fix: fail safely when AUR synchronization fails

sync-aur.sh

@@ -50,6 +50,16 @@ if git fetch --quiet aur-ro master 2>/dev/null; then
     log "AUR already in sync (${pkgname})"
     exit 0
   fi
+  # Established BEFORE pushing, rather than inferred from a failure afterwards.
+  # The AUR only ever accepts fast-forwards, so this is the whole decision --
+  # there is no force to fall back on.
+  if ! git merge-base --is-ancestor FETCH_HEAD HEAD; then
+    die "the AUR has commits origin does not, for ${pkgname}.
+     The AUR accepts fast-forwards only and refuses force pushes from anyone who
+     is not an AUR administrator, so this cannot be overwritten. Rebase or replay
+     the local changes onto the AUR's HEAD, then re-run."
+  fi
+
   log "AUR differs from origin; pushing (${pkgname})"
 else
   log "no AUR repo for ${pkgname} yet; the first push will create it"
@@ -60,20 +70,28 @@ if [ -n "${NO_GIT_PUSH:-}" ]; then
   exit 0
 fi
 
-if git push aur HEAD:master; then
+if push_output="$(git push aur HEAD:master 2>&1)"; then
+  printf '%s\n' "$push_output"
   log "pushed ${pkgname} to the AUR"
   exit 0
 fi
 
-# A rejected push here usually means the AUR has commits origin does not,
-# i.e. someone pushed directly. Overwriting that silently would destroy it.
-if [ -n "${AUR_FORCE:-}" ]; then
-  warn "AUR_FORCE set: force-pushing over the AUR history for ${pkgname}"
-  git push --force aur HEAD:master
-  exit 0
-fi
+# Report what git actually said, then name a remedy that can work. The previous
+# version blamed every failure on a diverged history and recommended a force
+# push -- wrong on both counts for what turned out to be an unregistered SSH key.
+printf '%s\n' "$push_output" >&2
 
-die "push to the AUR was rejected for ${pkgname}.
-     The AUR history has probably diverged from origin (someone pushed there
-     directly). Inspect it, then re-run with AUR_FORCE=1 to overwrite.
-     This run leaves origin untouched and will be retried on the next pass."
+case "$push_output" in
+  *"Permission denied"*|*"publickey"*|*"Could not read from remote repository"*)
+    die "the AUR rejected the SSH key for ${pkgname}.
+     Check that AUR_SSH_KEY holds a valid private key and that its public half is
+     saved on your AUR account (My Account -> SSH Public Key; the form needs your
+     account password before it will save)." ;;
+  *"non-fast-forward"*|*"fetch first"*)
+    die "the AUR moved between the pre-check and the push for ${pkgname}.
+     Nothing to do: the next run re-reads the AUR and reconciles." ;;
+  *"Host key verification failed"*|*"Could not resolve hostname"*)
+    die "could not reach aur.archlinux.org for ${pkgname}; see the error above." ;;
+  *)
+    die "push to the AUR failed for ${pkgname}; see the git error above." ;;
+esac