fix undefined variables in the wrap_outputs script

BUILD_TMPDIR and EXT_BUILD_DEPS were used in the wrapper script, but
they weren't actually defined. This wasn't caught because the script
prelude (which sets bash -u) wasn't used either.

This fix:
- Adds the missing script prelude to the wrapper script
- Generalizes get_env_prelude slightly so built_tools can use it
  directly, without needing to override variables
- Passes the env prelude to wrapped_outputs so it can set those same
  variables
- Prints out the paths to the build and deps dir at the end of a failed
  script, to make it easier to debug failed builds.

foreign_cc/built_tools/private/built_tools_framework.bzl

@@ -70,15 +70,13 @@ def built_tool_rule_impl(ctx, script_lines, out_dir, mnemonic, additional_tools
 
     root = detect_root(ctx.attr.srcs)
     lib_name = ctx.attr.name
-    env_prelude = get_env_prelude(ctx, lib_name, [], "")
+    env_prelude = get_env_prelude(ctx, out_dir.path, "")
 
     cc_toolchain = find_cpp_toolchain(ctx)
 
-    script = env_prelude + [
+    script = [
         "##script_prelude##",
-        "export EXT_BUILD_ROOT=##pwd##",
-        "export INSTALLDIR=$$EXT_BUILD_ROOT$$/{}".format(out_dir.path),
-        "export BUILD_TMPDIR=$$INSTALLDIR$$.build_tmpdir",
+    ] + env_prelude + [
         "##rm_rf## $$INSTALLDIR$$",
         "##rm_rf## $$BUILD_TMPDIR$$",
         "##mkdirs## $$INSTALLDIR$$",
@@ -97,7 +95,13 @@ def built_tool_rule_impl(ctx, script_lines, out_dir, mnemonic, additional_tools
         "",
     ])
 
-    wrapped_outputs = wrap_outputs(ctx, lib_name, mnemonic, script_text)
+    wrapped_outputs = wrap_outputs(
+        ctx,
+        lib_name = lib_name,
+        configure_name = mnemonic,
+        env_prelude = env_prelude,
+        script_text = script_text,
+    )
 
     tools = depset(
         [wrapped_outputs.wrapper_script_file, wrapped_outputs.script_file],

foreign_cc/private/framework.bzl

@@ -290,22 +290,21 @@ dependencies.""",
 def _is_msvc_var(var):
     return var == "INCLUDE" or var == "LIB"
 
-def get_env_prelude(ctx, lib_name, data_dependencies, target_root):
+def get_env_prelude(ctx, installdir, data_dependencies):
     """Generate a bash snippet containing environment variable definitions
 
     Args:
         ctx (ctx): The rule's context object
-        lib_name (str): The name of the target being built
+        installdir (str): The path from the root target's directory in the build output
         data_dependencies (list): A list of targets representing dependencies
-        target_root (str): The path from the root target's directory in the build output
 
     Returns:
         tuple: A list of environment variables to define in the build script and a dict
             of environment variables
     """
     env_snippet = [
         "export EXT_BUILD_ROOT=##pwd##",
-        "export INSTALLDIR=$$EXT_BUILD_ROOT$$/" + target_root + "/" + lib_name,
+        "export INSTALLDIR=$$EXT_BUILD_ROOT$$/" + installdir,
         "export BUILD_TMPDIR=$$INSTALLDIR$$.build_tmpdir",
         "export EXT_BUILD_DEPS=$$INSTALLDIR$$.ext_build_deps",
     ]
@@ -439,7 +438,8 @@ def cc_external_rule_impl(ctx, attrs):
     # Also add legacy dependencies while they're still available
     data_dependencies += ctx.attr.tools_deps + ctx.attr.additional_tools
 
