add CI leg to verify assemblies aren't unnecessarily being rebuilt

azure-pipelines.yml

@@ -233,6 +233,19 @@ jobs:
         - script: eng\CIBuild.cmd -configuration Release -noSign /p:DotNetBuildFromSource=true /p:FSharpSourceBuild=true
           displayName: Build
 
+      # Up-to-date
+      - job: UpToDate_Windows
+        pool:
+          vmImage: windows-2019
+        steps:
+        - checkout: self
+          clean: true
+        - task: PowerShell@2
+          displayName: Run up-to-date build check
+          inputs:
+            filePath: eng\tests\UpToDate.ps1
+            arguments: -configuration $(_BuildConfig) -ci -binaryLog
+
 #---------------------------------------------------------------------------------------------------------------------#
 #                                                    FCS builds                                                       #
 #---------------------------------------------------------------------------------------------------------------------#

eng/tests/UpToDate.ps1

@@ -0,0 +1,70 @@
+# This script verifies that subsequent calls to `Build.cmd` don't cause assemblies to be unnecessarily rebuilt.
+
+[CmdletBinding(PositionalBinding=$false)]
+param (
+    [string][Alias('c')]$configuration = "Debug",
+    [parameter(ValueFromRemainingArguments=$true)][string[]]$properties
+)
+
+Set-StrictMode -version 2.0
+$ErrorActionPreference = "Stop"
+
+try {
+    $RepoRoot = Join-Path $PSScriptRoot ".." | Join-Path -ChildPath ".." -Resolve
+    $BuildScript = Join-Path $RepoRoot "Build.cmd"
+
+    # do first build
+    & $BuildScript -configuration $configuration @properties
+    if ($LASTEXITCODE -ne 0) {
+        Write-Host "Error running first build."
+        exit 1
+    }
+
+    # gather assembly timestamps
+    $ArtifactsBinDir = Join-Path $RepoRoot "artifacts" | Join-Path -ChildPath "bin" -Resolve
+    $FSharpAssemblyDirs = Get-ChildItem -Path $ArtifactsBinDir -Filter "FSharp.*"
+    $FSharpAssemblyPaths = $FSharpAssemblyDirs | ForEach-Object { Get-ChildItem -Path (Join-Path $ArtifactsBinDir $_) -Recurse -Filter "$_.dll" } | ForEach-Object { $_.FullName }
+
+    $InitialAssembliesAndTimes = @{}
+    foreach ($asm in $FSharpAssemblyPaths) {
+        $LastWriteTime = (Get-Item $asm).LastWriteTimeUtc
+        $InitialAssembliesAndTimes.Add($asm, $LastWriteTime)
+    }
+
+    $InitialCompiledCount = $FSharpAssemblyPaths.Length
+
+    # build again
+    & $BuildScript -configuration $configuration @properties
+    if ($LASTEXITCODE -ne 0) {
+        Write-Host "Error running second build."
+        exit 1
+    }
+
+    # gather assembly timestamps again
+    $FinalAssembliesAndTimes = @{}
+    foreach ($asm in $FSharpAssemblyPaths) {
+        $LastWriteTime = (Get-Item $asm).LastWriteTimeUtc
+        $FinalAssembliesAndTimes.Add($asm, $LastWriteTime)
+    }
+
+    # validate that assembly timestamps haven't changed
+    $RecompiledFiles = @()
+    foreach ($asm in $InitialAssembliesAndTimes.keys) {
+        $InitialTime = $InitialAssembliesAndTimes[$asm]
+        $FinalTime = $FinalAssembliesAndTimes[$asm]
+        if ($InitialTime -ne $FinalTime) {
+            $RecompiledFiles += $asm
+        }
+    }
+
+    $RecompiledCount = $RecompiledFiles.Length
+    Write-Host "$RecompiledCount of $InitialCompiledCount assemblies were re-compiled"
+    $RecompiledFiles | ForEach-Object { Write-Host "    $_" }
+    exit $RecompiledCount
+}
+catch {
+    Write-Host $_
+    Write-Host $_.Exception
+    Write-Host $_.ScriptStackTrace
+    exit 1
+}