docs: add A23 runner-doctor failure mode for arc-dind credential overlay EROFS - #8020
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[WIP] Add knowledge base entry for A23 in Runner Doctor
docs: add A23 runner-doctor failure mode for arc-dind credential overlay EROFS
Sep 2, 2026
lpcox
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September 2, 2026 17:18
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🟡 Changes recommended
The generated workflow is stale, the portable playbook is incomplete, and the documented probe describes the wrong mount shape.
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Pull request overview
Documents A23, an ARC/DinD credential-overlay EROFS failure fixed by #7998.
Changes:
- Adds A23 catalog and lookup entries.
- Updates the workflow diagnostic playbook and portable agent.
- Adds alignment assertions.
File summaries
| File | Description |
|---|---|
.github/workflows/shared/self-hosted-failure-modes.md |
Adds canonical A23 documentation. |
.github/workflows/self-hosted-runner-doctor.md |
Adds A23 diagnostic matching. |
.github/agents/self-hosted-runner-doctor.md |
Mirrors A23 catalog content. |
scripts/ci/self-hosted-runner-doctor-workflow.test.ts |
Tests A23 catalog alignment. |
Review details
Suppressed comments (1)
.github/agents/self-hosted-runner-doctor.md:266
- This mirrored probe has the same incorrect mount description as the shared catalog: the failing overlay is
/dev/null:/host${effectiveHome}/<credential>:ro, whileworkDirbelongs to the separate prefixed chroot-home mount source. Update the probe to inspect those two surviving mounts rather than an overlay targeting an unprefixed home/workDir path.
| A23 | On `runner.topology: arc-dind` with `--docker-host-path-prefix` set, `docker compose up` fails with `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system` (also seen for `.docker/config.json`, `.composer/auth.json`) | `filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) is meant to drop the bogus AWF-owned chroot-home mount that the DinD daemon can't resolve, but it compared already-prefixed mount sources (from `buildAgentVolumes()`, which applies `--docker-host-path-prefix` as its final step) against the *raw, unprefixed* `config.workDir`/`effectiveHome`. Once a host-path prefix is set — always true on real ARC/DinD — the comparison silently stopped matching, so the bogus chroot-home mount survived filtering and Docker tried to create a `.npmrc` credential-hiding overlay mountpoint inside a path the daemon couldn't write to (EROFS). Distinct from A20 (which was about legitimate caller-supplied home mounts being dropped); A23 is the inverse case — the *bogus* mount not being dropped. | **Fixed in AWF (PR github/gh-aw-firewall#7998, merged 2026-09-02):** extracted `prefixHostPath()` in `src/services/host-path-prefix.ts` from `translateBindMountHostPath()` so bare paths can be prefixed consistently; `filterAgentVolumesForSysroot()` now prefixes `config.workDir`/`effectiveHome` before comparing against mount sources, restoring correct detection of daemon-invisible mounts. The existing safe fallback (`dropUnbackedHostHomeOverlays`, warn + skip masking) now engages correctly instead of silently failing; explicit writable `--mount` for the home root still preserves credential masking. Upgrade AWF to include github/gh-aw-firewall#7998. | Reproduce with `runnerTopology: 'arc-dind'` + `dockerHostPathPrefix` set (e.g. `/host`) and inspect generated compose for a `.npmrc`/`.docker/config.json`/`.composer/auth.json` `/dev/null` overlay mountpoint targeting an unprefixed `effectiveHome`/`workDir` path; on unpatched AWF, `docker compose up` fails with the EROFS mounting error above | github/gh-aw#57468, github/gh-aw-firewall#7994, github/gh-aw-firewall#7998 |
- Files reviewed: 4/4 changed files
- Comments generated: 4
- Review effort level: Balanced
