[disk][linux] add SELF_MOUNTINFO to use /proc/self

The changes to gopsutil for reading /proc/1/mountinfo affected
applications running under restricted environments that disallows access
to /proc/1/mountinfo. shirou#1159 was filed for android but other restricted
environments are also affected (eg, snaps)). The fix for shirou#1159 addressed
the application behavior to work under confinement for non-android as
well. However, depending on the system, the attempt to read
/proc/1/mountinfo could cause a sandbox denial in the logs which can be
quite noisy if using gopsutil as part of a monitoring solution that
polls often.

This introduces the SELF_MOUNTINFO to force reading from /proc/self
instead of first trying /proc/1. When unset or set to anything other
than '1', retain the current behavior with fallback. This allows people
to set SELF_MOUNTINFO=1 when gopsutil is running under these restricted
environments.

README.md

@@ -90,6 +90,9 @@ environment variable.
 You can set an alternative location to `/dev` by setting the `HOST_DEV`
 environment variable.
 
+You can force use of `/proc/self` for mount info by setting the
+`SELF_MOUNTINFO` environment variable.
+
 ## Documentation
 
 see http://godoc.org/github.com/shirou/gopsutil

disk/disk_linux.go

@@ -244,8 +244,17 @@ func readMountFile(root string) (lines []string, useMounts bool, filename string
 }
 
 func PartitionsWithContext(ctx context.Context, all bool) ([]PartitionStat, error) {
-	lines, useMounts, filename, err := readMountFile("1")
+	root := "1"
+	if os.Getenv("SELF_MOUNTINFO") == "1" {
+		// force preference for "/proc/self/mountinfo"  #1271
+		root = "self"
+	}
+
+	lines, useMounts, filename, err := readMountFile(root)
 	if err != nil {
+		if root == "self" {
+			return nil, err
+		}
 		// fallback to "/proc/self/mountinfo"  #1159
 		lines, useMounts, filename, err = readMountFile("self")
 		if err != nil {