Update appveyor.yml (#467)

appveyor.yml

@@ -24,6 +24,17 @@ init:
   - SET JAVA_HOME=C:\Program Files\Java\jdk19
   - SET PATH=%JAVA_HOME%\bin;%PATH%
 
+install:
+# If a newer build is queued for the same PR, cancel this one.
+# The AppVeyor 'rollout builds' option is supposed to serve the same
+# purpose, but it is problematic because it tends to cancel builds pushed
+# directly to main/master instead of just PR builds (or the converse).
+# credits: JuliaLang developers.
+- ps: if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `
+        https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/history?recordsNumber=50).builds | `
+        Where-Object pullRequestId -eq $env:APPVEYOR_PULL_REQUEST_NUMBER)[0].buildNumber) { `
+          throw "There are newer queued builds for this pull request, failing early." }
+
 before_build:
   - ps: $env:SOLUTION_NAME = $([io.path]::GetFileNameWithoutExtension($(Get-ChildItem -Path .\* -Include *.sln)))
   - ps: $env:SONAR_PROJECT = "$env:APPVEYOR_REPO_NAME" -replace "/","_"