Skip release branches for same-milestone Skia updates

.agents/skills/merge-skia-update/SKILL.md

@@ -46,32 +46,39 @@ Only after the maintainer sees the exact selected pair, ask them to confirm:
 If they do not confirm, stop. Do not independently re-audit reviews, labels,
 milestones, CI, packages, DEPS, bindings, or release notes.
 
-## 2. Preserve the previous release line when targeting main
+## 2. Preserve the previous release line for milestone bumps
 
 Run the script with the branch names already resolved by the AI:
 
 ```powershell
 pwsh .agents/skills/merge-skia-update/scripts/Prepare-SkiaReleaseBranches.ps1 `
   -SkiaSharpBaseBranch <parent-base> `
+  -SkiaSharpHeadBranch <parent-head> `
   -SkiaBaseBranch <native-base>
 ```
 
 The script determines the release action from the parent base branch:
 
-- `main`: reads the committed SkiaSharp package version from
-  `scripts/VERSIONS.txt` and derives `release/A.B.x` for the current line;
+- `main`: compares the committed `chrome_milestone` on the parent base and head;
+  when the milestone increases, it reads the base's committed SkiaSharp package
+  version from `scripts/VERSIONS.txt` and derives `release/A.B.x` for the
+  previous line;
+- `main` with no milestone change: reports a same-milestone sync and exits
+  without creating refs;
 - `release/A.B.x`: reports a servicing sync and exits without creating refs.
 
-The script always defaults to a dry run. When targeting `main`, it reads the
-current SkiaSharp base tip and the exact mono/skia commit referenced by its
-`externals/skia` gitlink. It requires the supplied mono/skia base branch to
-point at that commit and preflights the derived release branch in both
+The script always defaults to a dry run. For a milestone bump targeting `main`,
+it reads the current SkiaSharp base tip and the exact mono/skia commit referenced
+by its `externals/skia` gitlink. It requires the supplied mono/skia base branch
+to point at that commit and preflights the derived release branch in both
 repositories.
 
-Show the output and obtain confirmation. Rerun with the same inputs plus
-`-Push`. The script checks the source and destination refs again, creates the
-mono/skia release branch first, then the mono/SkiaSharp release branch, and
-verifies both. It never moves an existing branch.
+When the script reports a milestone bump, show the output and obtain
+confirmation. Rerun with the same inputs plus `-Push`. The script checks the
+source and destination refs again, creates the mono/skia release branch first,
+then the mono/SkiaSharp release branch, and verifies both. It never moves an
+existing branch. When it reports a same-milestone or servicing sync, continue
+without a push confirmation.
 
 ## 3. Prepare the mono/skia merge
 
@@ -136,8 +143,8 @@ and body to the maintainer.
 
 Tell the maintainer to merge it manually using the repository's normal merge
 method. Do not wait for another PR CI run: the repin script proved that the
-merged mono/skia tree is identical to the reviewed PR tree. Main CI validates
-the resulting merge.
+merged mono/skia tree is identical to the reviewed PR tree. CI on the resolved
+parent base branch validates the resulting merge.
 
 Stop. Continue only after the maintainer says mono/SkiaSharp is merged.
 
@@ -146,12 +153,12 @@ Stop. Continue only after the maintainer says mono/SkiaSharp is merged.
 Confirm:
 
 - both PRs are merged;
-- mono/SkiaSharp main points its gitlink and cgmanifest at the actual mono/skia
-  merge commit;
-- main CI has started for the resulting SkiaSharp commit.
+- the resolved mono/SkiaSharp parent base branch points its gitlink and
+  cgmanifest at the actual mono/skia merge commit;
+- CI has started on that parent base branch for the resulting SkiaSharp commit.
 
 Report both merged SHAs and the preserved release branch, if created. Do not
-wait for main CI to finish.
+wait for the parent base branch CI to finish.
 
