Functions `GetMounts()` and `parseMountTable()` return all the entries
as read and parsed from /proc/self/mountinfo. In many cases the caller
is only interested only one or a few entries, not all of them.

One good example is `Mounted()` function, which looks for a specific
entry only. Another example is `RecursiveUnmount()` which is only
interested in mount under a specific path.

This commit adds `filter` argument to `GetMounts()` to implement
two things:
 1. filter out entries a caller is not interested in
 2. stop processing if a caller is found what it wanted

`nil` can be passed to get a backward-compatible behavior, i.e. return
all the entries.

A few filters are implemented:
 - `PrefixFilter`: filters out all entries not under `prefix`
 - `SingleEntryFilter`: looks for a specific entry

Finally, `Mounted()` is modified to use `SingleEntryFilter()`, and
`RecursiveUnmount()` is using `PrefixFilter()`.

Unit tests are added to check filters are working.

[v2: ditch NoFilter, use nil]
[v3: ditch GetMountsFiltered()]
[v4: add unit test for filters]
[v5: switch to gotestyourself]

Signed-off-by: Kir Kolyshkin <[email protected]>

Use mount.SingleEntryFilter as we're only interested in a single entry.

Test case data of TestShouldUnmountRoot is modified accordingly, as
from now on:

1. `info` can't be nil;

2. the mountpoint check is not performed (as SingleEntryFilter
   guarantees it to be equal to daemon.root).

Signed-off-by: Kir Kolyshkin <[email protected]>

The flow of getSourceMount was:
 1 get all entries from /proc/self/mountinfo
 2 do a linear search for the `source` directory
 3 if found, return its data
 4 get the parent directory of `source`, goto 2

The repeated linear search through the whole mountinfo (which can have
thousands of records) is inefficient. Instead, let's just

 1 collect all the relevant records (only those mount points
   that can be a parent of `source`)
 2 find the record with the longest mountpath, return its data

This was tested manually with something like

```go
func TestGetSourceMount(t *testing.T) {
	mnt, flags, err := getSourceMount("/sys/devices/msr/")
	assert.NoError(t, err)
	t.Logf("mnt: %v, flags: %v", mnt, flags)
}
```

...but it relies on having a specific mount points on the system
being used for testing.

[v2: add unit tests for ParentsFilter]

Signed-off-by: Kir Kolyshkin <[email protected]>

The mountinfo parser implemented via `fmt.Sscanf()` is slower than the one
using `strings.Split()` and `strconv.Atoi()`. This rewrite helps to speed it
up to a factor of 8x, here is a result from go bench:

> BenchmarkParsingScanf-4      	     300	  22294112 ns/op
> BenchmarkParsingSplit-4      	    3000	   2780703 ns/op

I tried other approaches, such as using `fmt.Sscanf()` for the first
three (integer) fields and `strings.Split()` for the rest, but it slows
things down considerably:

> BenchmarkParsingMixed-4      	    1000	   8827058 ns/op

Note the old code uses `fmt.Sscanf`, when a linear search for '-' field,
when a split for the last 3 fields. The new code relies on a single
split.

I have also added more comments to aid in future development.

Finally, the test data is fixed to now have white space before the first field.

Signed-off-by: Kir Kolyshkin <[email protected]>

Now, every Unmount() call takes a burden to parse the whole nine yards
of /proc/self/mountinfo to figure out whether the given mount point is
mounted or not (and returns an error in case parsing fails somehow).

Instead, let's just call umount() and ignore EINVAL, which results
in the same behavior, but much better performance.

Note that EINVAL is returned from umount(2) not only in the case when
`target` is not mounted, but also for invalid flags. As the flags are
hardcoded here, it can't be the case.

Signed-off-by: Kir Kolyshkin <[email protected]>

There is no need to parse mount table and iterate through the list of
mounts, and then call Unmount() which again parses the mount table and
iterates through the list of mounts.

It is totally OK to call Unmount() unconditionally.

Signed-off-by: Kir Kolyshkin <[email protected]>

This is not for the sake of test to run faster of course;
this is to simplify the code as well as have some more
testing for mount.SingleEntryFilter().

Signed-off-by: Kir Kolyshkin <[email protected]>