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| - `awf-agent` fails to start under `runner.topology: arc-dind` (runc cannot create the `/dev/null` credential-hiding overlay mountpoints under `/host$HOME`), or the entrypoint aborts with `mkdir -p /host$HOME/.m2` failing under `set -e` → A20 (sysroot filter dropped every mount targeting `/host$HOME`, including a caller-supplied writable home; fixed in github/gh-aw-firewall#7244) | ||
| - `mkdirat ... : read-only file system` at agent container startup while a `filesystem.allowWrite` policy is active (not the `chroot.binariesSourcePath`-specific A12 case) → A21; `[entrypoint][WARN] Could not copy one-shot-token library to /tmp/awf-lib` followed by `Token protection will be disabled` → A21 (silent security-feature degradation under `filesystem.allowWrite`, fixed in github/gh-aw-firewall#7679) | ||
| - `invalid CapDrop: capability not supported by your kernel or not available in the current environment` → A22 (host capability bounding set trimmed below AWF's hardcoded `cap_drop` list, e.g. Talos; fixed in github/gh-aw-firewall#7795) | ||
| - `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system` (or `.docker/config.json`, `.composer/auth.json`) on `arc-dind` with `--docker-host-path-prefix` set → A23 (`filterAgentVolumesForSysroot()` compared prefixed mount sources against unprefixed `workDir`/`effectiveHome`, so the bogus chroot-home mount wasn't dropped; fixed in github/gh-aw-firewall#7998) |
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| // A23 new failure mode (arc-dind /dev/null credential overlay EROFS with --docker-host-path-prefix) | ||
| expect(content).toContain('| A23 | On `runner.topology: arc-dind` with `--docker-host-path-prefix` set, `docker compose up` fails with `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system`'); | ||
| expect(content).toContain('`filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) is meant to drop the bogus AWF-owned chroot-home mount'); | ||
| expect(content).toContain('extracted `prefixHostPath()` in `src/services/host-path-prefix.ts`'); | ||
| expect(content).toContain('`filterAgentVolumesForSysroot()` now prefixes `config.workDir`/`effectiveHome` before comparing against mount sources'); | ||
| expect(content).toContain('github/gh-aw#57468, github/gh-aw-firewall#7994, github/gh-aw-firewall#7998'); | ||
| expect(content).toContain('| `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system` (or `.docker/config.json`, `.composer/auth.json`) on `arc-dind` with `--docker-host-path-prefix` set | A23'); |
| | A20 | Under `runner.topology: arc-dind`, `awf-agent` fails to start (runc cannot create the ~30 credential-hiding `/dev/null` overlay mountpoints under `/host$HOME`), or once worked around, the entrypoint aborts with `mkdir -p /host$HOME/.m2` failing under `set -e` | `filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) dropped every mount targeting `/host$HOME`, including the compiler-supplied writable home (`${RUNNER_TEMP}/gh-aw/home`), because it could not distinguish AWF's own unshared `${workDir}-chroot-home` mount (correctly dropped) from a caller-supplied, daemon-visible home mount | **Fixed in AWF (PR github/gh-aw-firewall#7244, merged 2026-08-11):** home mounts whose target matches an explicitly supplied `--mount`/`config.volumeMounts` spec now survive the sysroot filter (the caller vouches for daemon-visibility); AWF's own workDir-based chroot-home mount is still dropped. If no writable `/host$HOME` survives, `/dev/null` credential overlays under that path are skipped with a warning instead of failing runc startup (overlays at the un-prefixed `$HOME` path are unaffected). `containers/agent/entrypoint.sh`'s JVM proxy pre-seeding now guards its `mkdir -p .../.m2` call and logs+skips instead of aborting under `set -e` when the chroot home is read-only. `buildCustomVolumeMounts()` (`src/services/agent-volumes/workspace-mounts.ts`) also stops re-prefixing targets that already start with `/host`, fixing a related double-`/host` bug for `--mount src:/host/path:ro` specs. Upgrade AWF to include github/gh-aw-firewall#7244. **Not addressed:** gh-aw emitting `${RUNNER_TEMP}/gh-aw` read-only over the chroot home, and its `DOCKER_HOST` gate on the chroot config patch — both require changes in `github/gh-aw`. | Inspect `docker-compose.redacted.yml` for a writable `/host$HOME` (or its explicit-mount target) under `runner.topology: arc-dind`; check agent startup logs for the "no writable home survived, skipping overlays" warning vs. a runc mountpoint-creation failure; check entrypoint logs for the "Cannot create .../.m2 (read-only home)" skip message | github/gh-aw-firewall#7239, github/gh-aw-firewall#7244 | | ||
