Fix HOST_MOUNT_PREFIX in docker with disk input (#3529)

(cherry picked from commit 7f66863)

Author: danielnelson

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`
+```
+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"
@@ -84,9 +85,14 @@ func (s *systemPS) DiskUsage(
 	for _, filter := range fstypeExclude {
 		fstypeExcludeSet[filter] = true
 	}
+	paths := make(map[string]bool)
+	for _, part := range parts {
+		paths[part.Mountpoint] = true
+	}
 
 	var usage []*disk.UsageStat
 	var partitions []*disk.PartitionStat
+	hostMountPrefix := s.OSGetenv("HOST_MOUNT_PREFIX")
 
 	for i := range parts {
 		p := parts[i]
@@ -105,15 +111,20 @@ 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)