strconv: add QuotedPrefix

QuotedPrefix is similar to Unquote, but returns the quoted string verbatim
and ignores any data after the quoted string.

Fixes #45033

Change-Id: I9f69fe9e3e45cbe9e63581cf1b457facb625045d
Reviewed-on: https://go-review.googlesource.com/c/go/+/314775
Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

4 files changed, 153 insertions, 85 deletions

diff --git a/src/strconv/bytealg.go b/src/strconv/bytealg.go
index 9780c28ef3..a2bb12c5f2 100644
--- a/src/strconv/bytealg.go
+++ b/src/strconv/bytealg.go
@@ -9,7 +9,7 @@ package strconv
 
 import "internal/bytealg"
 
-// contains reports whether the string contains the byte c.
-func contains(s string, c byte) bool {
-	return bytealg.IndexByteString(s, c) != -1
+// index returns the index of the first instance of c in s, or -1 if missing.
+func index(s string, c byte) int {
+	return bytealg.IndexByteString(s, c)
 }
diff --git a/src/strconv/bytealg_bootstrap.go b/src/strconv/bytealg_bootstrap.go
index 875a0eb147..0ed79f4de7 100644
--- a/src/strconv/bytealg_bootstrap.go
+++ b/src/strconv/bytealg_bootstrap.go
@@ -7,12 +7,12 @@
 
 package strconv
 
-// contains reports whether the string contains the byte c.
-func contains(s string, c byte) bool {
+// index returns the index of the first instance of c in s, or -1 if missing.
+func index(s string, c byte) int {
 	for i := 0; i < len(s); i++ {
 		if s[i] == c {
-			return true
+			return i
 		}
 	}
-	return false
+	return -1
 }
diff --git a/src/strconv/quote.go b/src/strconv/quote.go
index db0dbb288b..b3bbb1612b 100644
--- a/src/strconv/quote.go
+++ b/src/strconv/quote.go
@@ -15,6 +15,11 @@ const (
 	upperhex = "0123456789ABCDEF"
 )
 
+// contains reports whether the string contains the byte c.
+func contains(s string, c byte) bool {
+	return index(s, c) != -1
+}
+
 func quoteWith(s string, quote byte, ASCIIonly, graphicOnly bool) string {
 	return string(appendQuotedWith(make([]byte, 0, 3*len(s)/2), s, quote, ASCIIonly, graphicOnly))
 }
@@ -359,80 +364,132 @@ func UnquoteChar(s string, quote byte) (value rune, multibyte bool, tail string,
 	return
 }
 
+// QuotedPrefix returns the quoted string (as understood by Unquote) at the prefix of s.
+// If s does not start with a valid quoted string, QuotedPrefix returns an error.
+func QuotedPrefix(s string) (string, error) {
+	out, _, err := unquote(s, false)
+	return out, err
+}
+
 // Unquote interprets s as a single-quoted, double-quoted,
 // or backquoted Go string literal, returning the string value
 // that s quotes.  (If s is single-quoted, it would be a Go
 // character literal; Unquote returns the corresponding
 // one-character string.)
 func Unquote(s string) (string, error) {
-	n := len(s)
-	if n < 2 {
+	out, rem, err := unquote(s, true)
+	if len(rem) > 0 {
 		return "", ErrSyntax
 	}
-	quote := s[0]
-	if quote != s[n-1] {
-		return "", ErrSyntax
+	return out, err
+}
+
+// unquote parses a quoted string at the start of the input,
+// returning the parsed prefix, the remaining suffix, and any parse errors.
+// If unescape is true, the parsed prefix is unescaped,
+// otherwise the input prefix is provided verbatim.
+func unquote(in string, unescape bool) (out, rem string, err error) {
+	// Determine the quote form and optimistically find the terminating quote.
+	if len(in) < 2 {
+		return "", in, ErrSyntax
+	}
+	quote := in[0]
+	end := index(in[1:], quote)
+	if end < 0 {
+		return "", in, ErrSyntax
 	}
-	s = s[1 : n-1]
+	end += 2 // position after terminating quote; may be wrong if escape sequences are present
 
