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Standardize Dependabot auto-merge safeguards #167
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chore/standardize-dependabot-auto-merge-20260905
Sep 6, 2026
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,204 @@ | ||
| /** Validate that a dependency pull request remains safe to auto-merge. */ | ||
| import { existsSync, readFileSync, statSync } from "node:fs"; | ||
| import { isAbsolute, relative, resolve, sep } from "node:path"; | ||
| import { fileURLToPath } from "node:url"; | ||
|
|
||
| const DEPENDABOT = "dependabot[bot]"; | ||
| const WEB_FLOW = "web-flow"; | ||
|
|
||
| function refuse(message) { | ||
| throw new Error(`Refusing auto-merge; ${message}`); | ||
| } | ||
|
|
||
| function dependencyEcosystem(headRef) { | ||
| if (headRef.startsWith("dependabot/uv/")) return "uv"; | ||
| if (headRef.startsWith("dependabot/npm_and_yarn/")) return "npm"; | ||
| if (headRef.startsWith("dependabot/github_actions/")) return "github-actions"; | ||
| refuse(`unsupported Dependabot branch: ${headRef}`); | ||
| } | ||
|
|
||
| function isTrustedBaseFile(trustedBaseDirectory, path) { | ||
| const base = resolve(trustedBaseDirectory); | ||
| const candidate = resolve(base, path); | ||
| const pathFromBase = relative(base, candidate); | ||
| if ( | ||
| pathFromBase === "" || | ||
| isAbsolute(pathFromBase) || | ||
| pathFromBase === ".." || | ||
| pathFromBase.startsWith(`..${sep}`) | ||
| ) | ||
| return false; | ||
| return existsSync(candidate) && statSync(candidate).isFile(); | ||
| } | ||
|
|
||
| function assertAllowedFiles(ecosystem, changedFiles, trustedBaseDirectory) { | ||
| if (changedFiles.length === 0) | ||
| refuse("the pull request has no changed files."); | ||
| if (ecosystem === "uv") { | ||
| if (!isTrustedBaseFile(trustedBaseDirectory, "uv.lock")) | ||
| refuse("the trusted base does not use uv."); | ||
| if (changedFiles.length !== 1 || changedFiles[0] !== "uv.lock") | ||
| refuse("uv updates must change only uv.lock."); | ||
| return; | ||
| } | ||
| if (ecosystem === "npm") { | ||
| if ( | ||
| !isTrustedBaseFile(trustedBaseDirectory, "package.json") || | ||
| !isTrustedBaseFile(trustedBaseDirectory, "package-lock.json") | ||
| ) | ||
| refuse("the trusted base does not use npm."); | ||
| const isPackageFile = (path) => | ||
| path === "package.json" || path === "package-lock.json"; | ||
| const isLockfileOnly = | ||
| changedFiles.length === 1 && changedFiles[0] === "package-lock.json"; | ||
| const isManifestAndLockfile = | ||
| changedFiles.length === 2 && | ||
| changedFiles.includes("package.json") && | ||
| changedFiles.includes("package-lock.json") && | ||
| changedFiles.every(isPackageFile); | ||
| if (!isLockfileOnly && !isManifestAndLockfile) | ||
| refuse( | ||
| "npm updates must change only package.json and package-lock.json.", | ||
| ); | ||
| return; | ||
| } | ||
|
|
||
| const isExistingAllowedFile = (path) => | ||
| (/^\.github\/workflows\/[^/]+\.ya?ml$/.test(path) || | ||
| /(^|\/)action\.ya?ml$/.test(path)) && | ||
| isTrustedBaseFile(trustedBaseDirectory, path); | ||
| if (!changedFiles.every(isExistingAllowedFile)) | ||
| refuse("the update changes a file outside the trusted dependency scope."); | ||
| } | ||
|
|
||
| function isVerifiedDependabotCommit(commit) { | ||
| return ( | ||
| commit?.author?.login === DEPENDABOT && | ||
| commit?.committer?.login === WEB_FLOW && | ||
| commit?.commit?.verification?.verified === true | ||
| ); | ||
| } | ||
|
|
||
| function assertDirectDependabotHistory(event, commits) { | ||
| const [commit] = commits; | ||
| if ( | ||
| commits.length !== 1 || | ||
| !isVerifiedDependabotCommit(commit) || | ||
| commit?.sha !== event.pull_request.head.sha | ||
| ) | ||
| refuse("the pull request does not have a verified Dependabot head commit."); | ||
| } | ||
|
|
||
| function assertMergeParentAncestry(event, commits, ancestryProofs) { | ||
| const mergeCommits = commits.slice(1); | ||
| const currentBase = event.pull_request.base.sha; | ||
| if ( | ||
| !Array.isArray(ancestryProofs) || | ||
| ancestryProofs.length !== mergeCommits.length | ||
