ECPG gets embedded quotes wrong

A recent user complaint [1] led me to investigate what ECPG does with
embedded quotes (that is, quotes-meant-to-be-data) in SQL identifiers
and strings.  AFAICS, it gets it wrong.  For example, if you write
the literal 'abc''def' in an EXEC SQL command, that will come out the
other end as 'abc'def', triggering a syntax error in the backend.
Likewise, "abc""def" is reduced to "abc"def" which is wrong syntax.

It looks to me like a sufficient fix is just to keep these quote
sequences as-is within a converted string, so that the attached
appears to fix it.  I added some documentation too, since there
doesn't seem to be anything there now explaining how it's supposed
to work.

I doubt this is safely back-patchable, since anybody who's working
around the existing misbehavior (as I see sql/dyntest.pgc is doing)
would not appreciate it changing under them in a minor release.
But I think we can fix it in v14.

            regards, tom lane

[1]
https://www.postgresql.org/message-id/flat/CA%2B4qtLct1L%3DgUordX4c_AdctJ%2BvZBsebYYLBk18LX8dLHthktg%40mail.gmail.com

diff --git a/doc/src/sgml/ecpg.sgml b/doc/src/sgml/ecpg.sgml
index 6e3ca788f6..aa1499bcea 100644
--- a/doc/src/sgml/ecpg.sgml
+++ b/doc/src/sgml/ecpg.sgml
@@ -63,11 +63,22 @@ EXEC SQL ...;
 </programlisting>
    These statements syntactically take the place of a C statement.
    Depending on the particular statement, they can appear at the
-   global level or within a function.  Embedded
+   global level or within a function.
+  </para>
+
+  <para>
+   Embedded
    <acronym>SQL</acronym> statements follow the case-sensitivity rules of
    normal <acronym>SQL</acronym> code, and not those of C. Also they allow nested
    C-style comments that are part of the SQL standard. The C part of the
    program, however, follows the C standard of not accepting nested comments.
+   Embedded <acronym>SQL</acronym> statements likewise use SQL rules, not
+   C rules, for parsing quoted strings and identifiers.
+   (See <xref linkend="sql-syntax-strings"/> and
+   <xref linkend="sql-syntax-identifiers"/> respectively.  Note that
+   ECPG assumes that <varname>standard_conforming_strings</varname>
+   is <literal>on</literal>.)
+   Of course, the C part of the program follows C quoting rules.
   </para>

   <para>
diff --git a/src/interfaces/ecpg/preproc/pgc.l b/src/interfaces/ecpg/preproc/pgc.l
index 466bbac6a7..e98aa6c486 100644
--- a/src/interfaces/ecpg/preproc/pgc.l
+++ b/src/interfaces/ecpg/preproc/pgc.l
@@ -623,11 +623,8 @@ cppline            {space}*#([^i][A-Za-z]*|{if}|{ifdef}|{ifndef}|{import})((\/\*[^*/]*\*+
                     }
                 }

-<xq,xe,xn,xus>{xqdouble}    { addlitchar('\''); }
-<xqc>{xqcquote}    {
-                    addlitchar('\\');
-                    addlitchar('\'');
-                }
+<xq,xe,xn,xus>{xqdouble}    { addlit(yytext, yyleng); }
+<xqc>{xqcquote}                { addlit(yytext, yyleng); }
 <xq,xqc,xn,xus>{xqinside}    { addlit(yytext, yyleng); }
 <xe>{xeinside}  {
                     addlit(yytext, yyleng);
@@ -736,7 +733,7 @@ cppline            {space}*#([^i][A-Za-z]*|{if}|{ifdef}|{ifndef}|{import})((\/\*[^*/]*\*+
                     return UIDENT;
                 }
 <xd,xui>{xddouble}    {
-                    addlitchar('"');
+                    addlit(yytext, yyleng);
                 }
 <xd,xui>{xdinside}    {
                     addlit(yytext, yyleng);
diff --git a/src/interfaces/ecpg/test/expected/preproc-strings.c b/src/interfaces/ecpg/test/expected/preproc-strings.c
index e695007b13..1e50cd36c3 100644
--- a/src/interfaces/ecpg/test/expected/preproc-strings.c
+++ b/src/interfaces/ecpg/test/expected/preproc-strings.c
@@ -45,7 +45,7 @@ int main(void)
 #line 13 "strings.pgc"


