Better error messages for short reads/writes in SLRU

This avoids getting a

    Could not read from file ...: Success.

for a short read or write (since errno is not set in that case).
Instead, report a more specific error messages.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/5de61b6b-8be9-7771-0048-860328efe027%402ndquadrant.com

src/backend/access/transam/slru.c

@@ -920,18 +920,29 @@ SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid)
 							   path, offset)));
 			break;
 		case SLRU_READ_FAILED:
-			ereport(ERROR,
-					(errcode_for_file_access(),
-					 errmsg("could not access status of transaction %u", xid),
-					 errdetail("Could not read from file \"%s\" at offset %u: %m.",
-							   path, offset)));
+			if (errno)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not read from file \"%s\" at offset %u: %m.",
+								   path, offset)));
+			else
+				ereport(ERROR,
+						(errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not read from file \"%s\" at offset %u: read too few bytes.", path, offset)));
 			break;
 		case SLRU_WRITE_FAILED:
-			ereport(ERROR,
-					(errcode_for_file_access(),
-					 errmsg("could not access status of transaction %u", xid),
-					 errdetail("Could not write to file \"%s\" at offset %u: %m.",
-							   path, offset)));
+			if (errno)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not write to file \"%s\" at offset %u: %m.",
+								   path, offset)));
+			else
+				ereport(ERROR,
+						(errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not write to file \"%s\" at offset %u: wrote too few bytes.",
+								   path, offset)));
 			break;
 		case SLRU_FSYNC_FAILED:
 			ereport(data_sync_elevel(ERROR),