attestation: simplify gh bootstrapping

Signed-off-by: William Woodruff <william@yossarian.net>

Library/Homebrew/attestation.rb

@@ -22,6 +22,12 @@ module Homebrew
     # @api private
     BACKFILL_CUTOFF = DateTime.new(2024, 3, 14).freeze
 
+    def self.gh_executable
+      @gh_executable ||= with_env("HOMEBREW_VERIFY_ATTESTATIONS" => nil) do
+        ensure_executable!("gh")
+      end
+    end
+
     # Verifies the given bottle against a cryptographic attestation of build provenance.
     #
     # The provenance is verified as originating from `signing_repo`, which is a `String`
@@ -36,7 +42,8 @@ module Homebrew
     #
     # @api private
     def self.check_attestation(bottle, signing_repo, signing_workflow = nil)
-      cmd = [HOMEBREW_GH, "attestation", "verify", bottle.cached_download, "--repo", signing_repo, "--format", "json"]
+      cmd = [gh_executable, "attestation", "verify", bottle.cached_download, "--repo", signing_repo, "--format",
+             "json"]
 
       cmd += ["--cert-identity", signing_workflow] if signing_workflow.present?
 

Library/Homebrew/brew.sh

@@ -454,23 +454,8 @@ setup_git() {
   fi
 }
 
-setup_gh() {
-  # This is set by the user environment.
-  # shellcheck disable=SC2154
-  if [[ -n "${HOMEBREW_VERIFY_ATTESTATIONS}" && -x "${HOMEBREW_PREFIX}/opt/gh/bin/gh" ]]
-  then
-    HOMEBREW_GH="${HOMEBREW_PREFIX}/opt/gh/bin/gh"
-  elif [[ -n "${HOMEBREW_GH_PATH}" ]]
-  then
-    HOMEBREW_GH="${HOMEBREW_GH_PATH}"
-  else
-    HOMEBREW_GH="gh"
-  fi
-}
-
 setup_curl
 setup_git
-setup_gh
 
 HOMEBREW_VERSION="$("${HOMEBREW_GIT}" -C "${HOMEBREW_REPOSITORY}" describe --tags --dirty --abbrev=7 2>/dev/null)"
 HOMEBREW_USER_AGENT_VERSION="${HOMEBREW_VERSION}"
@@ -736,7 +721,6 @@ export HOMEBREW_API_DEFAULT_DOMAIN
 export HOMEBREW_BOTTLE_DEFAULT_DOMAIN
 export HOMEBREW_CURL_SPEED_LIMIT
 export HOMEBREW_CURL_SPEED_TIME
-export HOMEBREW_GH
 
 if [[ -n "${HOMEBREW_MACOS}" && -x "/usr/bin/xcode-select" ]]
 then

Library/Homebrew/cmd/update.sh

@@ -15,7 +15,7 @@
 # HOMEBREW_LIBRARY, HOMEBREW_PREFIX, HOMEBREW_REPOSITORY are set by bin/brew
 # HOMEBREW_BREW_DEFAULT_GIT_REMOTE, HOMEBREW_BREW_GIT_REMOTE, HOMEBREW_CACHE, HOMEBREW_CELLAR, HOMEBREW_CURL
 # HOMEBREW_DEV_CMD_RUN, HOMEBREW_FORCE_BREWED_CURL, HOMEBREW_FORCE_BREWED_GIT, HOMEBREW_SYSTEM_CURL_TOO_OLD
-# HOMEBREW_USER_AGENT_CURL, HOMEBREW_GH are set by brew.sh
+# HOMEBREW_USER_AGENT_CURL are set by brew.sh
 # shellcheck disable=SC2154
 source "${HOMEBREW_LIBRARY}/Homebrew/utils/lock.sh"
 
@@ -415,17 +415,6 @@ user account:
 EOS
   fi
 
-  # we need `gh` if the user enables attestation verification
-  if [[ -n "${HOMEBREW_VERIFY_ATTESTATIONS}" && ! -x "${HOMEBREW_GH}" ]]
-  then
-    # we cannot install `gh` if homebrew/core is unavailable.
-    # we don't enable attestations on `gh` itself, to prevent a bootstrap cycle.
-    if [[ -z "${HOMEBREW_CORE_AVAILABLE}" ]] || ! env -u HOMEBREW_VERIFY_ATTESTATIONS brew install gh
-    then
-      odie "'gh' must be installed and in your PATH!"
-    fi
-  fi
-
   # we may want to use Homebrew CA certificates
   if [[ -n "${HOMEBREW_FORCE_BREWED_CA_CERTIFICATES}" && ! -f "${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem" ]]
   then

Library/Homebrew/formula_installer.rb

@@ -1259,7 +1259,21 @@ on_request: installed_on_request?, options:)
   def pour
     if Homebrew::EnvConfig.verify_attestations? && formula.tap&.core_tap?
       ohai "Verifying attestation for #{formula.name}"
-      Homebrew::Attestation.check_core_attestation formula.bottle
+      begin
+        Homebrew::Attestation.check_core_attestation formula.bottle
+      rescue InvalidAttestationError => e
+        raise CannotInstallFormulaError, <<~EOS
+          The bottle for #{formula.name} has an invalid build provenance attestation.
+
+          This may indicate that the bottle was not produced by the expected
+          tap, or was maliciously inserted into the expected tap's bottle
+          storage.
+
+          Additional context:
+
+          #{e}
+        EOS
+      end
     end
 
     HOMEBREW_CELLAR.cd do

Library/Homebrew/global.rb

@@ -66,8 +66,6 @@ HOMEBREW_PULL_OR_COMMIT_URL_REGEX =
   %r[https://github\.com/([\w-]+)/([\w-]+)?/(?:pull/(\d+)|commit/[0-9a-fA-F]{4,40})]
 HOMEBREW_BOTTLES_EXTNAME_REGEX = /\.([a-z0-9_]+)\.bottle\.(?:(\d+)\.)?tar\.gz$/
 
-HOMEBREW_GH = Pathname(ENV.fetch("HOMEBREW_GH")).freeze
-
 require "env_config"
 require "macos_version"
 require "os"