Fix db_wal_test::TruncateLastLogAfterRecoverWithoutFlush failure (#6437)

Summary:
`TruncateLastLogAfterRecoverWithoutFlush` case depends on fallocate support
of underlying file system.

On a file system which lacks of this feature, like zfs, it will fail to allocate predefined file size as this test case intends to do;

So a check block is added to detect fallocate support and skip test if not.
The related work is done by JunHe77. Thanks!

Signed-off-by: Yuqi Gu <yuqi.gu@arm.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6437

Differential Revision: D20145032

Pulled By: pdillinger

fbshipit-source-id: c8b691dc508e95acfa2a004ddbc07e2faa76680d

db/db_wal_test.cc

@@ -1528,6 +1528,24 @@ TEST_F(DBWALTest, TruncateLastLogAfterRecoverWithoutFlush) {
   constexpr size_t kKB = 1024;
   Options options = CurrentOptions();
   options.avoid_flush_during_recovery = true;
+  // Test fallocate support of running file system.
+  // Skip this test if fallocate is not supported.
+  std::string fname_test_fallocate = dbname_ + "/preallocate_testfile";
+  int fd = -1;
+  do {
+    fd = open(fname_test_fallocate.c_str(), O_CREAT | O_RDWR | O_TRUNC, 0644);
+  } while (fd < 0 && errno == EINTR);
+  ASSERT_GT(fd, 0);
+  int alloc_status = fallocate(fd, 0, 0, 1);
+  int err_number = errno;
+  close(fd);
+  ASSERT_OK(options.env->DeleteFile(fname_test_fallocate));
+  if (err_number == ENOSYS || err_number == EOPNOTSUPP) {
+    fprintf(stderr, "Skipped preallocated space check: %s\n", strerror(err_number));
+    return;
+  }
+  ASSERT_EQ(0, alloc_status);
+
   DestroyAndReopen(options);
   size_t preallocated_size =
       dbfull()->TEST_GetWalPreallocateBlockSize(options.write_buffer_size);