-    env_prelude = get_env_prelude(ctx, lib_name, data_dependencies, target_root)
+    installdir = target_root + "/" + lib_name
+    env_prelude = get_env_prelude(ctx, installdir, data_dependencies)
 
     postfix_script = [attrs.postfix_script]
     if not attrs.postfix_script:
@@ -483,7 +483,13 @@ def cc_external_rule_impl(ctx, attrs):
         convert_shell_script(ctx, script_lines),
         "",
     ])
-    wrapped_outputs = wrap_outputs(ctx, lib_name, attrs.configure_name, script_text)
+    wrapped_outputs = wrap_outputs(
+        ctx,
+        lib_name = lib_name,
+        configure_name = attrs.configure_name,
+        script_text = script_text,
+        env_prelude = env_prelude,
+    )
 
     rule_outputs = outputs.declared_outputs + [installdir_copy.file]
     cc_toolchain = find_cpp_toolchain(ctx)
@@ -587,7 +593,7 @@ WrappedOutputs = provider(
 )
 
 # buildifier: disable=function-docstring
-def wrap_outputs(ctx, lib_name, configure_name, script_text, build_script_file = None):
+def wrap_outputs(ctx, lib_name, configure_name, script_text, env_prelude, build_script_file = None):
     extension = script_extension(ctx)
     build_log_file = ctx.actions.declare_file("{}_foreign_cc/{}.log".format(lib_name, configure_name))
     build_script_file = ctx.actions.declare_file("{}_foreign_cc/build_script{}".format(lib_name, extension))
@@ -602,34 +608,38 @@ def wrap_outputs(ctx, lib_name, configure_name, script_text, build_script_file =
     cleanup_on_success_function = create_function(
         ctx,
         "cleanup_on_success",
-        "rm -rf $BUILD_TMPDIR $EXT_BUILD_DEPS",
+        "rm -rf $$BUILD_TMPDIR$$ $$EXT_BUILD_DEPS$$",
     )
     cleanup_on_failure_function = create_function(
         ctx,
         "cleanup_on_failure",
         "\n".join([
             "##echo## \"rules_foreign_cc: Build failed!\"",
-            "##echo## \"rules_foreign_cc: Keeping temp build directory $$BUILD_TMPDIR$$ and dependencies directory $$EXT_BUILD_DEPS$$ for debug.\"",
-            "##echo## \"rules_foreign_cc: Please note that the directories inside a sandbox are still cleaned unless you specify '--sandbox_debug' Bazel command line flag.\"",
             "##echo## \"rules_foreign_cc: Printing build logs:\"",
             "##echo## \"_____ BEGIN BUILD LOGS _____\"",
             "##cat## $$BUILD_LOG$$",
             "##echo## \"_____ END BUILD LOGS _____\"",
             "##echo## \"rules_foreign_cc: Build wrapper script location: $$BUILD_WRAPPER_SCRIPT$$\"",
             "##echo## \"rules_foreign_cc: Build script location: $$BUILD_SCRIPT$$\"",
             "##echo## \"rules_foreign_cc: Build log location: $$BUILD_LOG$$\"",
+            "##echo## \"rules_foreign_cc: Keeping these below directories for debug, but note that the directories inside a sandbox\"",
+            "##echo## \"rules_foreign_cc: are still cleaned unless you specify the '--sandbox_debug' Bazel command line flag.\"",
+            "##echo## \"rules_foreign_cc: Build Dir: $$BUILD_TMPDIR$$\"",
+            "##echo## \"rules_foreign_cc: Deps Dir: $$EXT_BUILD_DEPS$$\"",
             "##echo## \"\"",
         ]),
     )
     trap_function = "##cleanup_function## cleanup_on_success cleanup_on_failure"
 
     build_command_lines = [
+        "##script_prelude##",
         "##assert_script_errors##",
         cleanup_on_success_function,
         cleanup_on_failure_function,
         # the call trap is defined inside, in a way how the shell function should be called
         # see, for instance, linux_commands.bzl
         trap_function,
+    ] + env_prelude + [
         "export BUILD_WRAPPER_SCRIPT=\"{}\"".format(wrapper_script_file.path),
         "export BUILD_SCRIPT=\"{}\"".format(build_script_file.path),
         "export BUILD_LOG=\"{}\"".format(build_log_file.path),