increment-version-only-master.yml

#WARNING: this may work unpredictable with parallel build of the same project (parallel jobs in matrix are fine)
#Note $separator variable. If "-" can be used earlier in build version, replace it to something unique

version: 0.1.{build}

environment:
  API_TOKEN:
    secure: SlDEDtAKuvsy+VErEuc/4DxdhPeKu2WRqvYzEh/fiH4=

for:
-
  branches:
    only:
      - master      
  init:
    - ps: |
        echo "common init"

init:
- ps: |
    echo "common init"
    $token = $env:API_TOKEN #should be defined as a secure variable
    $separator = "-"
    $headers = @{
      "Authorization" = "Bearer $token"
      "Content-type" = "application/json"
    }
    $apiURL = "https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG"
    $history = Invoke-RestMethod -Uri "$apiURL/history?recordsNumber=2" -Headers $headers  -Method Get          
    if ($history.builds.Count -eq 2) {
      $s = Invoke-RestMethod -Uri "https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/settings" -Headers $headers  -Method Get
      $s.settings.nextBuildNumber = ($s.settings.nextBuildNumber - 1)
      Invoke-RestMethod -Uri 'https://ci.appveyor.com/api/projects' -Headers $headers  -Body ($s.settings | ConvertTo-Json -Depth 10) -Method Put
      $previousVersion = $history.builds[1].version
      if ($previousVersion.IndexOf("$separator") -ne "-1") {$previousVersion = $previousVersion.SubString(0, $previousVersion.IndexOf("$separator"))}
      Update-AppveyorBuild -Version $previousVersion$separator$((New-Guid).ToString().SubString(0,8))
    }