Re-allow custom GUC names that have more than two components.

Commit 3db826bd5 disallowed this case, but it turns out that some
people are depending on it.  Since the core grammar has allowed
it since 3dc37cd8d, it seems like this code should fall in line.

Per bug #17045 from Robert Sosinski.

Discussion: https://postgr.es/m/17045-6a4a9f0d1513f72b@postgresql.org

src/backend/utils/misc/guc.c

@@ -5368,13 +5368,14 @@ add_guc_variable(struct config_generic *var, int elevel)
 /*
  * Decide whether a proposed custom variable name is allowed.
  *
- * It must be "identifier.identifier", where the rules for what is an
+ * It must be two or more identifiers separated by dots, where the rules
- * identifier agree with scan.l.
+ * for what is an identifier agree with scan.l.  (If you change this rule,
+ * adjust the errdetail in find_option().)
  */
 static bool
 valid_custom_variable_name(const char *name)
 {
-	int			num_sep = 0;
+	bool		saw_sep = false;
 	bool		name_start = true;
 
 	for (const char *p = name; *p; p++)
@@ -5383,7 +5384,7 @@ valid_custom_variable_name(const char *name)
 		{
 			if (name_start)
 				return false;	/* empty name component */
-			num_sep++;
+			saw_sep = true;
 			name_start = true;
 		}
 		else if (strchr("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
@@ -5400,8 +5401,8 @@ valid_custom_variable_name(const char *name)
 	}
 	if (name_start)
 		return false;			/* empty name component */
-	/* OK if we had exactly one separator */
+	/* OK if we found at least one separator */
-	return (num_sep == 1);
+	return saw_sep;
 }
 
 /*
@@ -5516,7 +5517,7 @@ find_option(const char *name, bool create_placeholders, bool skip_errors,
 						(errcode(ERRCODE_INVALID_NAME),
 						 errmsg("invalid configuration parameter name \"%s\"",
 								name),
-						 errdetail("Custom parameter names must be of the form \"identifier.identifier\".")));
+						 errdetail("Custom parameter names must be two or more simple identifiers separated by dots.")));
 			return NULL;
 		}
 	}

src/test/regress/expected/guc.out

@@ -515,6 +515,8 @@ SET no_such_variable TO 42;
 ERROR:  unrecognized configuration parameter "no_such_variable"
 -- Test "custom" GUCs created on the fly (which aren't really an
 -- intended feature, but many people use them).
+SHOW custom.my_guc;  -- error, not known yet
+ERROR:  unrecognized configuration parameter "custom.my_guc"
 SET custom.my_guc = 42;
 SHOW custom.my_guc;
  custom.my_guc 
@@ -522,14 +524,28 @@ SHOW custom.my_guc;
  42
 (1 row)
 
+RESET custom.my_guc;  -- this makes it go to empty, not become unknown again
+SHOW custom.my_guc;
+ custom.my_guc 
+---------------
+ 
+(1 row)
+
+SET custom.my.qualified.guc = 'foo';
+SHOW custom.my.qualified.guc;
+ custom.my.qualified.guc 
+-------------------------
+ foo
+(1 row)
+
 SET custom."bad-guc" = 42;  -- disallowed because -c cannot set this name
 ERROR:  invalid configuration parameter name "custom.bad-guc"
-DETAIL:  Custom parameter names must be of the form "identifier.identifier".
+DETAIL:  Custom parameter names must be two or more simple identifiers separated by dots.
 SHOW custom."bad-guc";
 ERROR:  unrecognized configuration parameter "custom.bad-guc"
 SET special."weird name" = 'foo';  -- could be allowed, but we choose not to
 ERROR:  invalid configuration parameter name "special.weird name"
-DETAIL:  Custom parameter names must be of the form "identifier.identifier".
+DETAIL:  Custom parameter names must be two or more simple identifiers separated by dots.
 SHOW special."weird name";
 ERROR:  unrecognized configuration parameter "special.weird name"
 --

src/test/regress/sql/guc.sql

@@ -151,8 +151,13 @@ SET no_such_variable TO 42;
 
 -- Test "custom" GUCs created on the fly (which aren't really an
 -- intended feature, but many people use them).
+SHOW custom.my_guc;  -- error, not known yet
 SET custom.my_guc = 42;
 SHOW custom.my_guc;
+RESET custom.my_guc;  -- this makes it go to empty, not become unknown again
+SHOW custom.my_guc;
+SET custom.my.qualified.guc = 'foo';
+SHOW custom.my.qualified.guc;
 SET custom."bad-guc" = 42;  -- disallowed because -c cannot set this name
 SHOW custom."bad-guc";
 SET special."weird name" = 'foo';  -- could be allowed, but we choose not to