flaky test: env_test: GetUniqueId() doesn't return unique id

[jay-zhuang]

https://app.circleci.com/pipelines/github/facebook/rocksdb/3708/workflows/14702f71-5afb-4b50-b0b0-0344cecfb0d1/jobs/35850/steps

env/env_test.cc:1180: Failure
Value of: ids.count(unique_id) == 0
  Actual: false
Expected: true
[  FAILED  ] DefaultEnvWithoutDirectIO/EnvPosixTestWithParam.RandomAccessUniqueIDDeletes/0, where GetParam() = (0xb28860, false) (14 ms)

[jay-zhuang]

Seems like on ext4 filesystem, the GetUniqueId may return the same id.

rocksdb/include/rocksdb/env.h

Line 714 in e9befde

virtual size_t GetUniqueId(char* /*id*/, size_t /*max_size*/) const {

The id has 3 parts:
https://github.com/facebook/rocksdb/blob/e5f755f55598256ae6cf80e716738a7d15439ca5/env/io_posix.cc#L347-L349

1. st_dev: device number is const
2. st_ino: inode number is not changed for ext4, as in the test, it create file right after delete, it reuses the same inode number. (seems btrfs still bump the inode number, so it cannot be reproduced on devserver)
3. uversion: it's get from ioctl(FS_IOC_GETVERSION), on ext4, seems it could be duplicated.

[jay-zhuang]

Hi @anand1976 @pdillinger, do you think it could be a correctness issue, as GetUniqueId() may return the same id for different files? Seems the API is only used for cache prefix:

rocksdb/table/block_based/block_based_table_reader.h

Lines 453 to 456 in 27aa443

	static void GenerateCachePrefix(TCache* cc, TFile* file, char* buffer,
	size_t* size) {
	// generate an id from the file
	*size = file->GetUniqueId(buffer, kMaxCacheKeyPrefixSize);

[jay-zhuang]

The problem can be reproduced by:
jay-zhuang@4540eb3

[jay-zhuang]

Seems it's also causing #7405

[jay-zhuang]

As it's a known issue that is tracking in #7405. We're disabling this flaky test for now: #7511