-	if quote == '`' {
-		if contains(s, '`') {
-			return "", ErrSyntax
+	switch quote {
+	case '`':
+		switch {
+		case !unescape:
+			out = in[:end] // include quotes
+		case !contains(in[:end], '\r'):
+			out = in[len("`") : end-len("`")] // exclude quotes
+		default:
+			// Carriage return characters ('\r') inside raw string literals
+			// are discarded from the raw string value.
+			buf := make([]byte, 0, end-len("`")-len("\r")-len("`"))
+			for i := len("`"); i < end-len("`"); i++ {
+				if in[i] != '\r' {
+					buf = append(buf, in[i])
+				}
+			}
+			out = string(buf)
 		}
-		if contains(s, '\r') {
-			// -1 because we know there is at least one \r to remove.
-			buf := make([]byte, 0, len(s)-1)
-			for i := 0; i < len(s); i++ {
-				if s[i] != '\r' {
-					buf = append(buf, s[i])
+		// NOTE: Prior implementations did not verify that raw strings consist
+		// of valid UTF-8 characters and we continue to not verify it as such.
+		// The Go specification does not explicitly require valid UTF-8,
+		// but only mention that it is implicitly valid for Go source code
+		// (which must be valid UTF-8).
+		return out, in[end:], nil
+	case '"', '\'':
+		// Handle quoted strings without any escape sequences.
+		if !contains(in[:end], '\\') && !contains(in[:end], '\n') {
+			var valid bool
+			switch quote {
+			case '"':
+				valid = utf8.ValidString(in[len(`"`) : end-len(`"`)])
+			case '\'':
+				r, n := utf8.DecodeRuneInString(in[len("'") : end-len("'")])
+				valid = len("'")+n+len("'") == end && (r != utf8.RuneError || n != 1)
+			}
+			if valid {
+				out = in[:end]
+				if unescape {
+					out = out[1 : end-1] // exclude quotes
 				}
+				return out, in[end:], nil
 			}
-			return string(buf), nil
 		}
-		return s, nil
-	}
-	if quote != '"' && quote != '\'' {
-		return "", ErrSyntax
-	}
-	if contains(s, '\n') {
-		return "", ErrSyntax
-	}
 
-	// Is it trivial? Avoid allocation.
-	if !contains(s, '\\') && !contains(s, quote) {
-		switch quote {
-		case '"':
-			if utf8.ValidString(s) {
-				return s, nil
+		// Handle quoted strings with escape sequences.
+		var buf []byte
+		in0 := in
+		in = in[1:] // skip starting quote
+		if unescape {
+			buf = make([]byte, 0, 3*end/2) // try to avoid more allocations
+		}
+		for len(in) > 0 && in[0] != quote {
+			// Process the next character,
+			// rejecting any unescaped newline characters which are invalid.
+			r, multibyte, rem, err := UnquoteChar(in, quote)
+			if in[0] == '\n' || err != nil {
+				return "", in0, ErrSyntax
 			}
-		case '\'':
-			r, size := utf8.DecodeRuneInString(s)
-			if size == len(s) && (r != utf8.RuneError || size != 1) {
-				return s, nil
+			in = rem
+
+			// Append the character if unescaping the input.
+			if unescape {
+				if r < utf8.RuneSelf || !multibyte {
+					buf = append(buf, byte(r))
+				} else {
+					var arr [utf8.UTFMax]byte
+					n := utf8.EncodeRune(arr[:], r)
+					buf = append(buf, arr[:n]...)
+				}
 			}
-		}
-	}
 
-	var runeTmp [utf8.UTFMax]byte
-	buf := make([]byte, 0, 3*len(s)/2) // Try to avoid more allocations.
-	for len(s) > 0 {
-		c, multibyte, ss, err := UnquoteChar(s, quote)
-		if err != nil {
-			return "", err
+			// Single quoted strings must be a single character.
+			if quote == '\'' {
+				break
+			}
 		}
-		s = ss
-		if c < utf8.RuneSelf || !multibyte {
-			buf = append(buf, byte(c))
-		} else {
-			n := utf8.EncodeRune(runeTmp[:], c)
-			buf = append(buf, runeTmp[:n]...)
+
+		// Verify that the string ends with a terminating quote.
+		if !(len(in) > 0 && in[0] == quote) {
+			return "", in0, ErrSyntax
 		}
-		if quote == '\'' && len(s) != 0 {
-			// single-quoted must be single character
-			return "", ErrSyntax
+		in = in[1:] // skip terminating quote
+
+		if unescape {
+			return string(buf), in, nil
 		}
+		return in0[:len(in0)-len(in)], in, nil
+	default:
+		return "", in, ErrSyntax
 	}
-	return string(buf), nil
 }
 
