Description Karsten Thoms 2015-10-16 04:29:18 EDT

The Workbench's SaveManager saves and restores the persistent state of builders. Only if this state is present, an incremental build after start of Eclipse is possible. Otherwise builders will perform a full build.

If the auto build is disabled, it can happen that builders were never initialized, which will lead to the problem that no builder state is persisted on closing the workbench. In this case it is expected that the previous state is preserved.

The following steps will reproduce the problem:
1) Start a runtime Eclipse workspace
2) Create a simple Java project
3) Disable auto-build
4) Set breakpoint on WorkspaceTreeReader_2#readTree#line122
   setBuilderInfos(builderInfos);
5) Restart Eclipse
   => Process stops at breakpoint, the variable 'builderInfos' is not empty
6) Continue and wait for start of Eclipse
7) Restart Eclipse again without triggering any build
   => Process stops at breakpoint, the variable 'builderInfos' *is empty*
8) Set breakpoint at BuildManager#initializeBuilder#line876
   if (tree != null)
9) Continue and trigger a build
   => Process stops at breakpoint, variable 'tree' is null
10) Set breakpoint at BuildManager#basicBuild#line161
11) Continue
   => Process stops at breakpoint, the BuildManager decides to perform a full build for the builder

When the BuildManager initializes a builder after start of Eclipse, it will get the last persistent state from the loaded tree (.metadata/plugins/org.eclipse.core.resources/.root/NNN.tree)

If a builder was initialized by BuildManager#initializeBuilder, its latest ElementTree will be stored by the SaveManager into the persisted tree file. But the steps described above will lead to the situation, that builders are never initialized, and so there is no state to save.

Since nothing happened between loading the tree and storing it, it is expected that the last builder state is preserved. The SaveManager might remember the last loaded builder state and store it back for the case that builders were not initialized.

The problem was observed in a large project, where it is a policy to disable auto-build since it lasts several minutes for a full build of the projects in the workspace. The issue leads to the problem that after a plugin update, which usually happens once a day for each developer, the workbench restarts without a build and drops the builder persistent state. After restart, an unnecessary full build is performed, which blocks developers for minutes.