fix: backslash at end of string was misinterpreted (googleapis#651)

* fix: backslash at end of string was misinterpreted

A SQL string with a string literal that ended with an escaped backslash was misinterpreted
by the statement parser as an unclosed literal. E.g. the string `'test\\'` would be seen
as an invalid literal.

* chore: address review comments

Author: olavloite

parser/statement_parser.go

@@ -458,8 +458,8 @@ func (p *StatementParser) skipQuoted(sql []byte, pos int, quote byte) (int, int,
 				// This was the end quote.
 				return pos + 1, quoteLength, nil
 			}
-		} else if (p.supportsBackslashEscape() || isEscapeString) && len(sql) > pos+1 && c == '\\' && sql[pos+1] == quote {
-			// This is an escaped quote (e.g. 'foo\'bar').
+		} else if (p.supportsBackslashEscape() || isEscapeString) && len(sql) > pos+1 && c == '\\' && (sql[pos+1] == quote || sql[pos+1] == '\\') {
+			// This is an escaped quote (e.g. 'foo\'bar') or an escaped backslash (e.g 'test\\').
 			// Note that in raw strings, the \ officially does not start an
 			// escape sequence, but the result is still the same, as in a raw
 			// string 'both characters are preserved'.

parser/statement_parser_test.go

@@ -740,6 +740,17 @@ SELECT * FROM PersonsTable WHERE id=@id`,
 				databasepb.DatabaseDialect_POSTGRESQL: spanner.ToSpannerError(status.Errorf(codes.InvalidArgument, "SQL statement contains an unclosed literal: %s", `?"?it\"?s"?`)),
 			},
 		},
+		"backslash at end of string": {
+			input: `?'test\\'?`,
+			wantSQL: map[databasepb.DatabaseDialect]string{
+				databasepb.DatabaseDialect_GOOGLE_STANDARD_SQL: `@p1'test\\'@p2`,
+				databasepb.DatabaseDialect_POSTGRESQL:          `$1'test\\'$2`,
+			},
+			want: map[databasepb.DatabaseDialect][]string{
+				databasepb.DatabaseDialect_GOOGLE_STANDARD_SQL: {"p1", "p2"},
+				databasepb.DatabaseDialect_POSTGRESQL:          {"p1", "p2"},
+			},
+		},
 		"triple-quoted string": {
 			input: `?'''?it\'?s'''?`,
 			wantSQL: map[databasepb.DatabaseDialect]string{
@@ -1092,6 +1103,16 @@ SELECT * FROM PersonsTable WHERE id=$1`,
 			input:   `?"?it\"?s"?`,
 			wantErr: spanner.ToSpannerError(status.Errorf(codes.InvalidArgument, "SQL statement contains an unclosed literal: %s", `?"?it\"?s"?`)),
 		},
+		"backslash at end of string": {
+			input:   `?'test\\'?`,
+			wantSQL: `$1'test\\'$2`,
+			want:    []string{"p1", "p2"},
+		},
+		"backslash at end of double-quoted string": {
+			input:   `?"test\\"?`,
+			wantSQL: `$1"test\\"$2`,
+			want:    []string{"p1", "p2"},
+		},
 		"triple-quoted string": {
 			input:   `?'''?it\'?s'''?`,
 			wantErr: spanner.ToSpannerError(status.Errorf(codes.InvalidArgument, "SQL statement contains an unclosed literal: %s", `?'''?it\'?s'''?`)),
@@ -1263,6 +1284,16 @@ func TestFindParamsWithCommentsPostgreSQL(t *testing.T) {
 			wantSQL: `$1\"?it\\"\"?s\"%s$2`,
 			want:    []string{"p1", "p2"},
 		},
+		"backslash at end of string": {
+			input:   `?'test\\'%s?`,
+			wantSQL: `$1'test\\'%s$2`,
+			want:    []string{"p1", "p2"},
+		},
+		"backslash at end of double-quoted string": {
+			input:   `?"test\\"%s?`,
+			wantSQL: `$1"test\\"%s$2`,
+			want:    []string{"p1", "p2"},
+		},
 		"triple-quotes": {
 			input:   `?%s'''?it\''?s'''?`,
 			wantSQL: `$1%s'''?it\''?s'''$2`,
@@ -2028,6 +2059,13 @@ func TestSkip(t *testing.T) {
 				databasepb.DatabaseDialect_POSTGRESQL:          "'''foo\\'",
 			},
 		},
+		"escaped backslash at end of string literal": {
+			input: "'test\\\\' as foo",
+			skipped: map[databasepb.DatabaseDialect]string{
+				databasepb.DatabaseDialect_GOOGLE_STANDARD_SQL: "'test\\\\'",
+				databasepb.DatabaseDialect_POSTGRESQL:          "'test\\\\'",
+			},
+		},
 		"string with linefeed": {
 			input: "'foo\n'  ",
 			skipped: map[databasepb.DatabaseDialect]string{