| | A21 | `awf-agent` fails to start with `runc create failed: ... mkdirat /var/lib/docker/overlay2/<layer-id>/merged/tmp/awf-init: read-only file system` (or equivalent for `/tmp/awf-runner-bin`) when a `filesystem.allowWrite` policy narrows `/tmp` to read-only; most reliably reproduced on ARC/DinD split-filesystem topologies using `--docker-host-path-prefix` | runc creates missing bind mountpoints with `mkdirat` against whichever bind already covers the destination. AWF control-plane mountpoints (`/tmp/awf-init`, `/tmp/awf-runner-bin`) were nested under the user-narrowable `/tmp` bind, so narrowing `/tmp` to `ro` blocked nested mountpoint creation and failed startup with `EROFS`. `/tmp/awf-lib` was helper-copy staging rather than a nested mountpoint; narrowing `/tmp` could silently prevent those copies. On ARC/DinD with a `/tmp`-rooted `--docker-host-path-prefix`, shared-prefix detection also misclassified AWF workDir-derived binds as daemon-only and failed closed. | **Fixed in AWF (PR github/gh-aw-firewall#7679, merged 2026-08-24):** init-signal moved to `/run/awf-init`; a new `planNestedMountpoints()`/`ensureNestedMountpoints()` pass pre-creates mountpoints that would land inside read-only covers (or fails closed); `isSharedDockerHostPathPrefix` now treats only the literal `/tmp` prefix as shared for ARC/DinD detection; legacy `/tmp/awf-init` compatibility binds remain for older pinned agent images; and `/tmp/awf-lib` helper staging (one-shot token protection library, Claude API key helper, `gh` CLI proxy wrapper, CA bundles, runner shims) moved to `/run/awf-lib`, eliminating silent degradation under `filesystem.allowWrite`. Startup now fails closed if the one-shot token library cannot be staged or if CLI proxying is enabled but the `gh` wrapper cannot be installed. Upgrade AWF to include github/gh-aw-firewall#7679. | Check `awf --version` for github/gh-aw-firewall#7679; inspect startup logs for `mkdirat ... read-only file system` with active `filesystem.allowWrite`; inspect entrypoint logs for `[entrypoint][WARN] Could not copy one-shot-token library to /tmp/awf-lib` — on older AWF this confirms the silent-degradation mode; on ARC/DinD verify whether `--docker-host-path-prefix` is exactly `/tmp` (shared) vs. daemon-only (for example `/host`) | github/gh-aw-firewall#7678, github/gh-aw-firewall#7679, github/gh-aw-firewall#7681, github/gh-aw-firewall#7728 | | ||
| | A22 | `arc-dind` topology fails to start with Docker rejecting the compose `cap_drop` list: `invalid CapDrop: capability not supported by your kernel or not available in the current environment: "CAP_SYS_MODULE"` (or similar) on hosts, such as Talos Linux, that trim capabilities from the container capability bounding set | `src/services/squid-service.ts` and `src/services/agent-service.ts` hardcoded `cap_drop` lists (`NET_RAW`, `SYS_ADMIN`, `SYS_PTRACE`, `SYS_MODULE`, `MKNOD`, `AUDIT_WRITE`, `SETFCAP` for Squid; a similar list for the agent) for both the Squid and agent containers with no filtering against what the host/daemon's capability bounding set actually supports, so hosts with a trimmed bounding set (for example Talos `gha-runner-scale-set` with `docker:29-dind`) can never satisfy Docker's compose validation | **Fixed in AWF (PR github/gh-aw-firewall#7795, merged 2026-08-28):** `cap_drop` is now filtered against the effective host capability bounding set (read daemon-side from `/proc/self/status` `CapBnd` via a privileged probe container) before writing `docker-compose.yml`; capabilities already absent from the bounding set are silently omitted from `cap_drop` (a safe no-op, since a capability that can't be granted can't be exploited either). Upgrade AWF to include github/gh-aw-firewall#7795. | `getHostCapabilityBoundingSet()` (`src/capability-filter.ts:78-89`) runs `docker run --rm --privileged --network=none alpine:latest cat /proc/self/status` against the daemon and decodes `CapBnd`; reproduce with `docker compose up` on a host missing `CAP_SYS_MODULE` from the bounding set — pre-fix this fails with `invalid CapDrop`, post-fix compose starts normally | github/gh-aw#56127, github/gh-aw-firewall#7788, github/gh-aw-firewall#7795 | | ||
