templater: strip single backslash before quotation mark in quoted template

db7463aa080f fixed the issue of double escapes, but it made the following
template fail with syntax error because of <\">. Strictly speaking, <\">
appears to be invalid in non-string part, but we are likely to escape <">
if surrounded by quotes, and we are used to write such templates by trial
and error.

  [templates]
  sl = "{tags % \"{ifeq(tag,'tip','',label('log.tag', ' {tag}'))}\"}"

So, for backward compatibility between 2.8.1 and 3.4, a single backslash
before quotation mark is stripped only in quoted template. We don't care
for <\"> in string literal in quoted template, which never worked as expected
before.

  template  result
  --------- ------------------------
  {\"\"}    parse error
  "{""}"    {""} -> <>
  "{\"\"}"  {""} -> <>
  {"\""}    {"\""} -> <">
  '{"\""}'  {"\""} -> <">
  "{"\""}"  parse error (don't care)

mercurial/templater.py

@@ -623,7 +623,21 b' def parsestring(s, quoted=True):'
     if quoted:
         if len(s) < 2 or s[0] != s[-1]:
             raise SyntaxError(_('unmatched quotes'))
-        return s[1:-1]
+        # de-backslash-ify only <\">. it is invalid syntax in non-string part of
+        # template, but we are likely to escape <"> in quoted string and it was
+        # accepted before, thanks to issue4290. <\\"> is unmodified because it
+        # is ambiguous and it was processed as such before 2.8.1.
+        #
+        #  template  result
+        #  --------- ------------------------
+        #  {\"\"}    parse error
+        #  "{""}"    {""} -> <>
+        #  "{\"\"}"  {""} -> <>
+        #  {"\""}    {"\""} -> <">
+        #  '{"\""}'  {"\""} -> <">
+        #  "{"\""}"  parse error (don't care)
+        q = s[0]
+        return s[1:-1].replace('\\\\' + q, '\\\\\\' + q).replace('\\' + q, q)
 
     return s
 

tests/test-command-template.t

@@ -2313,6 +2313,25 b' Test string escaping:'
   <>\n<]>
   <>\n<
 
+Test exception in quoted template. single backslash before quotation mark is
+stripped before parsing:
+
+  $ cat <<'EOF' > escquotetmpl
+  > changeset = "\" \\" \\\" \\\\" {files % \"{file}\"}\n"
+  > EOF
+  $ cd latesttag
+  $ hg log -r 2 --style ../escquotetmpl
+  " \" \" \\" head1
+
+  $ hg log -r 2 -T esc --config templates.esc='{\"invalid\"}\n'
+  hg: parse error at 1: syntax error
+  [255]
+  $ hg log -r 2 -T esc --config templates.esc='"{\"valid\"}\n"'
+  valid
+  $ hg log -r 2 -T esc --config templates.esc="'"'{\'"'"'valid\'"'"'}\n'"'"
+  valid
+  $ cd ..
+
 Test leading backslashes:
 
   $ cd latesttag