Fix HOST_MOUNT_PREFIX in docker with disk input (#3529)

docs/FAQ.md

@@ -1,5 +1,24 @@
 # Frequently Asked Questions
 
+### Q: How can I monitor the Docker Engine Host from within a container?
+
+You will need to setup several volume mounts as well as some environment
+variables:
+```
+docker run --name telegraf
+	-v /:/hostfs:ro
+	-v /etc:/hostfs/etc:ro
+	-v /proc:/hostfs/proc:ro
+	-v /sys:/hostfs/sys:ro
+	-v /var/run/utmp:/var/run/utmp:ro
+	-e HOST_ETC=/hostfs/etc
+	-e HOST_PROC=/hostfs/proc
+	-e HOST_SYS=/hostfs/sys
+	-e HOST_MOUNT_PREFIX=/hostfs
+	telegraf
+```
+
+
 ### Q: Why do I get a "no such host" error resolving hostnames that other
 programs can resolve?
 

plugins/inputs/system/DISK_README.md

@@ -20,9 +20,11 @@ Additionally, the behavior of resolving the `mount_points` can be configured by
 When present, this variable is prepended to the mountpoints discovered by the plugin before retrieving stats.
 The prefix is stripped from the reported `path` in the measurement.
 This settings is useful when running `telegraf` inside a docker container to report host machine metrics.
-In this case, the host's root volume should be mounted into the container and the `HOST_MOUNT_PREFIX` and `HOST_ETC` environment variables set.
+In this case, the host's root volume should be mounted into the container and the `HOST_MOUNT_PREFIX` and `HOST_PROC` environment variables set.
 
-`docker run -v /:/hostfs:ro -e HOST_MOUNT_PREFIX=/hostfs -e HOST_ETC=/hostfs/etc telegraf`
+```
+docker run -v /:/hostfs:ro -e HOST_MOUNT_PREFIX=/hostfs -e HOST_PROC=/hostfs/proc telegraf
+```
 
 ### Measurements & Fields:
 

plugins/inputs/system/disk_test.go

@@ -62,8 +62,6 @@ func TestDiskUsage(t *testing.T) {
 
 	mps.On("Partitions", true).Return(psAll, nil)
 	mps.On("OSGetenv", "HOST_MOUNT_PREFIX").Return("")
-	mps.On("OSStat", "/").Return(MockFileInfo{}, nil)
-	mps.On("OSStat", "/home").Return(MockFileInfo{}, nil)
 	mps.On("PSDiskUsage", "/").Return(&duAll[0], nil)
 	mps.On("PSDiskUsage", "/home").Return(&duAll[1], nil)
 

plugins/inputs/system/ps.go

@@ -2,6 +2,7 @@ package system
 
 import (
 	"os"
+	"strings"
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/internal"
@@ -84,6 +85,10 @@ func (s *systemPS) DiskUsage(
 	for _, filter := range fstypeExclude {
 		fstypeExcludeSet[filter] = true
 	}
+	paths := make(map[string]bool)
+	for _, part := range parts {
+		paths[part.Mountpoint] = true
+	}
 
 	// Autofs mounts indicate a potential mount, the partition will also be
 	// listed with the actual filesystem when mounted.  Ignore the autofs
@@ -92,6 +97,7 @@ func (s *systemPS) DiskUsage(
 
 	var usage []*disk.UsageStat
 	var partitions []*disk.PartitionStat
+	hostMountPrefix := s.OSGetenv("HOST_MOUNT_PREFIX")
 
 	for i := range parts {
 		p := parts[i]
@@ -110,15 +116,20 @@ func (s *systemPS) DiskUsage(
 			continue
 		}
 
-		mountpoint := s.OSGetenv("HOST_MOUNT_PREFIX") + p.Mountpoint
-		if _, err := s.OSStat(mountpoint); err != nil {
+		// If there's a host mount prefix, exclude any paths which conflict
+		// with the prefix.
+		if len(hostMountPrefix) > 0 &&
+			!strings.HasPrefix(p.Mountpoint, hostMountPrefix) &&
+			paths[hostMountPrefix+p.Mountpoint] {
 			continue
 		}
-		du, err := s.PSDiskUsage(mountpoint)
+
+		du, err := s.PSDiskUsage(p.Mountpoint)
 		if err != nil {
 			continue
 		}
-		du.Path = p.Mountpoint
+
+		du.Path = strings.TrimPrefix(p.Mountpoint, hostMountPrefix)
 		du.Fstype = p.Fstype
 		usage = append(usage, du)
 		partitions = append(partitions, &p)