MM-9072/MM-10185 Force-convert the encoding of OpenGraph metadata to UTF-8 (#8631)

* Force-convert non-UTF8 HTML to UTF8 before opengraph processing

* Split the force-encoding function

* Add benchmark Test for the forceHTMLEncodingToUTF8()

```
Running tool: /home/comet/go-v1.9.2/bin/go test -benchmem -run=^$ github.com/mattermost/mattermost-server/app -bench ^BenchmarkForceHTMLEncodingToUTF8$

[03:32:58 KST 2018/04/21] [INFO] (github.com/mattermost/mattermost-server/app.TestMain:28) -test.run used, not creating temporary containers
goos: linux
goarch: amd64
pkg: github.com/mattermost/mattermost-server/app
BenchmarkForceHTMLEncodingToUTF8/with_converting-4         	  100000	     11201 ns/op	   18704 B/op	      32 allocs/op
BenchmarkForceHTMLEncodingToUTF8/without_converting-4      	  300000	      3931 ns/op	    4632 B/op	      13 allocs/op
PASS
ok  	github.com/mattermost/mattermost-server/app	2.703s
Success: Benchmarks passed.
```

* Remove an unnecessary constraint

* Add pre-check if content-type header is already utf-8

* Move the checking for utf-8 into forceHTMLEncodingToUTF8() for testing

* Revert df3f347213faa0d023c26d201fa6531f46391086..HEAD, without Gopkg.lock

1 files changed, 187 insertions, 0 deletions

diff --git a/vendor/golang.org/x/text/runes/cond.go b/vendor/golang.org/x/text/runes/cond.go
new file mode 100644
index 000000000..df7aa02db
--- /dev/null
+++ b/vendor/golang.org/x/text/runes/cond.go
@@ -0,0 +1,187 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runes
+
+import (
+	"unicode/utf8"
+
+	"golang.org/x/text/transform"
+)
+
+// Note: below we pass invalid UTF-8 to the tIn and tNotIn transformers as is.
+// This is done for various reasons:
+// - To retain the semantics of the Nop transformer: if input is passed to a Nop
+//   one would expect it to be unchanged.
+// - It would be very expensive to pass a converted RuneError to a transformer:
+//   a transformer might need more source bytes after RuneError, meaning that
+//   the only way to pass it safely is to create a new buffer and manage the
+//   intermingling of RuneErrors and normal input.
+// - Many transformers leave ill-formed UTF-8 as is, so this is not
+//   inconsistent. Generally ill-formed UTF-8 is only replaced if it is a
+//   logical consequence of the operation (as for Map) or if it otherwise would
+//   pose security concerns (as for Remove).
+// - An alternative would be to return an error on ill-formed UTF-8, but this
+//   would be inconsistent with other operations.
+
+// If returns a transformer that applies tIn to consecutive runes for which
+// s.Contains(r) and tNotIn to consecutive runes for which !s.Contains(r). Reset
+// is called on tIn and tNotIn at the start of each run. A Nop transformer will
+// substitute a nil value passed to tIn or tNotIn. Invalid UTF-8 is translated
+// to RuneError to determine which transformer to apply, but is passed as is to
+// the respective transformer.
+func If(s Set, tIn, tNotIn transform.Transformer) Transformer {
+	if tIn == nil && tNotIn == nil {
+		return Transformer{transform.Nop}
+	}
+	if tIn == nil {
+		tIn = transform.Nop
+	}
+	if tNotIn == nil {
+		tNotIn = transform.Nop
+	}
+	sIn, ok := tIn.(transform.SpanningTransformer)
+	if !ok {
+		sIn = dummySpan{tIn}
+	}
+	sNotIn, ok := tNotIn.(transform.SpanningTransformer)
+	if !ok {
+		sNotIn = dummySpan{tNotIn}
+	}
+
+	a := &cond{
