Add opt-in, resumable regression investigations

.gitattributes

@@ -30,10 +30,13 @@ mono/config.make text eol=lf
 targets.make text eol=lf
 
 *.sh text eol=lf
+eng/regression-bisect/fixtures/checkpoint.txt text eol=lf
 
 *.bsl linguist-vendored=true
 *.xlf linguist-generated=true
 
 *.png binary
 
-.github/workflows/*.lock.yml linguist-generated=true merge=ours
+.github/workflows/*.lock.yml linguist-generated=true merge=ours
+# Preserve this pinned compiler's verbatim output, including generated whitespace.
+.github/workflows/regression-bisect.lock.yml eol=lf whitespace=-blank-at-eol,-blank-at-eof

.github/policies/regression-bisect.json

@@ -0,0 +1,73 @@
+{
+  "schemaVersion": 1,
+  "repository": "dotnet/fsharp",
+  "selection": {
+    "state": "open",
+    "label": "Regression",
+    "createdAtOrAfter": "2026-09-01T00:00:00Z"
+  },
+  "authorization": {
+    "mode": "reviewed-manifests-only",
+    "policySource": "pinned-trusted-default-branch",
+    "approvalDirectory": ".github/regression-bisect/approved",
+    "assets": "reviewed-repository-paths-and-sha256"
+  },
+  "accounting": {
+    "scope": "issue-allowance",
+    "reserveBeforeExecution": true,
+    "retriesCharged": true,
+    "cancellationRefunds": false,
+    "inputEditsResetAllowance": false,
+    "labelChangesResetAllowance": false,
+    "dispatchResetsAllowance": false,
+    "newAllowanceRequires": "maintainer-authorization"
+  },
+  "limits": {
+    "sourceBuilds": 24,
+    "workerUnits": 40,
+    "reservedMinutes": 720,
+    "attemptsPerItem": 2,
+    "assertionRepetitions": 2,
+    "buildTimeoutMinutes": 90,
+    "reproductionTimeoutSeconds": 300,
+    "workerTimeoutMinutes": 110,
+    "discoveryIssues": 3,
+    "discoveryPages": 2,
+    "discoveryPageSize": 100
+  },
+  "evidenceBytesPerStream": 16384,
+  "toolchains": {
+    "publicFeeds": [
+      "https://builds.dotnet.microsoft.com/dotnet",
+      "https://ci.dot.net/public",
+      "https://dotnetcli.blob.core.windows.net/dotnet",
+      "https://download.visualstudio.microsoft.com/download/pr"
+    ],
+    "measuredArchives": [
+      {
+        "sourceSha": "5a8b9adb034f420a5c2c1f577670c5eeeb454b7e",
+        "product": "sdk",
+        "version": "11.0.100-rc.1.26420.103",
+        "rid": "win-x64",
+        "origin": "https://builds.dotnet.microsoft.com",
+        "url": "https://builds.dotnet.microsoft.com/dotnet/Sdk/11.0.100-rc.1.26420.103/dotnet-sdk-11.0.100-rc.1.26420.103-win-x64.zip",
+        "redirectUrls": [],
+        "maxBytes": 1073741824,
+        "timeoutMs": 300000,
+        "approvalRevision": "2ec6de6837b5b367e1f1186e850b8faa2d188b16"
+      },
+      {
+        "sourceSha": "5a8b9adb034f420a5c2c1f577670c5eeeb454b7e",
+        "product": "sdk",
+        "version": "11.0.100-rc.1.26420.103",
+        "rid": "win-x64",
+        "origin": "https://ci.dot.net",
+        "url": "https://ci.dot.net/public/Sdk/11.0.100-rc.1.26420.103/dotnet-sdk-11.0.100-rc.1.26420.103-win-x64.zip",
+        "redirectUrls": [],
+        "maxBytes": 1073741824,
+        "timeoutMs": 300000,
+        "approvalRevision": "2ec6de6837b5b367e1f1186e850b8faa2d188b16"
+      }
+    ]
+  }
+}