-  { ECPGdo(__LINE__, 0, 1, NULL, 0, ECPGst_normal, "select 'abcdef' , N'abcdef' as foo , E'abc\\bdef' as \"foo\" ,
U&'d\\0061t\\0061'as U&\"foo\" , U&'d!+000061t!+000061' UESCAPE '!' , $foo$abc$def$foo$", ECPGt_EOIT,  
+  { ECPGdo(__LINE__, 0, 1, NULL, 0, ECPGst_normal, "select 'abc''d\\ef' , N'abc''d\\ef' as foo , E'abc''d\\\\ef' as
\"foo\"\"bar\", U&'d\\0061t\\0061' as U&\"foo\"\"bar\" , U&'d!+000061t!+000061' UESCAPE '!' , $foo$abc$def$foo$",
ECPGt_EOIT, 
     ECPGt_char,&(s1),(long)0,(long)1,(1)*sizeof(char),
     ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L,
     ECPGt_char,&(s2),(long)0,(long)1,(1)*sizeof(char),
diff --git a/src/interfaces/ecpg/test/expected/preproc-strings.stderr
b/src/interfaces/ecpg/test/expected/preproc-strings.stderr
index dbc9e5c0b8..4c3a8eee5a 100644
--- a/src/interfaces/ecpg/test/expected/preproc-strings.stderr
+++ b/src/interfaces/ecpg/test/expected/preproc-strings.stderr
@@ -8,7 +8,7 @@
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_process_output on line 13: OK: SET
 [NO_PID]: sqlca: code: 0, state: 00000
-[NO_PID]: ecpg_execute on line 15: query: select 'abcdef' , N'abcdef' as foo , E'abc\bdef' as "foo" , U&'d\0061t\0061'
asU&"foo" , U&'d!+000061t!+000061' UESCAPE '!' , $foo$abc$def$foo$; with 0 parameter(s) on connection ecpg1_regression 
+[NO_PID]: ecpg_execute on line 15: query: select 'abc''d\ef' , N'abc''d\ef' as foo , E'abc''d\\ef' as "foo""bar" ,
U&'d\0061t\0061'as U&"foo""bar" , U&'d!+000061t!+000061' UESCAPE '!' , $foo$abc$def$foo$; with 0 parameter(s) on
connectionecpg1_regression 
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_execute on line 15: using PQexec
 [NO_PID]: sqlca: code: 0, state: 00000
@@ -16,15 +16,15 @@
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_store_result on line 15: allocating memory for 1 tuples
 [NO_PID]: sqlca: code: 0, state: 00000
-[NO_PID]: ecpg_get_data on line 15: RESULT: abcdef offset: -1; array: no
+[NO_PID]: ecpg_get_data on line 15: RESULT: abc'd\ef offset: -1; array: no
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_store_result on line 15: allocating memory for 1 tuples
 [NO_PID]: sqlca: code: 0, state: 00000
-[NO_PID]: ecpg_get_data on line 15: RESULT: abcdef offset: -1; array: no
+[NO_PID]: ecpg_get_data on line 15: RESULT: abc'd\ef offset: -1; array: no
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_store_result on line 15: allocating memory for 1 tuples
 [NO_PID]: sqlca: code: 0, state: 00000
-[NO_PID]: ecpg_get_data on line 15: RESULT: abcdef offset: -1; array: no
+[NO_PID]: ecpg_get_data on line 15: RESULT: abc'd\ef offset: -1; array: no
 [NO_PID]: sqlca: code: 0, state: 00000
 [NO_PID]: ecpg_store_result on line 15: allocating memory for 1 tuples
 [NO_PID]: sqlca: code: 0, state: 00000
diff --git a/src/interfaces/ecpg/test/expected/preproc-strings.stdout
b/src/interfaces/ecpg/test/expected/preproc-strings.stdout
index 730d72dd64..1456b152d7 100644
--- a/src/interfaces/ecpg/test/expected/preproc-strings.stdout
+++ b/src/interfaces/ecpg/test/expected/preproc-strings.stdout
@@ -1 +1 @@
-abcdef abcdef abcdef data data abc$def
+abc'd\ef abc'd\ef abc'd\ef data data abc$def
diff --git a/src/interfaces/ecpg/test/preproc/strings.pgc b/src/interfaces/ecpg/test/preproc/strings.pgc
index f004ddf6dc..25157f136c 100644
--- a/src/interfaces/ecpg/test/preproc/strings.pgc
+++ b/src/interfaces/ecpg/test/preproc/strings.pgc
@@ -12,10 +12,10 @@ int main(void)

   exec sql set standard_conforming_strings to on;

-  exec sql select 'abcdef',
-                  N'abcdef' AS foo,
-                  E'abc\bdef' AS "foo",
-                  U&'d\0061t\0061' AS U&"foo",
+  exec sql select 'abc''d\ef',
+                  N'abc''d\ef' AS foo,
+                  E'abc''d\\ef' AS "foo""bar",
+                  U&'d\0061t\0061' AS U&"foo""bar",
                   U&'d!+000061t!+000061' uescape '!',
                   $foo$abc$def$foo$
                   into :s1, :s2, :s3, :s4, :s5, :s6;
diff --git a/src/interfaces/ecpg/test/sql/dyntest.pgc b/src/interfaces/ecpg/test/sql/dyntest.pgc
index 5f02fd5dd6..0222c89851 100644
--- a/src/interfaces/ecpg/test/sql/dyntest.pgc
+++ b/src/interfaces/ecpg/test/sql/dyntest.pgc
@@ -51,7 +51,7 @@ main ()
   exec sql create table dyntest (name char (14), d float8, i int,
                  bignumber int8, b boolean, comment text,
                  day date);
-  exec sql insert into dyntest values ('first entry', 14.7, 14, 123045607890, true, 'The world''''s most advanced open
sourcedatabase.', '1987-07-14'); 
+  exec sql insert into dyntest values ('first entry', 14.7, 14, 123045607890, true, 'The world''s most advanced open
sourcedatabase.', '1987-07-14'); 
   exec sql insert into dyntest values ('second entry', 1407.87, 1407, 987065403210, false, 'The elephant never
forgets.','1999-11-5'); 

   exec sql prepare MYQUERY from :QUERY;