Let Python stubs respect RUNFILES_* while finding the module space

When invoking a py_binary() through an sh_binary() using the Bash
runfiles library, the location of the py_binary() will be resolved from
the runfiles manifest file. This means that the argv[0] of the Python
stub script will not point to a location under the runfiles directory of
the shell script, but to a location in Bazel's execroot.

Normally this does not lead to any issues, as argv[0] + ".runfiles"
happens to point to be a valid runfiles directory as well. It does
become problematic when --nobuild_runfile_links is provided, as in that
case only the outer shell script is guaranteed to have a runfiles
directory; not the inner Python script.

This change extends the Python stub template to also consider
RUNFILES_DIR when no runfiles directory can be found. Even though it's
not technically correct, we also attempt to derive the runfiles
directory from RUNFILES_MANIFEST_FILE. I suspect that this is a
necessity as long as py_binary()s cannot operate purely using a manifest
file. They currently depend on a concrete instantiation of the runfiles
directory.

Fixes: bazelbuild#11997

src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt

@@ -95,8 +95,22 @@ def CreatePythonPathEntries(python_imports, module_space):
   parts = python_imports.split(':')
   return [module_space] + ['%s/%s' % (module_space, path) for path in parts]
 
-def FindModuleSpace():
+def FindModuleSpace(rel_path):
   """Finds the runfiles tree."""
+  # When the calling process used the runfiles manifest to resolve the
+  # location of this stub script, the path may be expanded. This means
+  # argv[0] may no longer point to a location inside the runfiles
+  # directory. We should therefore respect RUNFILES_DIR and
+  # RUNFILES_MANIFEST_FILE set by the caller.
+  runfiles_dir = os.environ.get('RUNFILES_DIR', None)
+  if not runfiles_dir:
+    runfiles_manifest_file = os.environ.get('RUNFILES_MANIFEST_FILE', '')
+    if (runfiles_manifest_file.endswith('.runfiles_manifest') or
+        runfiles_manifest_file.endswith('.runfiles/MANIFEST')):
+      runfiles_dir = runfiles_manifest_file[:-9]
+  if runfiles_dir and os.path.exists(os.path.join(runfiles_dir, rel_path)):
+    return runfiles_dir
+
   stub_filename = sys.argv[0]
   if not os.path.isabs(stub_filename):
     stub_filename = os.path.join(os.getcwd(), stub_filename)
@@ -286,10 +300,14 @@ def Main():
 
   new_env = {}
 
+  rel_path = '%main%'
+  if IsWindows():
+    rel_path = rel_path.replace('/', os.sep)
+
   if IsRunningFromZip():
     module_space = CreateModuleSpace()
   else:
-    module_space = FindModuleSpace()
+    module_space = FindModuleSpace(rel_path)
 
   python_imports = '%imports%'
   python_path_entries = CreatePythonPathEntries(python_imports, module_space)
@@ -317,10 +335,6 @@ def Main():
   # Now look for my main python source file.
   # The magic string percent-main-percent is replaced with the filename of the
   # main file of the Python binary in BazelPythonSemantics.java.
-  rel_path = '%main%'
-  if IsWindows():
-    rel_path = rel_path.replace('/', os.sep)
-
   main_filename = os.path.join(module_space, rel_path)
   main_filename = GetWindowsPathWithUNCPrefix(main_filename)
   assert os.path.exists(main_filename), \

src/test/shell/integration/python_stub_test.sh

@@ -178,4 +178,51 @@ EOF
       &> $TEST_log || fail "bazel run failed"
 }
 
+# When invoking a Python binary using the runfiles manifest, the stub
+# script's argv[0] will point to a location in the execroot; not the
+# runfiles directory of the caller. The stub script should still be
+# capable of finding its runfiles directory by considering RUNFILES_DIR
+# and RUNFILES_MANIFEST_FILE set by the caller.
+function test_python_through_bash_without_runfile_links() {
+  mkdir -p python_through_bash
+
+  cat > python_through_bash/BUILD << EOF
+py_binary(
+    name = "inner",
+    srcs = ["inner.py"],
+)
+
+sh_binary(
+    name = "outer",
+    srcs = ["outer.sh"],
+    data = [":inner"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)
+EOF
+
+  cat > python_through_bash/outer.sh << 'EOF'
+#!/bin/bash
+
+# --- begin runfiles.bash initialization v2 ---
+# Copy-pasted from the Bazel Bash runfiles library v2.
+set -uo pipefail; f=bazel_tools/tools/bash/runfiles/runfiles.bash
+source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$0.runfiles/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
+# --- end runfiles.bash initialization v2 ---
+
+exec "$(rlocation main/python_through_bash/inner)"
+EOF
+  chmod +x python_through_bash/outer.sh
+
+  touch python_through_bash/inner.py
+
+  bazel run --nobuild_runfile_links //python_through_bash:outer \
+      &> $TEST_log || fail "bazel run failed"
+  expect_log "I am Python"
+}
+
 run_suite "Tests for the Python rules without Python execution"