Query with timer can throw IllegalThreadStateException

[maiken2051]

Running a query that uses a timer can cause an IllegalThreadStateException if the underlying ThreadGroup has been destroyed.

This error happens when the caller executes a query with a timeout and then doesn't execute another query with a timeout until some time has elapsed. The JVM can destroy the ThreadGroup behind the TimeoutTimer (because it contains no active threads) which causes any new query with a timeout to throw an IllegalThreadStateException.

Once the JDBC driver has entered this state any new query with a timeout will throw the exception. It's especially easy to run into this problem in demanding / multi-threaded applications.

My change makes sure the ThreadGroup is not destroyed before creating a new timer thread. If the ThreadGroup is destroyed, the code will create a new ThreadGroup.

[codecov-io]

Codecov Report

Merging #474 into dev will decrease coverage by 0.03%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##                dev     #474      +/-   ##
============================================
- Coverage     46.25%   46.21%   -0.04%     
+ Complexity     2206     2195      -11     
============================================
  Files           108      108              
  Lines         25231    25215      -16     
  Branches       4173     4166       -7     
============================================
- Hits          11671    11654      -17     
+ Misses        11541    11535       -6     
- Partials       2019     2026       +7

Flag	Coverage Δ	Complexity Δ
#JDBC41	46.04% <83.33%> (+0.03%)	2184 <0> (-9)	⬇️
#JDBC42	46.12% <83.33%> (-0.05%)	2192 <0> (-11)

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/microsoft/sqlserver/jdbc/IOBuffer.java	53.7% <83.33%> (+0.34%)	0 <0> (ø)	⬇️
...rc/main/java/com/microsoft/sqlserver/jdbc/DDC.java	43.37% <0%> (-2.03%)	103% <0%> (-2%)
.../com/microsoft/sqlserver/jdbc/SQLServerDriver.java	75.44% <0%> (-1.52%)	25% <0%> (ø)
...oft/sqlserver/jdbc/SQLServerPreparedStatement.java	50.91% <0%> (-0.66%)	161% <0%> (-4%)
...oft/sqlserver/jdbc/SQLServerParameterMetaData.java	23.98% <0%> (-0.38%)	31% <0%> (ø)
...m/microsoft/sqlserver/jdbc/SQLServerStatement.java	59.06% <0%> (-0.31%)	129% <0%> (-1%)
...soft/sqlserver/jdbc/SQLServerDatabaseMetaData.java	29.47% <0%> (-0.23%)	48% <0%> (-1%)
...n/java/com/microsoft/sqlserver/jdbc/DataTypes.java	77.62% <0%> (-0.17%)	4% <0%> (-1%)
...om/microsoft/sqlserver/jdbc/SQLServerBulkCopy.java	52.48% <0%> (-0.17%)	239% <0%> (-2%)
.../microsoft/sqlserver/jdbc/SQLServerConnection.java	45.71% <0%> (-0.1%)	273% <0%> (-1%)
... and 16 more

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[AfsanehR-zz]

@maiken2051 Thank you for your effort in creating this pr. Could you please open this to dev branch?

[ajlam]

Hi @maiken2051, I just rebased this against the dev branch.

[maiken2051]

Thanks for rebasing the PR...

[ajlam]

@maiken2051, just wanted to keep you posted. We are going to be doing a code review soon. Thanks again for your contributions.

[cheenamalhotra]

Hi @maiken2051 Please sign CLA so that we can proceed with merging your PR.

[maiken2051]

I think the CLA should be OK now?

[peterbae]

Hi @maiken2051, thanks for the contribution. The changes look good, and I approved it - but I'm curious as to where I can find the information regarding JVM destroying a ThreadGroup that contains no active threads. Could you point me to where I can read more about this?

[maiken2051]

Check out the source for ThreadGroup.threadTerminated. This is from OpenJDK but I assume all JVM implementations are similar. The docs say:

Notifies the group that the thread t has terminated. Destroy the group if all of the following conditions are true: this is a daemon thread group; there are no more alive or unstarted threads in the group; there are no subgroups in this thread group.

When a thread exits it will call this method to remove itself from the active list on its owner ThreadGroup. If it's the last active thread the group will be destroyed.

[peterbae]

I see, thanks - please merge this pull request 👍