 ## Stop conditions
 
@@ -166,6 +173,7 @@ Stop when:
 - a supplied native base branch does not match the parent base gitlink;
 - an existing release branch points at a different SHA;
 - a release source changes between dry run and `-Push`;
+- the parent head milestone regresses;
 - mono/skia was not merged with a two-parent merge commit;
 - that merge does not contain the parent PR's reviewed native commit;
 - the merged and reviewed native trees differ;

.agents/skills/merge-skia-update/evals/evals.json

@@ -9,7 +9,7 @@
       "expectations": [
         "Does not repeat the deep review.",
         "Verifies that the two supplied PRs reciprocally link to each other.",
-        "Passes AI-resolved branch names to Prepare-SkiaReleaseBranches.ps1 without -Push before applying.",
+        "Passes AI-resolved parent base/head and native base branch names to Prepare-SkiaReleaseBranches.ps1 without -Push before applying.",
         "Invokes pr-commit-message for mono/skia.",
         "Does not merge either PR."
       ]
@@ -41,16 +41,29 @@
     {
       "id": 3,
       "prompt": "The M160 bump is approved. Create the previous release branches before I merge.",
-      "expected_output": "The AI passes the resolved main and native base branches; the script derives release/A.B.x from the current SkiaSharp package version committed on main, then uses the parent base tip and its exact externals/skia gitlink to create matching branches, native first, after a confirmed dry run.",
+      "expected_output": "The AI passes the resolved parent base/head and native base branches; the script verifies that the parent milestone increases, derives release/A.B.x from the current SkiaSharp package version committed on main, then uses the parent base tip and its exact externals/skia gitlink to create matching branches, native first, after a confirmed dry run.",
       "files": [],
       "expectations": [
-        "The AI resolves the base branches but does not calculate or pass the release line.",
+        "The AI resolves the parent base/head and native base branches but does not calculate or pass the release line.",
+        "The script confirms that chrome_milestone increases from the parent base to the parent head.",
         "The script derives release/A.B.x from scripts/VERSIONS.txt on the exact parent base commit.",
         "Uses the native SHA referenced by the parent base.",
         "Preflights both destinations before either write.",
         "Never moves an existing release branch."
       ]
     },
+    {
+      "id": 11,
+      "prompt": "This approved Skia update targets main but stays on the current milestone. Prepare the merge.",
+      "expected_output": "Passes the resolved parent base/head and native base branches to the preparation script, which compares their committed chrome_milestone values, reports a same-milestone sync, creates no release refs, and continues to the native merge-message handoff.",
+      "files": [],
+      "expectations": [
+        "Does not create a release branch for a same-milestone main update.",
+        "Does not ask for a release-branch push confirmation.",
+        "Generates the native merge message.",
+        "Does not merge the native PR."
+      ]
+    },
     {
       "id": 4,
       "prompt": "This is an approved same-milestone servicing sync. Prepare the merge.",
@@ -115,12 +128,12 @@
     {
       "id": 8,
       "prompt": "The repin is pushed. Wait for all PR CI before giving me the SkiaSharp merge message.",
-      "expected_output": "Generates the SkiaSharp merge message without waiting for another PR CI run because tree equality proved the native content unchanged; it notes that main CI validates the resulting merge.",
+      "expected_output": "Generates the SkiaSharp merge message without waiting for another PR CI run because tree equality proved the native content unchanged; it notes that CI on the resolved parent base branch validates the resulting merge.",
       "files": [],
       "expectations": [
         "Does not rerun the deep review.",
         "Does not wait for refreshed PR CI.",
-        "Generates the parent merge message.",
+        "Generates the parent merge message and refers to CI on the resolved parent base branch rather than assuming main.",
         "Leaves the parent merge to the maintainer."
       ]
     }

.agents/skills/merge-skia-update/scripts/Prepare-SkiaReleaseBranches.ps1

@@ -6,6 +6,9 @@ param(
     [Parameter(Mandatory)]
     [string] $SkiaSharpBaseBranch,
 
+    [Parameter(Mandatory)]
+    [string] $SkiaSharpHeadBranch,
+
     [Parameter(Mandatory)]
     [string] $SkiaBaseBranch,
 
@@ -35,6 +38,29 @@ function Get-GitHubRepository {
     return $match.Groups['repository'].Value
 }
 
