plugin/hosts: don't drop entries after an over-long line (#8496)

bufio.Scanner stops at the first line longer than its 64KiB default
buffer and reports bufio.ErrTooLong from Err(). parse() never checked
Err(), so that line and every entry after it were dropped silently: the
hosts file simply looked shorter than it is, with nothing in the log.

Raise the scanner's limit to 1MiB (the scanner still grows its buffer
lazily, so nothing is preallocated up front) and log an error if the
scan does stop early, so the truncation is at least visible.

Signed-off-by: Paco Cartones <pacocartones@users.noreply.github.com>
Co-authored-by: Paco Cartones <pacocartones@users.noreply.github.com>

plugin/hosts/hostsfile_test.go

@@ -290,3 +290,19 @@ func TestLookupStaticHostReloadRace(t *testing.T) {
 
 	wg.Wait()
 }
+
+func TestParseLineLongerThanDefaultScanBuffer(t *testing.T) {
+	// A line longer than bufio.Scanner's default 64KiB buffer must not stop
+	// the scan: the entries that follow it still have to be parsed.
+	long := strings.Repeat("a", 70*1024)
+	h := testHostsfile("127.0.0.1 before.example.org\n" +
+		"127.0.0.2 " + long + ".example.org\n" +
+		"127.0.0.3 after.example.org\n")
+
+	if addrs := h.LookupStaticHostV4("before.example.org."); len(addrs) != 1 || addrs[0].String() != "127.0.0.1" {
+		t.Errorf("LookupStaticHostV4(before.example.org.) = %v, want [127.0.0.1]", addrs)
+	}
+	if addrs := h.LookupStaticHostV4("after.example.org."); len(addrs) != 1 || addrs[0].String() != "127.0.0.3" {
+		t.Errorf("LookupStaticHostV4(after.example.org.) = %v, want [127.0.0.3]", addrs)
+	}
+}