Reject an Xcode whose xcodebuild cannot exit cleanly, and retire the probe

The probe answered what it was for, on the real image:

  Xcode_27_Release_Candidate.app -> prints "Xcode 27.0", exit 134
  Xcode_27.0.0.app / Xcode_27.0.app / Xcode.app -> 27.0, exit 0

Two traps in one line, not one. The crashing app is FIRST in glob order, so
without `|| true` the failing status propagates out of the assignment and
`set -e` kills selection on candidate one -- before any working Xcode is tried,
and with no output at all, because the crash text went to /dev/null. That much
the previous commit fixed.

But it also prints a perfectly good version BEFORE aborting, so testing the
string alone accepts an Xcode whose xcodebuild dies. On this image that was
survivable only because all four report 27.0 and the tie-break keeps the last
one, which is luck: had the crashing app sorted last, it would have been chosen
and every build on it would have failed somewhere less obvious. Candidates are
now judged on their exit status too.

A/B against a fake /Applications holding a crasher that sorts first and prints a
version, plus a working Xcode: master exits 134 and selects nothing; this selects
the working one and exits 0.

The probe workflow goes, as its header said it would.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: shai-almog

.github/workflows/ios-xcode27-probe.yml

@@ -58,17 +58,32 @@ cn1_select_xcode() {
         local app version major best_version="" seen=""
         for app in /Applications/Xcode*.app; do
             [ -x "$app/Contents/Developer/usr/bin/xcodebuild" ] || continue
-            # `|| true` is load-bearing. A caller running under `set -e` -- which
-            # every CI step here does -- is killed by this assignment the moment
-            # one candidate's xcodebuild exits non-zero, before the emptiness
-            # check below ever runs. Measured on the GitHub xcode-27 image, where
-            # /Applications holds the real Xcode plus three symlinks to it and one
-            # of those invocations aborts (SIGABRT, status 134): selection died
-            # with no output at all, because the crash text goes to /dev/null and
-            # the status goes to `set -e`. A candidate that cannot report its
-            # version is one to SKIP, which is what the next line is for.
+            # Run it, then judge it on BOTH its output and its exit status, and
+            # let neither kill the caller.
+            #
+            # Measured on the GitHub xcode-27 image, where /Applications holds the
+            # real Xcode plus three symlinks to it:
+            #
+            #   Xcode_27_Release_Candidate.app -> prints "Xcode 27.0", exit 134
+            #   Xcode_27.0.0.app / Xcode_27.0.app / Xcode.app -> 27.0, exit 0
+            #
+            # Two separate traps in that one line. The crashing app is FIRST in
+            # glob order, and without the `|| true` the failing status propagates
+            # out of the assignment and `set -e` -- which every CI step here runs
+            # with -- kills selection on candidate one, before any working Xcode
+            # is tried, with no output at all because the crash text went to
+            # /dev/null. And it prints a perfectly good version BEFORE aborting,
+            # so checking the string alone accepts an Xcode whose xcodebuild dies;
+            # here that was survivable only because all four report 27.0 and the
+            # tie-break happens to keep the last, which is luck rather than logic.
+            #
+            # An xcodebuild that cannot exit cleanly is not one to build with.
             version="$("$app/Contents/Developer/usr/bin/xcodebuild" -version 2>/dev/null \
                         | awk '/^Xcode /{print $2; exit}' || true)"
+            if ! "$app/Contents/Developer/usr/bin/xcodebuild" -version >/dev/null 2>&1; then
+                seen="$seen ${version:-?}($app, unusable)"
+                continue
+            fi
             [ -n "$version" ] || continue
             seen="$seen $version($app)"
             major="${version%%.*}"