mountinfo: GetMountsFromReader() remove redundant switch, fix dropped fields

This switch was implementing the exact same check as is done in
strings.Join(); https://cs.opensource.google/go/go/+/refs/tags/go1.16.7:src/strings/strings.go;l=421-427

This only difference removing this switch makes is that it will assign
an empty string to p.Optional if no optional fields are present, but
given that an empty string is the default value for p.Optional, this
should make no difference.

This also revealed an off-by-one bug:

Before this change:

- we return an error if `sepIdx == 5` (earlier check)
- for `sepIdx == 6` we skipped (in this switch)
- for `7`, we take field 6 (equivalent to `fields[6:7]`, or `fields[6:sepIdx]`)
- for the "default" (else), we use `fields[6:sepIdx-1]`, which for (e.g.) `sepIndex=8`
  would do the same as `sepIdx=7`, so we'd drop one value.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

mountinfo/mountinfo_linux.go

@@ -105,16 +105,9 @@ func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
 		p.Options = fields[5]
 
 		// zero or more optional fields
-		switch {
-		case sepIdx == 6:
-			// zero, do nothing
-		case sepIdx == 7:
-			p.Optional = fields[6]
-		default:
-			p.Optional = strings.Join(fields[6:sepIdx-1], " ")
-		}
+		p.Optional = strings.Join(fields[6:sepIdx], " ")
 
-		// Run the filter after parsing all of the fields.
+		// Run the filter after parsing all fields.
 		var skip, stop bool
 		if filter != nil {
 			skip, stop = filter(p)

mountinfo/mountinfo_linux_test.go

@@ -682,7 +682,7 @@ func TestParseMountinfoExtraCases(t *testing.T) {
 			name:  "extra optional fields", // which we carefully gather
 			entry: `251 15 0:3573 / /mnt/point rw,relatime shared:123 extra:tag what:ever key:value - aufs none rw`,
 			valid: true,
-			exp:   &Info{Mountpoint: "/mnt/point", FSType: "aufs", Source: "none", Optional: "shared:123 extra:tag what:ever"},
+			exp:   &Info{Mountpoint: "/mnt/point", FSType: "aufs", Source: "none", Optional: "shared:123 extra:tag what:ever key:value"},
 		},
 		{
 			name:  "empty source field (kernel < 5.1 bug)",