Improve error reporting for unsupported effective_io_concurrency setting.

Give a specific error complaining about lack of posix_fadvise() when
someone tries to set effective_io_concurrency > 0 on platforms
without that.

This probably isn't worth extensive back-patching, but I (tgl) felt
cramming it into v11 was reasonable.

James Robinson

Discussion: https://postgr.es/m/153771876450.14994.560017943128223619@wrigleys.postgresql.org
Discussion: https://postgr.es/m/A3942987-5BC7-4F05-B54D-2A0EC2914B33@jlr-photo.com

Author: tglsfdc

src/backend/utils/misc/guc.c

@@ -2641,10 +2641,11 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&effective_io_concurrency,
 #ifdef USE_PREFETCH
-		1, 0, MAX_IO_CONCURRENCY,
+		1,
 #else
-		0, 0, 0,
+		0,
 #endif
+		0, MAX_IO_CONCURRENCY,
 		check_effective_io_concurrency, assign_effective_io_concurrency, NULL
 	},
 
@@ -10681,6 +10682,11 @@ check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 	else
 		return false;
 #else
+	if (*newval != 0)
+	{
+		GUC_check_errdetail("effective_io_concurrency must be set to 0 on platforms that lack posix_fadvise()");
+		return false;
+	}
 	return true;
 #endif							/* USE_PREFETCH */
 }

src/include/pg_config_manual.h

@@ -136,7 +136,9 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs.
+ * If you change this, you probably need to adjust the error message in
+ * check_effective_io_concurrency.)
  */
 #ifdef USE_POSIX_FADVISE
 #define USE_PREFETCH