| | A23 | On `runner.topology: arc-dind` with `--docker-host-path-prefix` set, `docker compose up` fails with `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system` (also seen for `.docker/config.json`, `.composer/auth.json`) | `filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) is meant to drop the bogus AWF-owned chroot-home mount that the DinD daemon can't resolve, but it compared already-prefixed mount sources (from `buildAgentVolumes()`, which applies `--docker-host-path-prefix` as its final step) against the *raw, unprefixed* `config.workDir`/`effectiveHome`. Once a host-path prefix is set — always true on real ARC/DinD — the comparison silently stopped matching, so the bogus chroot-home mount survived filtering and Docker tried to create a `.npmrc` credential-hiding overlay mountpoint inside a path the daemon couldn't write to (EROFS). Distinct from A20 (which was about legitimate caller-supplied home mounts being dropped); A23 is the inverse case — the *bogus* mount not being dropped. | **Fixed in AWF (PR github/gh-aw-firewall#7998, merged 2026-09-02):** extracted `prefixHostPath()` in `src/services/host-path-prefix.ts` from `translateBindMountHostPath()` so bare paths can be prefixed consistently; `filterAgentVolumesForSysroot()` now prefixes `config.workDir`/`effectiveHome` before comparing against mount sources, restoring correct detection of daemon-invisible mounts. The existing safe fallback (`dropUnbackedHostHomeOverlays`, warn + skip masking) now engages correctly instead of silently failing; explicit writable `--mount` for the home root still preserves credential masking. Upgrade AWF to include github/gh-aw-firewall#7998. | Reproduce with `runnerTopology: 'arc-dind'` + `dockerHostPathPrefix` set (e.g. `/host`) and inspect generated compose for a `.npmrc`/`.docker/config.json`/`.composer/auth.json` `/dev/null` overlay mountpoint targeting an unprefixed `effectiveHome`/`workDir` path; on unpatched AWF, `docker compose up` fails with the EROFS mounting error above | github/gh-aw#57468, github/gh-aw-firewall#7994, github/gh-aw-firewall#7998 | |
| | A20 | Under `runner.topology: arc-dind`, `awf-agent` fails to start (runc cannot create the ~30 credential-hiding `/dev/null` overlay mountpoints under `/host$HOME`), or once worked around, the entrypoint aborts with `mkdir -p /host$HOME/.m2` failing under `set -e` | `filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) dropped every mount targeting `/host$HOME`, including the compiler-supplied writable home (`${RUNNER_TEMP}/gh-aw/home`), because it could not distinguish AWF's own unshared `${workDir}-chroot-home` mount (correctly dropped) from a caller-supplied, daemon-visible home mount | **Fixed in AWF (PR github/gh-aw-firewall#7244, merged 2026-08-11):** home mounts whose target matches an explicitly supplied `--mount`/`config.volumeMounts` spec now survive the sysroot filter (the caller vouches for daemon-visibility); AWF's own workDir-based chroot-home mount is still dropped. If no writable `/host$HOME` survives, `/dev/null` credential overlays under that path are skipped with a warning instead of failing runc startup (overlays at the un-prefixed `$HOME` path are unaffected). `containers/agent/entrypoint.sh`'s JVM proxy pre-seeding now guards its `mkdir -p .../.m2` call and logs+skips instead of aborting under `set -e` when the chroot home is read-only. `buildCustomVolumeMounts()` (`src/services/agent-volumes/workspace-mounts.ts`) also stops re-prefixing