Rename io_direct to debug_io_direct.

Give the new GUC introduced by d4e71df6 a name that is clearly not
intended for mainstream use quite yet.

Future proposals would drop the prefix only after adding infrastructure
to make it efficient.  Having the switch in the tree sooner is good
because it might lead to new discoveries about the hazards awaiting us
on a wide range of systems, but that name was too enticing and could
lead to cross-version confusion in future, per complaints from Noah and
Justin.

Suggested-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com> (the idea, not the patch)
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (ditto)
Discussion: https://postgr.es/m/20230430041106.GA2268796%40rfd.leadboat.com

doc/src/sgml/config.sgml

@@ -11160,6 +11160,38 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-debug-io-direct" xreflabel="debug_io_direct">
+      <term><varname>debug_io_direct</varname> (<type>string</type>)
+      <indexterm>
+        <primary><varname>debug_io_direct</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Ask the kernel to minimize caching effects for relation data and WAL
+        files using <literal>O_DIRECT</literal> (most Unix-like systems),
+        <literal>F_NOCACHE</literal> (macOS) or
+        <literal>FILE_FLAG_NO_BUFFERING</literal> (Windows).
+       </para>
+       <para>
+        May be set to an empty string (the default) to disable use of direct
+        I/O, or a comma-separated list of operations that should use direct I/O.
+        The valid options are <literal>data</literal> for
+        main data files, <literal>wal</literal> for WAL files, and
+        <literal>wal_init</literal> for WAL files when being initially
+        allocated.
+       </para>
+       <para>
+        Some operating systems and file systems do not support direct I/O, so
+        non-default settings may be rejected at startup or cause errors.
+       </para>
+       <para>
+        Currently this feature reduces performance, and is intended for
+        developer testing only.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-debug-parallel-query" xreflabel="debug_parallel_query">
       <term><varname>debug_parallel_query</varname> (<type>enum</type>)
       <indexterm>
@@ -11221,38 +11253,6 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-io-direct" xreflabel="io_direct">
-      <term><varname>io_direct</varname> (<type>string</type>)
-      <indexterm>
-        <primary><varname>io_direct</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Ask the kernel to minimize caching effects for relation data and WAL
-        files using <literal>O_DIRECT</literal> (most Unix-like systems),
-        <literal>F_NOCACHE</literal> (macOS) or
-        <literal>FILE_FLAG_NO_BUFFERING</literal> (Windows).
-       </para>
-       <para>
-        May be set to an empty string (the default) to disable use of direct
-        I/O, or a comma-separated list of operations that should use direct I/O.
-        The valid options are <literal>data</literal> for
-        main data files, <literal>wal</literal> for WAL files, and
-        <literal>wal_init</literal> for WAL files when being initially
-        allocated.
-       </para>
-       <para>
-        Some operating systems and file systems do not support direct I/O, so
-        non-default settings may be rejected at startup or cause errors.
-       </para>
-       <para>
-        Currently this feature reduces performance, and is intended for
-        developer testing only.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry id="guc-post-auth-delay" xreflabel="post_auth_delay">
       <term><varname>post_auth_delay</varname> (<type>integer</type>)
       <indexterm>

src/backend/storage/file/fd.c

@@ -3844,7 +3844,7 @@ check_io_direct(char **newval, void **extra, GucSource source)
 #if PG_O_DIRECT == 0
 	if (strcmp(*newval, "") != 0)
 	{
-		GUC_check_errdetail("io_direct is not supported on this platform.");
+		GUC_check_errdetail("debug_io_direct is not supported on this platform.");
 		result = false;
 	}
 	flags = 0;
@@ -3859,7 +3859,7 @@ check_io_direct(char **newval, void **extra, GucSource source)
 	if (!SplitGUCList(rawstring, ',', &elemlist))
 	{
 		GUC_check_errdetail("invalid list syntax in parameter \"%s\"",
-							"io_direct");
+							"debug_io_direct");
 		pfree(rawstring);
 		list_free(elemlist);
 		return false;
@@ -3891,14 +3891,14 @@ check_io_direct(char **newval, void **extra, GucSource source)
 #if XLOG_BLCKSZ < PG_IO_ALIGN_SIZE
 	if (result && (flags & (IO_DIRECT_WAL | IO_DIRECT_WAL_INIT)))
 	{
-		GUC_check_errdetail("io_direct is not supported for WAL because XLOG_BLCKSZ is too small");
+		GUC_check_errdetail("debug_io_direct is not supported for WAL because XLOG_BLCKSZ is too small");
 		result = false;
 	}
 #endif
 #if BLCKSZ < PG_IO_ALIGN_SIZE
 	if (result && (flags & IO_DIRECT_DATA))
 	{
-		GUC_check_errdetail("io_direct is not supported for data because BLCKSZ is too small");
+		GUC_check_errdetail("debug_io_direct is not supported for data because BLCKSZ is too small");
 		result = false;
 	}
 #endif

src/backend/utils/misc/guc_tables.c

@@ -4568,7 +4568,7 @@ struct config_string ConfigureNamesString[] =
 	},
 
 	{
-		{"io_direct", PGC_POSTMASTER, DEVELOPER_OPTIONS,
+		{"debug_io_direct", PGC_POSTMASTER, DEVELOPER_OPTIONS,
 			gettext_noop("Use direct I/O for file access."),
 			NULL,
 			GUC_LIST_INPUT | GUC_NOT_IN_SAMPLE

src/test/modules/test_misc/t/004_io_direct.pl

@@ -40,7 +40,7 @@ my $node = PostgreSQL::Test::Cluster->new('main');
 $node->init;
 $node->append_conf(
 	'postgresql.conf', qq{
-io_direct = 'data,wal,wal_init'
+debug_io_direct = 'data,wal,wal_init'
 shared_buffers = '256kB' # tiny to force I/O
 wal_level = replica # minimal runs out of shared_buffers when set so tiny
 });