fix(inputs.nfsclient): Avoid panics, better error messages (#13982)

plugins/inputs/nfsclient/README.md

@@ -64,6 +64,16 @@ _N.B._ the `include_mounts` and `exclude_mounts` arguments are both applied to
 the local mount location (e.g. /mnt/NFS), not the server export
 (e.g. nfsserver:/vol/NFS).  Go regexp patterns can be used in either.
 
+## Location of mountstats
+
+If you have mounted the /proc file system in a container, to tell this plugin
+where to find the new location, set the `MOUNT_PROC` environment variable. For
+example, in a Docker compose file, if /proc is mounted to /host/proc, then use:
+
+```yaml
+MOUNT_PROC: /host/proc/self/mountstats
+```
+
 ### References
 
 1. [nfsiostat](http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary)

plugins/inputs/nfsclient/nfsclient.go

@@ -296,6 +296,16 @@ func (*NFSClient) SampleConfig() string {
 }
 
 func (n *NFSClient) Gather(acc telegraf.Accumulator) error {
+	if _, err := os.Stat(n.mountstatsPath); os.IsNotExist(err) {
+		return err
+	}
+
+	// Attempt to read the file to see if we have permissions before opening
+	// which can lead to a panic
+	if _, err := os.ReadFile(n.mountstatsPath); err != nil {
+		return err
+	}
+
 	file, err := os.Open(n.mountstatsPath)
 	if err != nil {
 		n.Log.Errorf("Failed opening the %q file: %v ", file.Name(), err)

plugins/inputs/nfsclient/nfsclient_test.go

@@ -205,3 +205,10 @@ func TestNFSClientProcessFull(t *testing.T) {
 	acc.AssertContainsFields(t, "nfs_bytes", fieldsBytes)
 	acc.AssertContainsFields(t, "nfs_xprt_tcp", fieldsXprtTCP)
 }
+
+func TestNFSClientFileDoesNotExist(t *testing.T) {
+	var acc testutil.Accumulator
+	nfsclient := NFSClient{Fullstat: true}
+	nfsclient.mountstatsPath = "/does_not_exist"
+	require.Error(t, nfsclient.Gather(&acc))
+}