targets that already start with `/host`, fixing a related double-`/host` bug for `--mount src:/host/path:ro` specs. Upgrade AWF to include github/gh-aw-firewall#7244. **Not addressed:** gh-aw emitting `${RUNNER_TEMP}/gh-aw` read-only over the chroot home, and its `DOCKER_HOST` gate on the chroot config patch — both require changes in `github/gh-aw`. | Inspect `docker-compose.redacted.yml` for a writable `/host$HOME` (or its explicit-mount target) under `runner.topology: arc-dind`; check agent startup logs for the "no writable home survived, skipping overlays" warning vs. a runc mountpoint-creation failure; check entrypoint logs for the "Cannot create .../.m2 (read-only home)" skip message | github/gh-aw-firewall#7239, github/gh-aw-firewall#7244 | | ||
| | A21 | `awf-agent` fails to start with `runc create failed: ... mkdirat /var/lib/docker/overlay2/<layer-id>/merged/tmp/awf-init: read-only file system` (or equivalent for `/tmp/awf-runner-bin`) when a `filesystem.allowWrite` policy narrows `/tmp` to read-only; most reliably reproduced on ARC/DinD split-filesystem topologies using `--docker-host-path-prefix` | runc creates missing bind mountpoints with `mkdirat` against whichever bind already covers the destination. AWF control-plane mountpoints (`/tmp/awf-init`, `/tmp/awf-runner-bin`) were nested under the user-narrowable `/tmp` bind, so narrowing `/tmp` to `ro` blocked nested mountpoint creation and failed startup with `EROFS`. `/tmp/awf-lib` was helper-copy staging rather than a nested mountpoint; narrowing `/tmp` could silently prevent those copies. On ARC/DinD with a `/tmp`-rooted `--docker-host-path-prefix`, shared-prefix detection also misclassified AWF workDir-derived binds as daemon-only and failed closed. | **Fixed in AWF (PR github/gh-aw-firewall#7679, merged 2026-08-24):** init-signal moved to `/run/awf-init`; a new `planNestedMountpoints()`/`ensureNestedMountpoints()` pass pre-creates mountpoints that would land inside read-only covers (or fails closed); `isSharedDockerHostPathPrefix` now treats only the literal `/tmp` prefix as shared for ARC/DinD detection; legacy `/tmp/awf-init` compatibility binds remain for older pinned agent images; and `/tmp/awf-lib` helper staging (one-shot token protection library, Claude API key helper, `gh` CLI proxy wrapper, CA bundles, runner shims) moved to `/run/awf-lib`, eliminating silent degradation under `filesystem.allowWrite`. Startup now fails closed if the one-shot token library cannot be staged or if CLI proxying is enabled but the `gh` wrapper cannot be installed. Upgrade AWF to include github/gh-aw-firewall#7679. | Check `awf --version` for github/gh-aw-firewall#7679; inspect startup logs for `mkdirat ... read-only file system` with active `filesystem.allowWrite`; inspect entrypoint logs for `[entrypoint][WARN] Could not copy one-shot-token library to /tmp/awf-lib` — on older AWF this confirms the silent-degradation mode; on ARC/DinD verify whether `--docker-host-path-prefix` is exactly `/tmp` (shared) vs. daemon-only (for example `/host`) | github/gh-aw-firewall#7678, github/gh-aw-firewall#7679, github/gh-aw-firewall#7681, github/gh-aw-firewall#7728 | | ||
| | A22 | `arc-dind` topology fails to start with Docker rejecting the compose `cap_drop` list: `invalid CapDrop: capability not supported by your kernel or not available in the current environment: "CAP_SYS_MODULE"` (or similar) on hosts, such as Talos Linux, that trim capabilities from the container capability bounding set | `src/services/squid-service.ts` and `src/services/agent-service.ts` hardcoded `cap_drop` lists (`NET_RAW`, `SYS_ADMIN`, `SYS_PTRACE`, `SYS_MODULE`, `MKNOD`, `AUDIT_WRITE`, `SETFCAP` for Squid; a similar list for the agent) for both the Squid and agent containers with no filtering against what the host/daemon's capability bounding set actually supports, so hosts with a trimmed bounding set (for example Talos `gha-runner-scale-set` with `docker:29-dind`) can never satisfy Docker's compose validation | **Fixed in AWF (PR github/gh-aw-firewall#7795, merged 2026-08-28):** `cap_drop` is