fix: avoid ln race condition in bootstrap script

Avoid startup failures when multiple processes attempt to create the same
symlinks simultaneously.

Ensure symlinks are created and ignore errors if they already exist, which
can happen in a race condition during startup.

Author: rickeylev

python/private/stage1_bootstrap_template.sh

@@ -6,6 +6,30 @@ if [[ -n "${RULES_PYTHON_BOOTSTRAP_VERBOSE:-}" ]]; then
   set -x
 fi
 
+# Creates a symlink. If the symlink already exists, it is tolerated to avoid
+# race conditions during startup.
+function _symlink() {
+  local target="$1"
+  local link="$2"
+  if ln -s "$target" "$link" 2>/dev/null; then
+    return 0
+  fi
+  # If it failed, maybe it already exists because of a race.
+  if [[ -L "$link" || -e "$link" ]]; then
+    return 0
+  fi
+  # If it doesn't exist, maybe we don't have write permission in the directory.
+  local dir
+  dir=$(dirname "$link")
+  if [[ ! -w "$dir" ]]; then
+    echo >&2 "ERROR: Cannot create symlink $link: Directory $dir is not writable"
+  else
+    echo >&2 "ERROR: Failed to create symlink $link -> $target"
+  fi
+  return 1
+}
+
+
 # runfiles-root-relative path
 STAGE2_BOOTSTRAP="%stage2_bootstrap%"
 
@@ -153,7 +177,7 @@ if [[ "$IS_ZIPFILE" == "1" ]]; then
   fi
   # The bin/ directory may not exist if it is empty.
   mkdir -p "$(dirname $python_exe)"
-  ln -s "$symlink_to" "$python_exe"
+  _symlink "$symlink_to" "$python_exe"
 elif [[ "$RECREATE_VENV_AT_RUNTIME" == "1" ]]; then
   if [[ -n "$RULES_PYTHON_EXTRACT_ROOT" ]]; then
     use_exec=1
@@ -226,16 +250,16 @@ EOF
     fi
 
     mkdir -p "$venv/bin"
-    ln -s "$python_exe_actual" "$python_exe"
+    _symlink "$python_exe_actual" "$python_exe"
 
     if [[ ! -e "$venv_site_packages" ]]; then
       mkdir -p $(dirname $venv_site_packages)
-      ln -s "$runfiles_venv_site_packages" "$venv_site_packages"
+      _symlink "$runfiles_venv_site_packages" "$venv_site_packages"
     fi
   fi
 
   if [[ ! -e "$venv/pyvenv.cfg" ]]; then
-    ln -s "$runfiles_venv/pyvenv.cfg" "$venv/pyvenv.cfg"
+    _symlink "$runfiles_venv/pyvenv.cfg" "$venv/pyvenv.cfg"
   fi
 else
   use_exec=1