.github/regression-bisect/discovery.schema.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "Reviewed issue-linked regression discovery registration (not execution approval)",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["schemaVersion", "repository", "issueNumber", "commentId", "freshnessCommentId", "checkpointAnchorSha", "bootstrapDigest", "freshnessBootstrapDigest", "provisionDeadline", "workflowPath", "workflowGeneration"],
+  "properties": {
+    "schemaVersion": { "const": 2 },
+    "repository": { "const": "dotnet/fsharp" },
+    "issueNumber": { "type": "integer", "minimum": 1 },
+    "commentId": { "type": "integer", "minimum": 1 },
+    "freshnessCommentId": { "type": "integer", "minimum": 1 },
+    "checkpointAnchorSha": { "type": "string", "pattern": "^[a-f0-9]{40}$" },
+    "bootstrapDigest": { "type": "string", "pattern": "^[a-f0-9]{64}$" },
+    "freshnessBootstrapDigest": { "type": "string", "pattern": "^[a-f0-9]{64}$" },
+    "provisionDeadline": { "type": "string", "format": "date-time" },
+    "workflowPath": { "const": ".github/workflows/regression-bisect.lock.yml" },
+    "workflowGeneration": { "type": "integer", "minimum": 1 }
+  }
+}

.github/regression-bisect/registration.schema.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "Reviewed regression allowance registration (not an execution approval)",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["allowanceId", "deadline", "checkpointAnchorSha", "checkpointCommentId", "checkpointBootstrapDigest", "resultCommentId", "resultBootstrapDigest"],
+  "properties": {
+    "allowanceId": { "type": "string", "pattern": "^[a-zA-Z0-9_-]{1,128}$" },
+    "deadline": { "type": "string", "format": "date-time" },
+    "checkpointAnchorSha": { "type": "string", "pattern": "^[a-f0-9]{40}$" },
+    "checkpointCommentId": { "type": "integer", "minimum": 1 },
+    "checkpointBootstrapDigest": { "type": "string", "pattern": "^[a-f0-9]{64}$" },
+    "resultCommentId": { "type": "integer", "minimum": 1 },
+    "resultBootstrapDigest": { "type": "string", "pattern": "^[a-f0-9]{64}$" }
+  }
+}

.github/workflows/regression-bisect-controller.yml

@@ -0,0 +1,61 @@
+name: Regression bisect controller
+on:
+  workflow_dispatch:
+    inputs:
+      issue:
+        description: Approved issue number
+        type: number
+        required: true
+      mode:
+        description: One deterministic transaction
+        type: choice
+        options: [bootstrap, advance, publish]
+        required: true
+      routingKey:
+        description: Durable trusted routing identity (empty for an operator transaction)
+        type: string
+        default: ""
+      routingGeneration:
+        description: Exact checkpoint generation selected by trusted routing
+        type: string
+        default: ""
+permissions:
+  contents: read
+  actions: read
+  issues: write
+  statuses: write
+concurrency:
+  group: regression-bisect-dotnet-fsharp-${{ inputs.issue }}
+  cancel-in-progress: false
+jobs:
+  controller:
+    if: github.repository == 'dotnet/fsharp' && github.ref == 'refs/heads/main' && vars.REGRESSION_CONTROLLER_ENABLED == 'true'
+    environment: regression-controller
+    runs-on: [self-hosted, linux, x64, regression-container-v1]
+    timeout-minutes: 135
+    steps:
+      - name: Require the reviewed immutable workflow revision
+        shell: bash
+        env:
+          RB_TRUSTED_REVISION: ${{ vars.REGRESSION_CONTROLLER_REVISION }}
+        run: |
+          if ! [[ "$RB_TRUSTED_REVISION" =~ ^[0-9a-f]{40}$ && "$RB_TRUSTED_REVISION" == "$GITHUB_WORKFLOW_SHA" ]]; then
+            echo "::error::The reviewed helper revision must be the immutable workflow SHA"
+            exit 1
+          fi
+      - name: Immutable trusted helpers, never the historical source
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          repository: dotnet/fsharp
+          ref: ${{ vars.REGRESSION_CONTROLLER_REVISION }}
+          persist-credentials: false
+          fetch-depth: 1
+      - name: One controller transaction
+        uses: ./eng/regression-bisect
+        env:
+          RB_ACTION_INPUTS: ${{ toJSON(inputs) }}
+          RB_TRUSTED_REVISION: ${{ vars.REGRESSION_CONTROLLER_REVISION }}
+          RB_STATE_DIRECTORY: /var/lib/fsharp-regression-controller
+          RB_CONTAINMENT_DIRECTORY: /var/lib/fsharp-regression-containment
+          RB_GITHUB_TOKEN: ${{ secrets.REGRESSION_CONTROLLER_TOKEN }}
+          RB_SIGNING_KEYS: ${{ secrets.REGRESSION_CONTROLLER_SIGNING_KEYS }}