fix: backslash at end of string literal was misinterpreted (#4246)

SQL strings that contained a string literal with a backslash as the last character were
misinterpreted by the statement parser as containing unclosed string literals.

Fixes googleapis/java-spanner-jdbc#2303

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/AbstractStatementParser.java

@@ -1033,8 +1033,9 @@ int skipQuoted(
       } else if (supportsBackslashEscape()
           && currentChar == BACKSLASH
           && length > currentIndex + 1
-          && sql.charAt(currentIndex + 1) == startQuote) {
-        // This is an escaped quote (e.g. 'foo\'bar').
+          && (sql.charAt(currentIndex + 1) == startQuote
+              || sql.charAt(currentIndex + 1) == BACKSLASH)) {
+        // 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'.
         appendIfNotNull(result, currentChar);

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/SpannerStatementParserTest.java

@@ -54,6 +54,7 @@ public void testSkip() {
     assertEquals("'foo\"bar\"'", skip("'foo\"bar\"'  ", 0));
     assertEquals("\"foo'bar'\"", skip("\"foo'bar'\"  ", 0));
     assertEquals("`foo'bar'`", skip("`foo'bar'`  ", 0));
+    assertEquals("'test\\\\'", skip("'test\\\\'", 0));
 
     assertEquals("'''foo'bar'''", skip("'''foo'bar'''  ", 0));
     assertEquals("'''foo\\'bar'''", skip("'''foo\\'bar'''  ", 0));

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/StatementParserTest.java

@@ -1268,6 +1268,10 @@ public void testGoogleStandardSQLDialectConvertPositionalParametersToNamedParame
         "@p1'''?it\\'?s \n ?it\\'?s'''@p2",
         parser.convertPositionalParametersToNamedParameters('?', "?'''?it\\'?s \n ?it\\'?s'''?")
             .sqlWithNamedParameters);
+    assertEquals(
+        "@p1'?test?\\\\'@p2",
+        parser.convertPositionalParametersToNamedParameters('?', "?'?test?\\\\'?")
+            .sqlWithNamedParameters);
 
     assertUnclosedLiteral(parser, "?'?it\\'?s \n ?it\\'?s'?");
     assertUnclosedLiteral(parser, "?'?it\\'?s \n ?it\\'?s?");