+		tIn:    sIn,
+		tNotIn: sNotIn,
+		f:      s.Contains,
+	}
+	a.Reset()
+	return Transformer{a}
+}
+
+type dummySpan struct{ transform.Transformer }
+
+func (d dummySpan) Span(src []byte, atEOF bool) (n int, err error) {
+	return 0, transform.ErrEndOfSpan
+}
+
+type cond struct {
+	tIn, tNotIn transform.SpanningTransformer
+	f           func(rune) bool
+	check       func(rune) bool               // current check to perform
+	t           transform.SpanningTransformer // current transformer to use
+}
+
+// Reset implements transform.Transformer.
+func (t *cond) Reset() {
+	t.check = t.is
+	t.t = t.tIn
+	t.t.Reset() // notIn will be reset on first usage.
+}
+
+func (t *cond) is(r rune) bool {
+	if t.f(r) {
+		return true
+	}
+	t.check = t.isNot
+	t.t = t.tNotIn
+	t.tNotIn.Reset()
+	return false
+}
+
+func (t *cond) isNot(r rune) bool {
+	if !t.f(r) {
+		return true
+	}
+	t.check = t.is
+	t.t = t.tIn
+	t.tIn.Reset()
+	return false
+}
+
+// This implementation of Span doesn't help all too much, but it needs to be
+// there to satisfy this package's Transformer interface.
+// TODO: there are certainly room for improvements, though. For example, if
+// t.t == transform.Nop (which will a common occurrence) it will save a bundle
+// to special-case that loop.
+func (t *cond) Span(src []byte, atEOF bool) (n int, err error) {
+	p := 0
+	for n < len(src) && err == nil {
+		// Don't process too much at a time as the Spanner that will be
+		// called on this block may terminate early.
+		const maxChunk = 4096
+		max := len(src)
+		if v := n + maxChunk; v < max {
+			max = v
+		}
+		atEnd := false
+		size := 0
+		current := t.t
+		for ; p < max; p += size {
+			r := rune(src[p])
+			if r < utf8.RuneSelf {
+				size = 1
+			} else if r, size = utf8.DecodeRune(src[p:]); size == 1 {
+				if !atEOF && !utf8.FullRune(src[p:]) {
+					err = transform.ErrShortSrc
+					break
+				}
+			}
+			if !t.check(r) {
+				// The next rune will be the start of a new run.
+				atEnd = true
+				break
+			}
+		}
+		n2, err2 := current.Span(src[n:p], atEnd || (atEOF && p == len(src)))
+		n += n2
+		if err2 != nil {
+			return n, err2
+		}
+		// At this point either err != nil or t.check will pass for the rune at p.
+		p = n + size
+	}
+	return n, err
+}
+
+func (t *cond) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	p := 0
+	for nSrc < len(src) && err == nil {
+		// Don't process too much at a time, as the work might be wasted if the
+		// destination buffer isn't large enough to hold the result or a
+		// transform returns an error early.
+		const maxChunk = 4096
+		max := len(src)
+		if n := nSrc + maxChunk; n < len(src) {
+			max = n
+		}
+		atEnd := false
+		size := 0
+		current := t.t
+		for ; p < max; p += size {
+			r := rune(src[p])
+			if r < utf8.RuneSelf {
+				size = 1
+			} else if r, size = utf8.DecodeRune(src[p:]); size == 1 {
+				if !atEOF && !utf8.FullRune(src[p:]) {
+					err = transform.ErrShortSrc
+					break
+				}
+			}
+			if !t.check(r) {
+				// The next rune will be the start of a new run.
+				atEnd = true
+				break
+			}
+		}
+		nDst2, nSrc2, err2 := current.Transform(dst[nDst:], src[nSrc:p], atEnd || (atEOF && p == len(src)))
+		nDst += nDst2
+		nSrc += nSrc2
+		if err2 != nil {
+			return nDst, nSrc, err2
+		}
+		// At this point either err != nil or t.check will pass for the rune at p.
+		p = nSrc + size
+	}
+	return nDst, nSrc, err
+}