| ) | ||
| refuse("the merge-parent ancestry evidence is incomplete."); | ||
|
|
||
| for (const [index, commit] of mergeCommits.entries()) { | ||
| const secondParent = commit?.parents?.[1]?.sha; | ||
| const proof = ancestryProofs[index]; | ||
| if ( | ||
| typeof secondParent !== "string" || | ||
| proof?.parent_sha !== secondParent || | ||
| proof?.base_sha !== currentBase || | ||
| proof?.base_commit !== secondParent || | ||
| proof?.head_commit !== currentBase || | ||
| proof?.merge_base_commit !== secondParent || | ||
| !["ahead", "identical"].includes(proof?.status) || | ||
| !Number.isInteger(proof?.ahead_by) || | ||
| proof.ahead_by < 0 || | ||
| proof?.behind_by !== 0 | ||
| ) | ||
| refuse("a merge second parent is not proven to be on the current base."); | ||
| } | ||
| } | ||
|
|
||
| function assertUpdateBranchHistory(event, commits, ancestryProofs) { | ||
| if (commits.length < 2) | ||
| refuse("the pull request is not a GitHub Update branch merge."); | ||
| if (!isVerifiedDependabotCommit(commits[0])) | ||
| refuse("the pull request history does not begin with Dependabot."); | ||
|
|
||
| for (let index = 1; index < commits.length; index += 1) { | ||
| const commit = commits[index]; | ||
| const previous = commits[index - 1]; | ||
| if ( | ||
| commit?.committer?.login !== WEB_FLOW || | ||
| commit?.commit?.verification?.verified !== true || | ||
| commit?.parents?.length !== 2 || | ||
| commit.parents[0]?.sha !== previous?.sha | ||
| ) | ||
| refuse("the pull request contains a non-Dependabot edit."); | ||
| } | ||
|
|
||
| assertMergeParentAncestry(event, commits, ancestryProofs); | ||
| const latest = commits.at(-1); | ||
| if ( | ||
| latest?.sha !== event.pull_request.head.sha || | ||
| latest.parents[1]?.sha !== event.pull_request.base.sha | ||
| ) | ||
| refuse("the latest commit is not an update from the current base branch."); | ||
| } | ||
|
|
||
| /** | ||
| * Authorize a Dependabot update or a chain containing only GitHub Update branch merges. | ||
| * | ||
| * @param {object} input Validation inputs from the pull-request event and API. | ||
| */ | ||
| export function authorizeDependabotUpdate({ | ||
| ancestryProofs = [], | ||
| changedFiles, | ||
| commits, | ||
| event, | ||
| trustedBaseDirectory = process.cwd(), | ||
| }) { | ||
| const pullRequest = event.pull_request; | ||
| if ( | ||
| event.repository?.fork !== false || | ||
| pullRequest?.user?.login !== DEPENDABOT || | ||
| pullRequest?.head?.repo?.full_name !== event.repository?.full_name || | ||
| pullRequest?.base?.ref !== event.repository?.default_branch | ||
| ) | ||
| refuse( | ||
| "the pull request does not have the required Dependabot provenance.", | ||
| ); | ||
|
|
||
| const ecosystem = dependencyEcosystem(pullRequest.head.ref); | ||
| if (commits.length === 1) assertDirectDependabotHistory(event, commits); | ||
| else assertUpdateBranchHistory(event, commits, ancestryProofs); | ||
| assertAllowedFiles(ecosystem, changedFiles, trustedBaseDirectory); | ||
| return ecosystem; | ||
| } | ||
|
|
||
| function main() { | ||
| const [, , eventPath, changedFilesPath, commitsPath, ancestryProofsPath] = | ||
| process.argv; | ||
| if (!eventPath || !changedFilesPath || !commitsPath || !ancestryProofsPath) | ||
| throw new Error( | ||
| "Usage: dependabot-auto-merge.mjs EVENT CHANGED_FILES COMMITS ANCESTRY_PROOFS", | ||
| ); | ||
| const event = JSON.parse(readFileSync(eventPath, "utf8")); | ||
| const changedFiles = readFileSync(changedFilesPath, "utf8") | ||
| .split("\n") | ||
| .filter(Boolean); | ||
| const commitPages = JSON.parse(readFileSync(commitsPath, "utf8")); | ||
| const commits = commitPages.flat(); | ||
| const ancestryProofs = JSON.parse(readFileSync(ancestryProofsPath, "utf8")); | ||
| authorizeDependabotUpdate({ | ||
| ancestryProofs, | ||
| changedFiles, | ||
| commits, | ||
| event, | ||
| }); | ||
| console.log("Authorized dependency update files and commit history."); | ||
| } | ||
|
|
||
| if ( | ||
| process.argv[1] && | ||
| fileURLToPath(import.meta.url) === resolve(process.argv[1]) | ||
| ) | ||
| main(); | ||
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