Do not fail or print errors when Shellzelisk cannot find a requested ?

?Bazel binary, if tools/bazel exists.

Instead, we should do our best to populate the $BAZEL_REAL env var with some meaningful path, but then delegate to tools/bazel without aborting or printing anything, in case we couldn't find a requested binary or make sense of the contents of .bazelversion.

This is in response to #10356 (comment) and hopefully fixes the use case reported by the affected users.

@laurentlb This is the cherry-pick I mentioned. I will import it, if you LGTM this PR.
@Helcaraxan @davido FYI.

Closes #10664.

PiperOrigin-RevId: 291904662

scripts/packages/bazel.sh

@@ -131,13 +131,6 @@ function get_bazel_version() {
 
 get_bazel_version
 
-if [[ -z $bazel_version ]]; then
-  color "31" "ERROR: No installed Bazel version found, cannot continue."
-  (echo ""
-  echo "Bazel binaries have to be installed in ${wrapper_dir}, but none were found.") 2>&1
-  exit 1
-fi
-
 BAZEL_REAL="${wrapper_dir}/bazel-${bazel_version}-${os_arch_suffix}"
 
 # Try without the architecture suffix.
@@ -156,6 +149,23 @@ if [[ ! -x ${BAZEL_REAL} && -x ${bazel_real_path} ]]; then
   fi
 fi
 
+# If the repository contains a checked-in executable called tools/bazel, we
+# assume that they know what they're doing and have their own way of versioning
+# Bazel. Thus, we don't have to print our helpful messages or error out in case
+# we couldn't find a binary.
+readonly wrapper="${workspace_dir}/tools/bazel"
+if [[ -x "$wrapper" && -f "$wrapper" ]]; then
+  export BAZEL_REAL
+  exec -a "$0" "${wrapper}" "$@"
+fi
+
+if [[ -z $bazel_version ]]; then
+  color "31" "ERROR: No installed Bazel version found, cannot continue."
+  (echo ""
+  echo "Bazel binaries have to be installed in ${wrapper_dir}, but none were found.") 2>&1
+  exit 1
+fi
+
 if [[ ! -x $BAZEL_REAL ]]; then
   color "31" "ERROR: The project you're trying to build requires Bazel ${bazel_version} (${reason}), but it wasn't found in ${wrapper_dir}."
 
@@ -190,10 +200,4 @@ if [[ ! -x $BAZEL_REAL ]]; then
   exit 1
 fi
 
-readonly wrapper="${workspace_dir}/tools/bazel"
-if [[ -x "$wrapper" && -f "$wrapper" ]]; then
-  export BAZEL_REAL
-  exec -a "$0" "${wrapper}" "$@"
-fi
-
 exec -a "$0" "${BAZEL_REAL}" "$@"

src/test/shell/bazel/bazel_wrapper_test.sh

@@ -206,9 +206,43 @@ exit 0
 EOF
   chmod +x tools/bazel
 
-  USE_BAZEL_VERSION="0.29.1" ../bin/bazel build //src:bazel &> "$TEST_log"
+  # Due to https://github.com/bazelbuild/bazel/issues/10356, we have to ignore
+  # the .bazelversion file in case a tools/bazel executable is present.
+  # Even if the requested Bazel version doesn't exist, the wrapper still has to
+  # add the $BAZEL_REAL environment variable, so this will point to a
+  # non-existing file in that case. It's up to the owner of the repo to decide
+  # what to make of it - e.g. print an error message, fallback to something else
+  # or completely ignore the $BAZEL_REAL variable.
+  USE_BAZEL_VERSION="3.0.0" ../bin/bazel build //src:bazel &> "$TEST_log"
+  expect_log "Hello from the wrapper tools/bazel!"
+  expect_log "BAZEL_REAL = .*/bin/bazel-3.0.0"
+  expect_log "My args: build //src:bazel"
+}
+
+test_gracefully_handles_bogus_bazelversion() {
+  setup_mock
+
+  mkdir tools
+  cat > tools/bazel <<'EOF'
+#!/bin/bash
+set -euo pipefail
+echo "Hello from the wrapper tools/bazel!"
+echo "My args: $@"
+exit 0
+EOF
+  chmod +x tools/bazel
+
+  # Create a .bazelversion file that looks completely different than what we
+  # actually support. The wrapper is supposed to not crash and still call the
+  # tools/bazel executable. The content of the $BAZEL_REAL variable can be
+  # completely bogus, of course.
+  cat > .bazelversion <<'EOF'
+mirrors: [http://mirror.example/bazel-5.0.0, http://github.com/example/]
+# The above is our internal mirror. The syntax is only supported since
+# Bazelisk 42.0.
+EOF
+  ../bin/bazel build //src:bazel &> "$TEST_log"
   expect_log "Hello from the wrapper tools/bazel!"
-  expect_log "BAZEL_REAL = .*/bin/bazel-0.29.1"
   expect_log "My args: build //src:bazel"
 }