regexp: limit the capacity of slices of bytes returned by FindX

This change limits the capacity of the slices of bytes returned by:

- Find
- FindAll
- FindAllSubmatch

to be the same as their length.

Fixes #30169

Change-Id: I07b632757d2bfeab42fce0d42364e2a16c597360
Reviewed-on: https://go-review.googlesource.com/c/161877Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/find_test.go

@@ -161,6 +161,9 @@ func TestFind(t *testing.T) {
 			t.Errorf("expected match; got none: %s", test)
 		case test.matches != nil && result != nil:
 			expect := test.text[test.matches[0][0]:test.matches[0][1]]
+			if len(result) != cap(result) {
+				t.Errorf("expected capacity %d got %d: %s", len(result), cap(result), test)
+			}
 			if expect != string(result) {
 				t.Errorf("expected %q got %q: %s", expect, result, test)
 			}
@@ -242,9 +245,13 @@ func TestFindAll(t *testing.T) {
 				continue
 			}
 			for k, e := range test.matches {
+				got := result[k]
+				if len(got) != cap(got) {
+					t.Errorf("match %d: expected capacity %d got %d: %s", k, len(got), cap(got), test)
+				}
 				expect := test.text[e[0]:e[1]]
-				if expect != string(result[k]) {
-					t.Errorf("match %d: expected %q got %q: %s", k, expect, result[k], test)
+				if expect != string(got) {
+					t.Errorf("match %d: expected %q got %q: %s", k, expect, got, test)
 				}
 			}
 		}
@@ -323,9 +330,14 @@ func testSubmatchBytes(test *FindTest, n int, submatches []int, result [][]byte,
 			}
 			continue
 		}
+		got := result[k/2]
+		if len(got) != cap(got) {
+			t.Errorf("match %d: expected capacity %d got %d: %s", n, len(got), cap(got), test)
+			return
+		}
 		expect := test.text[submatches[k]:submatches[k+1]]
-		if expect != string(result[k/2]) {
-			t.Errorf("match %d: expected %q got %q: %s", n, expect, result, test)
+		if expect != string(got) {
+			t.Errorf("match %d: expected %q got %q: %s", n, expect, got, test)
 			return
 		}
 	}

src/regexp/regexp.go

@@ -761,7 +761,7 @@ func (re *Regexp) Find(b []byte) []byte {
 	if a == nil {
 		return nil
 	}
-	return b[a[0]:a[1]]
+	return b[a[0]:a[1]:a[1]]
 }
 
 // FindIndex returns a two-element slice of integers defining the location of
@@ -829,7 +829,7 @@ func (re *Regexp) FindSubmatch(b []byte) [][]byte {
 	ret := make([][]byte, 1+re.numSubexp)
 	for i := range ret {
 		if 2*i < len(a) && a[2*i] >= 0 {
-			ret[i] = b[a[2*i]:a[2*i+1]]
+			ret[i] = b[a[2*i]:a[2*i+1]:a[2*i+1]]
 		}
 	}
 	return ret
@@ -1025,7 +1025,7 @@ func (re *Regexp) FindAll(b []byte, n int) [][]byte {
 		if result == nil {
 			result = make([][]byte, 0, startSize)
 		}
-		result = append(result, b[match[0]:match[1]])
+		result = append(result, b[match[0]:match[1]:match[1]])
 	})
 	return result
 }
@@ -1100,7 +1100,7 @@ func (re *Regexp) FindAllSubmatch(b []byte, n int) [][][]byte {
 		slice := make([][]byte, len(match)/2)
 		for j := range slice {
 			if match[2*j] >= 0 {
-				slice[j] = b[match[2*j]:match[2*j+1]]
+				slice[j] = b[match[2*j]:match[2*j+1]:match[2*j+1]]
 			}
 		}
 		result = append(result, slice)