+function Get-SkiaMilestone {
+    param([string] $Commit)
+
+    $manifestText = git show "${Commit}:cgmanifest.json" | Out-String
+    $manifest = $manifestText | ConvertFrom-Json
+    $registrations = @(
+        $manifest.registrations | Where-Object {
+            $_.component.type -eq 'other' -and
+            $_.component.other.name -eq 'skia' -and
+            $null -ne $_.chrome_milestone
+        }
+    )
+    if ($registrations.Count -ne 1) {
+        throw "Cannot derive the Skia milestone from cgmanifest.json at $Commit."
+    }
+
+    $milestone = [int] $registrations[0].chrome_milestone
+    if ($milestone -le 0) {
+        throw "Invalid Skia milestone '$($registrations[0].chrome_milestone)' in cgmanifest.json at $Commit."
+    }
+    return $milestone
+}
+
 function Assert-Destination {
     param(
         [string] $Repository,
@@ -59,6 +85,7 @@ function New-RemoteBranch {
 $repoRoot = git rev-parse --show-toplevel
 Set-Location $repoRoot
 git check-ref-format --branch $SkiaSharpBaseBranch | Out-Null
+git check-ref-format --branch $SkiaSharpHeadBranch | Out-Null
 git check-ref-format --branch $SkiaBaseBranch | Out-Null
 
 $parentUrl = git remote get-url origin
@@ -75,6 +102,20 @@ if ($SkiaSharpBaseBranch -ne 'main') {
     throw "SkiaSharp base branch must be main or release/A.B.x; got $SkiaSharpBaseBranch."
 }
 
+git fetch --no-tags origin "+refs/heads/${SkiaSharpHeadBranch}:refs/remotes/origin/${SkiaSharpHeadBranch}" | Out-Null
+$parentHeadSha = git rev-parse "refs/remotes/origin/${SkiaSharpHeadBranch}^{commit}"
+
+$baseMilestone = Get-SkiaMilestone $parentBaseSha
+$headMilestone = Get-SkiaMilestone $parentHeadSha
+if ($headMilestone -lt $baseMilestone) {
+    throw "Skia milestone regresses from m$baseMilestone on $SkiaSharpBaseBranch to m$headMilestone on $SkiaSharpHeadBranch."
+}
+if ($headMilestone -eq $baseMilestone) {
+    Write-Host "Same-milestone sync: m$baseMilestone -> m$headMilestone."
+    Write-Host 'No release branches are needed.'
+    exit 0
+}
+
 $versions = (git show "${parentBaseSha}:scripts/VERSIONS.txt" | Out-String).Trim()
 $versionMatch = [regex]::Match($versions, '(?m)^SkiaSharp\s+nuget\s+(?<major>\d+)\.(?<minor>\d+)\.\d+(?:[-+]\S+)?\s*$')
 if (-not $versionMatch.Success) {
@@ -104,6 +145,7 @@ if ($nativeBranchSha -ne $nativeBaseSha) {
 $nativeExisting = Assert-Destination $nativeRepository $nativeUrl $ReleaseBranch $nativeBaseSha
 $parentExisting = Assert-Destination $parentRepository $parentUrl $ReleaseBranch $parentBaseSha
 
+Write-Host "Milestone bump:     m$baseMilestone -> m$headMilestone"
 Write-Host "Release branch:     $ReleaseBranch"
 Write-Host "$nativeRepository source: $SkiaBaseBranch @ $nativeBaseSha$(if ($nativeExisting) { ' (release branch already exists)' })"
 Write-Host "$parentRepository source: $SkiaSharpBaseBranch @ $parentBaseSha$(if ($parentExisting) { ' (release branch already exists)' })"
@@ -115,8 +157,9 @@ if (-not $Push) {
 
 # Re-read every source and destination immediately before the first write.
 $parentBaseNow = Get-RemoteBranchSha $parentUrl $SkiaSharpBaseBranch
+$parentHeadNow = Get-RemoteBranchSha $parentUrl $SkiaSharpHeadBranch
 $nativeBaseNow = Get-RemoteBranchSha $nativeUrl $SkiaBaseBranch
-if ($parentBaseNow -ne $parentBaseSha -or $nativeBaseNow -ne $nativeBaseSha) {
+if ($parentBaseNow -ne $parentBaseSha -or $parentHeadNow -ne $parentHeadSha -or $nativeBaseNow -ne $nativeBaseSha) {
     throw 'A source branch changed after preflight. Run the script without -Push again.'
 }
 $nativeExisting = Assert-Destination $nativeRepository $nativeUrl $ReleaseBranch $nativeBaseSha