Fix failure-to-read-man-page in commit 899bd785c.

posix_fallocate() is not quite a drop-in replacement for fallocate(),
because it is defined to return the error code as its function result,
not in "errno".  I (tgl) missed this because RHEL6's version seems
to set errno as well.  That is not the case on more modern Linuxen,
though, as per buildfarm results.

Aside from fixing the return-convention confusion, remove the test
for ENOSYS; we expect that glibc will mask that for posix_fallocate,
though it does not for fallocate.  Keep the test for EINTR, because
POSIX specifies that as a possible result, and buildfarm results
suggest that it can happen in practice.

Back-patch to 9.4, like the previous commit.

Thomas Munro

Discussion: https://postgr.es/m/1002664500.12301802.1471008223422.JavaMail.yahoo@mail.yahoo.com

src/backend/storage/ipc/dsm_impl.c

@@ -425,17 +425,14 @@ dsm_impl_posix_resize(int fd, off_t size)
 		do
 		{
 			rc = posix_fallocate(fd, 0, size);
-		} while (rc == -1 && errno == EINTR);
+		} while (rc == EINTR);
 
-		if (rc != 0 && errno == ENOSYS)
-		{
-			/*
-			 * Kernel too old (< 2.6.23).  Rather than fail, just trust that
-			 * we won't hit the problem (it typically doesn't show up without
-			 * many-GB-sized requests, anyway).
-			 */
-			rc = 0;
-		}
+		/*
+		 * The caller expects errno to be set, but posix_fallocate() doesn't
+		 * set it.  Instead it returns error numbers directly.  So set errno,
+		 * even though we'll also return rc to indicate success or failure.
+		 */
+		errno = rc;
 	}
 #endif							/* HAVE_POSIX_FALLOCATE && __linux__ */