Introduce framework for parallelizing various pg_upgrade tasks.

A number of pg_upgrade steps require connecting to every database
in the cluster and running the same query in each one.  When there
are many databases, these steps are particularly time-consuming,
especially since they are performed sequentially, i.e., we connect
to a database, run the query, and process the results before moving
on to the next database.

This commit introduces a new framework that makes it easy to
parallelize most of these once-in-each-database tasks by processing
multiple databases concurrently.  This framework manages a set of
slots that follow a simple state machine, and it uses libpq's
asynchronous APIs to establish the connections and run the queries.
The --jobs option is used to determine the number of slots to use.
To use this new task framework, callers simply need to provide the
query and a callback function to process its results, and the
framework takes care of the rest.  A more complete description is
provided at the top of the new task.c file.

None of the eligible once-in-each-database tasks are converted to
use this new framework in this commit.  That will be done via
several follow-up commits.

Reviewed-by: Jeff Davis, Robert Haas, Daniel Gustafsson, Ilya Gladyshev, Corey Huinker
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13

Author: nathan-bossart

doc/src/sgml/ref/pgupgrade.sgml

@@ -118,7 +118,7 @@ PostgreSQL documentation
      <varlistentry>
       <term><option>-j <replaceable class="parameter">njobs</replaceable></option></term>
       <term><option>--jobs=<replaceable class="parameter">njobs</replaceable></option></term>
-      <listitem><para>number of simultaneous processes or threads to use
+      <listitem><para>number of simultaneous connections and processes/threads to use
       </para></listitem>
      </varlistentry>
 
@@ -587,8 +587,8 @@ NET STOP postgresql-&majorversion;
 
     <para>
      The <option>--jobs</option> option allows multiple CPU cores to be used
-     for copying/linking of files and to dump and restore database schemas
-     in parallel;  a good place to start is the maximum of the number of
+     for copying/linking of files, dumping and restoring database schemas
+     in parallel, etc.;  a good place to start is the maximum of the number of
      CPU cores and tablespaces.  This option can dramatically reduce the
      time to upgrade a multi-database server running on a multiprocessor
      machine.

src/bin/pg_upgrade/Makefile

@@ -25,6 +25,7 @@ OBJS = \
 	relfilenumber.o \
 	server.o \
 	tablespace.o \
+	task.o \
 	util.o \
 	version.o
 

src/bin/pg_upgrade/meson.build

@@ -14,6 +14,7 @@ pg_upgrade_sources = files(
   'relfilenumber.c',
   'server.c',
   'tablespace.c',
+  'task.c',
   'util.c',
   'version.c',
 )

src/bin/pg_upgrade/pg_upgrade.h

@@ -494,3 +494,24 @@ void		parallel_transfer_all_new_dbs(DbInfoArr *old_db_arr, DbInfoArr *new_db_arr
 										  char *old_pgdata, char *new_pgdata,
 										  char *old_tablespace);
 bool		reap_child(bool wait_for_child);
+
+/* task.c */
+
+typedef void (*UpgradeTaskProcessCB) (DbInfo *dbinfo, PGresult *res, void *arg);
+
+/* struct definition is private to task.c */
+typedef struct UpgradeTask UpgradeTask;
+
+UpgradeTask *upgrade_task_create(void);
+void		upgrade_task_add_step(UpgradeTask *task, const char *query,
+								  UpgradeTaskProcessCB process_cb, bool free_result,
+								  void *arg);
+void		upgrade_task_run(const UpgradeTask *task, const ClusterInfo *cluster);
+void		upgrade_task_free(UpgradeTask *task);
+
+/* convenient type for common private data needed by several tasks */
+typedef struct
+{
+	FILE	   *file;
+	char		path[MAXPGPATH];
+} UpgradeTaskReport;