Build perf is negatively impacted

Running PerfView during the build of the PInvoke project, I see that almost a full second goes to the GetBuildVersion task, which seems to be doing some repeat work perhaps within the same task execution, and certainly across projects. Can we cache the results across projects?

Name                                                                                                                                                            	Inc %	   Inc
 nerdbank.gitversioning.tasks!GetBuildVersion.Execute                                                                                                           	  2.2	   939
+ nerdbank.gitversioning!VersionOracle.Create                                                                                                                   	  2.2	   935
|+ nerdbank.gitversioning!Nerdbank.GitVersioning.VersionOracle..ctor(class System.String,class LibGit2Sharp.Repository,class Nerdbank.GitVersioning.ICloudBuild)	  2.2	   919
||+ nerdbank.gitversioning!GitExtensions.GetVersionHeight                                                                                                       	  0.9	   381
||+ nerdbank.gitversioning!GitExtensions.GetIdAsVersion                                                                                                         	  0.8	   340
||+ nerdbank.gitversioning!VersionFile.GetVersion                                                                                                               	  0.4	   181

Opportunities:

• Add an incremental build aspect to the GetBuildVersion target where the results are saved to a file with the cache key being the tuple: version.json timestamp (and path?) and commit id. (@japj’s idea)
• Cache the height for one or more commits so even if the commit advances, we quickly find a commit whose height was predetermined in the cache file and cut off the potentially long git history traversal (@djluck’s idea)
• #508 Cut down on redundant GetBuildVersion executions per-project that @djluck identified, possibly by using the MSBuild task call to invoke another target but with a reduced set of global properties to improve target skipping around P2P and TargetFramework invocations.
• #508 Build on the shared MSBuild target invocation within a single project to share it across projects based on a project property that specifies the directory path for which version.json will be searched for and used as the ‘cache key’ for reuse. This key can be a global property in the MSBuild task invocation to automatically cache at the right level, and this same property can serve as the input for the version.json search.
• #507 Never parse the same version.json content twice
• Tell libgit2sharp to diff without considering file renames per @djluck’s comment.

[AArnott]

I’m preparing a change that should bring the number of GetBuildVersion invocations from (projects * targetframeworks) to just (projects) (which should help @djluck) and down to exactly 1 when there is just one version.json file in the repo and the repo ‘opts in’ to sharing version info across projects.

[filipnavara]

I tried version 3.3.22-alpha with the added GitVersionBaseDirectory property and I no longer see the GetBuildVersion task in the MSBuild performance traces at all; neither for the full rebuild nor for the incremental one. There are two invocations of GetBuildVersionCore that together consume around 126ms of time: