bytes: set cap of slices returned by Split and Fields and friends

This avoids the problem in which appending to a slice returned by
Split can affect subsequent slices.

Fixes #21149.

Change-Id: Ie3df2b9ceeb9605d4625f47d49073c5f348cf0a1
Reviewed-on: https://go-review.googlesource.com/74510Reviewed-by: Jelte Fennema <github-tech@jeltef.nl>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/bytes/bytes.go

@@ -39,7 +39,7 @@ func explode(s []byte, n int) [][]byte {
 			break
 		}
 		_, size = utf8.DecodeRune(s)
-		a[na] = s[0:size]
+		a[na] = s[0:size:size]
 		s = s[size:]
 		na++
 	}
@@ -219,7 +219,7 @@ func genSplit(s, sep []byte, sepSave, n int) [][]byte {
 		if m < 0 {
 			break
 		}
-		a[i] = s[:m+sepSave]
+		a[i] = s[: m+sepSave : m+sepSave]
 		s = s[m+len(sep):]
 		i++
 	}
@@ -302,7 +302,7 @@ func Fields(s []byte) [][]byte {
 			i++
 			continue
 		}
-		a[na] = s[fieldStart:i]
+		a[na] = s[fieldStart:i:i]
 		na++
 		i++
 		// Skip spaces in between fields.
@@ -312,7 +312,7 @@ func Fields(s []byte) [][]byte {
 		fieldStart = i
 	}
 	if fieldStart < len(s) { // Last field might end at EOF.
-		a[na] = s[fieldStart:]
+		a[na] = s[fieldStart:len(s):len(s)]
 	}
 	return a
 }
@@ -363,7 +363,7 @@ func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
 	// Create subslices from recorded field indices.
 	a := make([][]byte, len(spans))
 	for i, span := range spans {
-		a[i] = s[span.start:span.end]
+		a[i] = s[span.start:span.end:span.end]
 	}
 
 	return a

src/bytes/bytes_test.go

@@ -736,6 +736,13 @@ var splittests = []SplitTest{
 func TestSplit(t *testing.T) {
 	for _, tt := range splittests {
 		a := SplitN([]byte(tt.s), []byte(tt.sep), tt.n)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
 		result := sliceOfString(a)
 		if !eq(result, tt.a) {
 			t.Errorf(`Split(%q, %q, %d) = %v; want %v`, tt.s, tt.sep, tt.n, result, tt.a)
@@ -744,6 +751,11 @@ func TestSplit(t *testing.T) {
 		if tt.n == 0 {
 			continue
 		}
+
+		if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+			t.Errorf("last appended result was %s; want %s", x, want)
+		}
+
 		s := Join(a, []byte(tt.sep))
 		if string(s) != tt.s {
 			t.Errorf(`Join(Split(%q, %q, %d), %q) = %q`, tt.s, tt.sep, tt.n, tt.sep, s)
@@ -782,11 +794,23 @@ var splitaftertests = []SplitTest{
 func TestSplitAfter(t *testing.T) {
 	for _, tt := range splitaftertests {
 		a := SplitAfterN([]byte(tt.s), []byte(tt.sep), tt.n)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
 		result := sliceOfString(a)
 		if !eq(result, tt.a) {
 			t.Errorf(`Split(%q, %q, %d) = %v; want %v`, tt.s, tt.sep, tt.n, result, tt.a)
 			continue
 		}
+
+		if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+			t.Errorf("last appended result was %s; want %s", x, want)
+		}
+
 		s := Join(a, nil)
 		if string(s) != tt.s {
 			t.Errorf(`Join(Split(%q, %q, %d), %q) = %q`, tt.s, tt.sep, tt.n, tt.sep, s)
@@ -821,12 +845,29 @@ var fieldstests = []FieldsTest{
 
 func TestFields(t *testing.T) {
 	for _, tt := range fieldstests {
-		a := Fields([]byte(tt.s))
+		b := []byte(tt.s)
+		a := Fields(b)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
 		result := sliceOfString(a)
 		if !eq(result, tt.a) {
 			t.Errorf("Fields(%q) = %v; want %v", tt.s, a, tt.a)
 			continue
 		}
+
+		if string(b) != tt.s {
+			t.Errorf("slice changed to %s; want %s", string(b), tt.s)
+		}
+		if len(tt.a) > 0 {
+			if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+				t.Errorf("last appended result was %s; want %s", x, want)
+			}
+		}
 	}
 }
 
@@ -847,11 +888,28 @@ func TestFieldsFunc(t *testing.T) {
 		{"aXXbXXXcX", []string{"a", "b", "c"}},
 	}
 	for _, tt := range fieldsFuncTests {
-		a := FieldsFunc([]byte(tt.s), pred)
+		b := []byte(tt.s)
+		a := FieldsFunc(b, pred)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
 		result := sliceOfString(a)
 		if !eq(result, tt.a) {
 			t.Errorf("FieldsFunc(%q) = %v, want %v", tt.s, a, tt.a)
 		}
+
+		if string(b) != tt.s {
+			t.Errorf("slice changed to %s; want %s", b, tt.s)
+		}
+		if len(tt.a) > 0 {
+			if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+				t.Errorf("last appended result was %s; want %s", x, want)
+			}
+		}
 	}
 }