now filtered against the effective host capability bounding set (read daemon-side from `/proc/self/status` `CapBnd` via a privileged probe container) before writing `docker-compose.yml`; capabilities already absent from the bounding set are silently omitted from `cap_drop` (a safe no-op, since a capability that can't be granted can't be exploited either). Upgrade AWF to include github/gh-aw-firewall#7795. | `getHostCapabilityBoundingSet()` (`src/capability-filter.ts:78-89`) runs `docker run --rm --privileged --network=none alpine:latest cat /proc/self/status` against the daemon and decodes `CapBnd`; reproduce with `docker compose up` on a host missing `CAP_SYS_MODULE` from the bounding set — pre-fix this fails with `invalid CapDrop`, post-fix compose starts normally | github/gh-aw#56127, github/gh-aw-firewall#7788, github/gh-aw-firewall#7795 | | ||
| | A23 | On `runner.topology: arc-dind` with `--docker-host-path-prefix` set, `docker compose up` fails with `error mounting "/dev/null" to .../home/.npmrc: create mountpoint ...: read-only file system` (also seen for `.docker/config.json`, `.composer/auth.json`) | `filterAgentVolumesForSysroot()` (`src/services/optional-services.ts`) is meant to drop the bogus AWF-owned chroot-home mount that the DinD daemon can't resolve, but it compared already-prefixed mount sources (from `buildAgentVolumes()`, which applies `--docker-host-path-prefix` as its final step) against the *raw, unprefixed* `config.workDir`/`effectiveHome`. Once a host-path prefix is set — always true on real ARC/DinD — the comparison silently stopped matching, so the bogus chroot-home mount survived filtering and Docker tried to create a `.npmrc` credential-hiding overlay mountpoint inside a path the daemon couldn't write to (EROFS). Distinct from A20 (which was about legitimate caller-supplied home mounts being dropped); A23 is the inverse case — the *bogus* mount not being dropped. | **Fixed in AWF (PR github/gh-aw-firewall#7998, merged 2026-09-02):** extracted `prefixHostPath()` in `src/services/host-path-prefix.ts` from `translateBindMountHostPath()` so bare paths can be prefixed consistently; `filterAgentVolumesForSysroot()` now prefixes `config.workDir`/`effectiveHome` before comparing against mount sources, restoring correct detection of daemon-invisible mounts. The existing safe fallback (`dropUnbackedHostHomeOverlays`, warn + skip masking) now engages correctly instead of silently failing; explicit writable `--mount` for the home root still preserves credential masking. Upgrade AWF to include github/gh-aw-firewall#7998. | Reproduce with `runnerTopology: 'arc-dind'` + `dockerHostPathPrefix` set (e.g. `/host`) and inspect generated compose for a `.npmrc`/`.docker/config.json`/`.composer/auth.json` `/dev/null` overlay mountpoint targeting an unprefixed `effectiveHome`/`workDir` path; on unpatched AWF, `docker compose up` fails with the EROFS mounting error above | github/gh-aw#57468, github/gh-aw-firewall#7994, github/gh-aw-firewall#7998 | |
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Add runner-doctor failure mode A23 documenting an arc-dind credential-overlay mounting error when
--docker-host-path-prefixis set. The issue occurs whenfilterAgentVolumesForSysroot()compares already-prefixed mount sources against unprefixedworkDir/effectiveHomepaths, causing the bogus chroot-home mount to survive filtering and Docker attempting to create/dev/nulloverlays inside a read-only daemon path (EROFS). The fix is already deployed (#7998, merged 2026-09-02); this PR documents the failure mode and fix in the runner-doctor knowledge base.Changes:
Shared failure-modes catalog (
.github/workflows/shared/self-hosted-failure-modes.md)Workflow playbook (
.github/workflows/self-hosted-runner-doctor.md)Portable agent documentation (
.github/agents/self-hosted-runner-doctor.md)Test suite (
scripts/ci/self-hosted-runner-doctor-workflow.test.ts)The three runner-doctor files are kept in sync by the updater workflow; this PR ensures new failure modes propagate correctly to all three locations.