Plugin is not thread safe

[gjoranv]

When testing this plugin in multithreaded maven builds, it failed in 3 out of 5 runs.

Maven creates a new instance of the mojo for each thread. So in general, for a plugin to be thread safe, it cannot use any system resources, e.g. writing to common tmp dirs or system properties. Also, it should not use any non-final static fields.

In all cases when the build fails, we see stack traces from two of our maven modules that run ToolFacade.run at exactly the same time (stack trace is identical for both modules):

09:35:05 Caused by: org.apache.maven.plugin.MojoExecutionException: Failed to execute JavaCC
09:35:05     at org.codehaus.mojo.javacc.ToolFacade.run (ToolFacade.java:100)
09:35:05     at org.codehaus.mojo.javacc.JavaCCMojo.processGrammar (JavaCCMojo.java:161)
09:35:05     at org.codehaus.mojo.javacc.AbstractJavaCCMojo.execute (AbstractJavaCCMojo.java:464)
09:35:05     at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
09:35:05     at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:208)
...

In about half the cases, we don't see any more details. But the remaining builds report a NullPointerException for one of the modules:

09:35:05 Caused by: java.lang.NullPointerException
09:35:05     at com.helger.pgcc.parser.JavaCCParserInternals.setinsertionpoint (JavaCCParserInternals.java:176)
09:35:05     at com.helger.pgcc.parser.JavaCCParser.ClassOrInterfaceDeclaration (JavaCCParser.java:1860)
09:35:05     at com.helger.pgcc.parser.JavaCCParser.TypeDeclaration (JavaCCParser.java:1780)
09:35:05     at com.helger.pgcc.parser.JavaCCParser.CompilationUnit (JavaCCParser.java:1629)
09:35:05     at com.helger.pgcc.parser.JavaCCParser.javacc_input (JavaCCParser.java:136)
09:35:05     at com.helger.pgcc.parser.Main.mainProgram (Main.java:296)
09:35:05     at org.codehaus.mojo.javacc.JavaCC.execute (JavaCC.java:557)
09:35:05     at org.codehaus.mojo.javacc.ToolFacade.run (ToolFacade.java:96)
09:35:05     at org.codehaus.mojo.javacc.JavaCCMojo.processGrammar[INFO] Downloading from local central mirror: http:/ (JavaCCMojo.java:161)
09:35:05    at org.codehaus.mojo.javacc.AbstractJavaCCMojo.execute (AbstractJavaCCMojo.java:464)
09:35:05     at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)

It seems that JavaCCMojo.processGrammar is a common entry point. A quick fix might be to synchronize the method if the root cause is difficult to find.

[phax]

Point taken.
Short term: synchronize call to e "pgcc.parser.Main"
Long term: avoid global variables there :)

[gjoranv]

@phax Any updates on this issue? You are of course right that synchronization must be placed in the static methods of parser.Main and not in the mojo like I suggested.

[phax]

Of course not :( I forgot - sorry.
I will fix this quickly and make you a 4.1.1 release.

[phax]

Version 4.1.1 will be on Maven Central within the next half hour (I hope).

[gjoranv]

@phax, thanks a lot! We'll test this and provide an update.

[gjoranv]

Your changes seems to do the trick. I ran 8 builds with no errors in the environment that was most prone to the issue. I sent a PR to mark the plugin as thread safe to get rid of the maven warning, see #16.
Thanks a lot!