (c *OSCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, er
 		return nil, err
 	}
 
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
+
 	ch <- prometheus.MustNewConstMetric(
 		c.PhysicalMemoryFreeBytes,
 		prometheus.GaugeValue,

collector/system.go

@@ -4,6 +4,7 @@
 package collector
 
 import (
+	"errors"
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -94,6 +95,9 @@ func (c *SystemCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.ContextSwitchesTotal,

collector/tcp.go

@@ -5,6 +5,7 @@
 package collector
 
 import (
+	"errors"
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -118,6 +119,9 @@ func (c *TCPCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	// Counters
 	ch <- prometheus.MustNewConstMetric(

collector/vmware.go

@@ -2,6 +2,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -201,6 +203,9 @@ func (c *VmwareCollector) collectMem(ch chan<- prometheus.Metric) (*prometheus.D
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.MemActive,
@@ -287,6 +292,9 @@ func (c *VmwareCollector) collectCpu(ch chan<- prometheus.Metric) (*prometheus.D
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.CpuLimitMHz,