HDFS-14668 Support Fuse with Users from multiple Security Realms

[fapifta]

The rationale behind the change is the following:
when a username is specified to the underlying calls of the FileSystem API, that is specified to the Java kerberos layer as a principal name, and if that does not match the principal in the ticket cache, authentication fails on the Java level. This renders FUSE usable in a kerberized environment, if and only if the user's ticket cache contains a principal who's name is matching the name of the OS user used to access the FUSE mount and the realm of the principal is the default realm per the /etc/krb5.conf file. Other cases have worked before the UserGroupInformation changes in HADOOP-9747, and after the change suggested by this PR.

How it was tested:

• In a non-kerberized environment after deploying the new compiled binary and mount hdfs via fuse:

  • a user can read/write any directory/file that is accessible by him based on his OS username
  • a user can't read/write any directory/file that is not accessible for him based on his OS username
  • username seems to be properly map to the Unix username and permission checks are performed as with the Java client if participating usernames and userids match on the mounting host and the NameNode.

• In a kerberized environment after deploying the new compiled binary and mount hdfs via fuse:

  • a principal is correctly recognized and authorized regardless of the OS username
  • tested read/write with a principal with the OS username in the default realm
  • tested read/write with a principal with the OS username in a non-default but trusted realm
  • tested read/write with a principal with a name different from the OS username in the default realm
  • tested read/write with a principal with a name different from the OS username in a non-default but trusted realm

The tests were running manually, as it requires a multiple realm setup with cross-realm trust, which we cannot emulate in the current test environment.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	1m 8s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s	The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
		_ trunk Compile Tests _
+1 💚	mvninstall	20m 22s	trunk passed
+1 💚	compile	1m 49s	trunk passed
+1 💚	mvnsite	0m 19s	trunk passed
+1 💚	shadedclient	36m 32s	branch has no errors when building and testing our client artifacts.
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 14s	the patch passed
+1 💚	compile	1m 39s	the patch passed
-1 ❌	cc	1m 39s	hadoop-hdfs-project_hadoop-hdfs-native-client generated 5 new + 14 unchanged - 5 fixed = 19 total (was 19)
+1 💚	golang	1m 39s	the patch passed
+1 💚	javac	1m 39s	the patch passed
+1 💚	mvnsite	0m 14s	the patch passed
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	shadedclient	14m 47s	patch has no errors when building and testing our client artifacts.
		_ Other Tests _
+1 💚	unit	6m 50s	hadoop-hdfs-native-client in the patch passed.
+1 💚	asflicense	0m 27s	The patch does not generate ASF License warnings.
		64m 10s

Subsystem	Report/Notes
Docker	Client=19.03.5 Server=19.03.5 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-1739/1/artifact/out/Dockerfile
GITHUB PR	#1739
Optional Tests	dupname asflicense compile cc mvnsite javac unit golang
uname	Linux a0ffb326dad1 4.15.0-66-generic #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / fccccc9
Default Java	1.8.0_222
cc	https://builds.apache.org/job/hadoop-multibranch/job/PR-1739/1/artifact/out/diff-compile-cc-hadoop-hdfs-project_hadoop-hdfs-native-client.txt
Test Results	https://builds.apache.org/job/hadoop-multibranch/job/PR-1739/1/testReport/
Max. process+thread count	307 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs-native-client U: hadoop-hdfs-project/hadoop-hdfs-native-client
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-1739/1/console
versions	git=2.7.4 maven=3.3.9
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[jojochuang]

+1 the fix is deployed and verified to work.