Further fixes for quoted-list GUC values in pg_dump and ruleutils.c.

Commits 742869946 et al turn out to be a couple bricks shy of a load.
We were dumping the stored values of GUC_LIST_QUOTE variables as they
appear in proconfig or setconfig catalog columns.  However, although that
quoting rule looks a lot like SQL-identifier double quotes, there are two
critical differences: empty strings ("") are legal, and depending on which
variable you're considering, values longer than NAMEDATALEN might be valid
too.  So the current technique fails altogether on empty-string list
entries (as reported by Steven Winfield in bug #15248) and it also risks
truncating file pathnames during dump/reload of GUC values that are lists
of pathnames.

To fix, split the stored value without any downcasing or truncation,
and then emit each element as a SQL string literal.

This is a tad annoying, because we now have three copies of the
comma-separated-string splitting logic in varlena.c as well as a fourth
one in dumputils.c.  (Not to mention the randomly-different-from-those
splitting logic in libpq...)  I looked at unifying these, but it would
be rather a mess unless we're willing to tweak the API definitions of
SplitIdentifierString, SplitDirectoriesString, or both.  That might be
worth doing in future; but it seems pretty unsafe for a back-patched
bug fix, so for now accept the duplication.

Back-patch to all supported branches, as the previous fix was.

Discussion: https://postgr.es/m/7585.1529435872@sss.pgh.pa.us

src/backend/utils/adt/ruleutils.c

@@ -2640,14 +2640,39 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 				/*
 				 * Variables that are marked GUC_LIST_QUOTE were already fully
 				 * quoted by flatten_set_variable_args() before they were put
-				 * into the proconfig array; we mustn't re-quote them or we'll
-				 * make a mess.  Variables that are not so marked should just
-				 * be emitted as simple string literals.  If the variable is
-				 * not known to guc.c, we'll do the latter; this makes it
-				 * unsafe to use GUC_LIST_QUOTE for extension variables.
+				 * into the proconfig array.  However, because the quoting
+				 * rules used there aren't exactly like SQL's, we have to
+				 * break the list value apart and then quote the elements as
+				 * string literals.  (The elements may be double-quoted as-is,
+				 * but we can't just feed them to the SQL parser; it would do
+				 * the wrong thing with elements that are zero-length or
+				 * longer than NAMEDATALEN.)
+				 *
+				 * Variables that are not so marked should just be emitted as
+				 * simple string literals.  If the variable is not known to
+				 * guc.c, we'll do that; this makes it unsafe to use
+				 * GUC_LIST_QUOTE for extension variables.
 				 */
 				if (GetConfigOptionFlags(configitem, true) & GUC_LIST_QUOTE)
-					appendStringInfoString(&buf, pos);
+				{
+					List	   *namelist;
+					ListCell   *lc;
+
+					/* Parse string into list of identifiers */
+					if (!SplitGUCList(pos, ',', &namelist))
+					{
+						/* this shouldn't fail really */
+						elog(ERROR, "invalid list syntax in proconfig item");
+					}
+					foreach(lc, namelist)
+					{
+						char	   *curname = (char *) lfirst(lc);
+
+						simple_quote_literal(&buf, curname);
+						if (lnext(lc))
+							appendStringInfoString(&buf, ", ");
+					}
+				}
 				else
 					simple_quote_literal(&buf, pos);
 				appendStringInfoChar(&buf, '\n');

src/backend/utils/adt/varlena.c

@@ -3503,6 +3503,118 @@ SplitDirectoriesString(char *rawstring, char separator,
 }
 
 
+/*
+ * SplitGUCList --- parse a string containing identifiers or file names
+ *
+ * This is used to split the value of a GUC_LIST_QUOTE GUC variable, without
+ * presuming whether the elements will be taken as identifiers or file names.
+ * We assume the input has already been through flatten_set_variable_args(),
+ * so that we need never downcase (if appropriate, that was done already).
+ * Nor do we ever truncate, since we don't know the correct max length.
+ * We disallow embedded whitespace for simplicity (it shouldn't matter,
+ * because any embedded whitespace should have led to double-quoting).
+ * Otherwise the API is identical to SplitIdentifierString.
+ *
+ * XXX it's annoying to have so many copies of this string-splitting logic.
+ * However, it's not clear that having one function with a bunch of option
+ * flags would be much better.
+ *
+ * XXX there is a version of this function in src/bin/pg_dump/dumputils.c.
+ * Be sure to update that if you have to change this.
+ *
+ * Inputs:
+ *	rawstring: the input string; must be overwritable!	On return, it's
+ *			   been modified to contain the separated identifiers.
+ *	separator: the separator punctuation expected between identifiers
+ *			   (typically '.' or ',').  Whitespace may also appear around
+ *			   identifiers.
+ * Outputs:
+ *	namelist: filled with a palloc'd list of pointers to identifiers within
+ *			  rawstring.  Caller should list_free() this even on error return.
+ *
+ * Returns true if okay, false if there is a syntax error in the string.
+ */
+bool
+SplitGUCList(char *rawstring, char separator,
+			 List **namelist)
+{
+	char	   *nextp = rawstring;
+	bool		done = false;
+
+	*namelist = NIL;
+
+	while (scanner_isspace(*nextp))
+		nextp++;				/* skip leading whitespace */
+
+	if (*nextp == '\0')
+		return true;			/* allow empty string */
+
+	/* At the top of the loop, we are at start of a new identifier. */
+	do
+	{
+		char	   *curname;
+		char	   *endp;
+
+		if (*nextp == '"')
+		{
+			/* Quoted name --- collapse quote-quote pairs */
+			curname = nextp + 1;
+			for (;;)
+			{
+				endp = strchr(nextp + 1, '"');
+				if (endp == NULL)
+					return false;	/* mismatched quotes */
+				if (endp[1] != '"')
+					break;		/* found end of quoted name */
+				/* Collapse adjacent quotes into one quote, and look again */
+				memmove(endp, endp + 1, strlen(endp));
+				nextp = endp;
+			}
+			/* endp now points at the terminating quote */
+			nextp = endp + 1;
+		}
+		else
+		{
+			/* Unquoted name --- extends to separator or whitespace */
+			curname = nextp;
+			while (*nextp && *nextp != separator &&
+				   !scanner_isspace(*nextp))
+				nextp++;
+			endp = nextp;
+			if (curname == nextp)
+				return false;	/* empty unquoted name not allowed */
+		}
+
+		while (scanner_isspace(*nextp))
+			nextp++;			/* skip trailing whitespace */
+
+		if (*nextp == separator)
+		{
+			nextp++;
+			while (scanner_isspace(*nextp))
+				nextp++;		/* skip leading whitespace for next */
+			/* we expect another name, so done remains false */
+		}
+		else if (*nextp == '\0')
+			done = true;
+		else
+			return false;		/* invalid syntax */
+
+		/* Now safe to overwrite separator with a null */
+		*endp = '\0';
+
+		/*
+		 * Finished isolating current name --- add it to list
+		 */
+		*namelist = lappend(*namelist, curname);
+
+		/* Loop back if we didn't reach end of string */
+	} while (!done);
+
+	return true;
+}
+
+
 /*****************************************************************************
  *	Comparison Functions used for bytea
  *