 // bsearch16 returns the smallest i such that a[i] >= x.
diff --git a/src/strconv/quote_test.go b/src/strconv/quote_test.go
index f1faf137bd..4750be2740 100644
--- a/src/strconv/quote_test.go
+++ b/src/strconv/quote_test.go
@@ -6,6 +6,7 @@ package strconv_test
 
 import (
 	. "strconv"
+	"strings"
 	"testing"
 	"unicode"
 )
@@ -297,6 +298,7 @@ var misquoted = []string{
 	`"\z"`,
 	"`",
 	"`xxx",
+	"``x\r",
 	"`\"",
 	`"\'"`,
 	`'\"'`,
@@ -307,22 +309,13 @@ var misquoted = []string{
 
 func TestUnquote(t *testing.T) {
 	for _, tt := range unquotetests {
-		if out, err := Unquote(tt.in); err != nil || out != tt.out {
-			t.Errorf("Unquote(%#q) = %q, %v want %q, nil", tt.in, out, err, tt.out)
-		}
+		testUnquote(t, tt.in, tt.out, nil)
 	}
-
-	// run the quote tests too, backward
 	for _, tt := range quotetests {
-		if in, err := Unquote(tt.out); in != tt.in {
-			t.Errorf("Unquote(%#q) = %q, %v, want %q, nil", tt.out, in, err, tt.in)
-		}
+		testUnquote(t, tt.out, tt.in, nil)
 	}
-
 	for _, s := range misquoted {
-		if out, err := Unquote(s); out != "" || err != ErrSyntax {
-			t.Errorf("Unquote(%#q) = %q, %v want %q, %v", s, out, err, "", ErrSyntax)
-		}
+		testUnquote(t, s, "", ErrSyntax)
 	}
 }
 
@@ -333,26 +326,44 @@ func TestUnquoteInvalidUTF8(t *testing.T) {
 
 		// one of:
 		want    string
-		wantErr string
+		wantErr error
 	}{
 		{in: `"foo"`, want: "foo"},
-		{in: `"foo`, wantErr: "invalid syntax"},
+		{in: `"foo`, wantErr: ErrSyntax},
 		{in: `"` + "\xc0" + `"`, want: "\xef\xbf\xbd"},
 		{in: `"a` + "\xc0" + `"`, want: "a\xef\xbf\xbd"},
 		{in: `"\t` + "\xc0" + `"`, want: "\t\xef\xbf\xbd"},
 	}
-	for i, tt := range tests {
-		got, err := Unquote(tt.in)
-		var gotErr string
-		if err != nil {
-			gotErr = err.Error()
-		}
-		if gotErr != tt.wantErr {
-			t.Errorf("%d. Unquote(%q) = err %v; want %q", i, tt.in, err, tt.wantErr)
-		}
-		if tt.wantErr == "" && err == nil && got != tt.want {
-			t.Errorf("%d. Unquote(%q) = %02x; want %02x", i, tt.in, []byte(got), []byte(tt.want))
-		}
+	for _, tt := range tests {
+		testUnquote(t, tt.in, tt.want, tt.wantErr)
+	}
+}
+
+func testUnquote(t *testing.T, in, want string, wantErr error) {
+	// Test Unquote.
+	got, gotErr := Unquote(in)
+	if got != want || gotErr != wantErr {
+		t.Errorf("Unquote(%q) = (%q, %v), want (%q, %v)", in, got, gotErr, want, wantErr)
+	}
+
+	// Test QuotedPrefix.
+	// Adding an arbitrary suffix should not change the result of QuotedPrefix
+	// assume that the suffix doesn't accidentally terminate a truncated input.
+	if gotErr == nil {
+		want = in
+	}
+	suffix := "\n\r\\\"`'" // special characters for quoted strings
+	if len(in) > 0 {
+		suffix = strings.ReplaceAll(suffix, in[:1], "")
+	}
+	in += suffix
+	got, gotErr = QuotedPrefix(in)
+	if gotErr == nil && wantErr != nil {
+		_, wantErr = Unquote(got) // original input had trailing junk, reparse with only valid prefix
+		want = got
+	}
+	if got != want || gotErr != wantErr {
+		t.Errorf("QuotedPrefix(%q) = (%q, %v), want (%q, %v)", in, got, gotErr, want, wantErr)
 	}
 }