List offending databases in pg_upgrade datallowconn check

The check for datallowconn being properly set on all databases in the
old cluster errored out on the first instance, rather than report the
set of problematic databases.  This adds reporting to a textfile like
how many other checks are performed.  While there, also add a comment
to the function as per how other checks are commented.

This check won't catch if template1 isn't allowing connections, since
that's used for connecting in the first place.  That error remains as
it is today:

connection to server on socket ".." failed: FATAL:  database "template1" is not currently accepting connections

Author: Jeevan Ladhe <jeevan.ladhe@enterprisedb.com>
Reviewed-by: Suraj Kharage <suraj.kharage@enterprisedb.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CAOgcT0McABqF_iFFQuuRf9is+gmYMsmUu_SWNikyr=2VGyP9Jw@mail.gmail.com

src/bin/pg_upgrade/check.c

@@ -682,6 +682,13 @@ check_is_install_user(ClusterInfo *cluster)
 }
 
 
+/*
+ *	check_proper_datallowconn
+ *
+ *	Ensure that all non-template0 databases allow connections since they
+ *	otherwise won't be restored; and that template0 explicitly doesn't allow
+ *	connections since it would make pg_dumpall --globals restore fail.
+ */
 static void
 check_proper_datallowconn(ClusterInfo *cluster)
 {
@@ -691,9 +698,16 @@ check_proper_datallowconn(ClusterInfo *cluster)
 	int			ntups;
 	int			i_datname;
 	int			i_datallowconn;
+	FILE	   *script = NULL;
+	char		output_path[MAXPGPATH];
+	bool		found = false;
 
 	prep_status("Checking database connection settings");
 
+	snprintf(output_path, sizeof(output_path), "%s/%s",
+			 log_opts.basedir,
+			 "databases_with_datallowconn_false.txt");
+
 	conn_template1 = connectToServer(cluster, "template1");
 
 	/* get database names */
@@ -724,8 +738,14 @@ check_proper_datallowconn(ClusterInfo *cluster)
 			 * restore
 			 */
 			if (strcmp(datallowconn, "f") == 0)
-				pg_fatal("All non-template0 databases must allow connections, "
-						 "i.e. their pg_database.datallowconn must be true\n");
+			{
+				found = true;
+				if (script == NULL && (script = fopen_priv(output_path, "w")) == NULL)
+					pg_fatal("could not open file \"%s\": %s\n",
+							 output_path, strerror(errno));
+
+				fprintf(script, "%s\n", datname);
+			}
 		}
 	}
 
@@ -733,7 +753,22 @@ check_proper_datallowconn(ClusterInfo *cluster)
 
 	PQfinish(conn_template1);
 
-	check_ok();
+	if (script)
+		fclose(script);
+
+	if (found)
+	{
+		pg_log(PG_REPORT, "fatal\n");
+		pg_fatal("All non-template0 databases must allow connections, i.e. their\n"
+				 "pg_database.datallowconn must be true.  Your installation contains\n"
+				 "non-template0 databases with their pg_database.datallowconn set to\n"
+				 "false.  Consider allowing connection for all non-template0 databases\n"
+				 "or drop the databases which do not allow connections.  A list of\n"
+				 "databases with the problem is in the file:\n"
+				 "    %s\n\n", output_path);
+	}
+	else
+		check_ok();
 }