[clang] [Driver] Disable default configs via envvar during testing

[mgorny]

Summary

Add support for a CLANG_NO_DEFAULT_CONFIG envvar that works like
the --no-default-config option when set to a non-empty value. Use it
to disable loading system configuration files during the test suite
runs.

Configuration files can change the driver behavior in extensive ways,
and it is neither really possible nor feasible to account for or undo
the effects of even the most common configuration uses. Therefore,
the most reasonable option seems to be to ignore configuration files
while running the majority of tests (with the notable exception of tests
for configuration file support).

Due to the diversity of ways that %clang is used in the test suite,
including using it to copy or symlink the clang executable, as well to
call -cc1 and -cc1as modes, it is not feasible to pass the explicit
options to disable config loading either. Using an environment variable
has the advantage of being easily applied across the test suite
and easily unset for default configuration file loading tests.

[mgorny]

I'm not sure if we should be documenting this in the user manual, or if it should be considered an internal-use envvar.

[MaskRay]

The concept LGTM.

clang/lib/Driver/Driver.cpp

1073

Looking at LLDB_LAUNCH_FLAG_DISABLE_ASLR/LIBCLANG_DISABLE_PCH_VALIDATION, they don't check whether the value is non-empty.

[mgorny]

clang/lib/Driver/Driver.cpp

1073

Well, my personal experience tells me that it's sometimes non-trivial to unset var while setting it to empty value is easier, e.g.:

# undo global
CLANG_NO_DEFAULT_CONFIG= clang ...
# vs. env -u (not in POSIX)
env -u CLANG_NO_DEFAULT_CONFIG ...

[MaskRay]

during testing

This part of the subject is probably not needed. I think exposing the idea to user can probably be useful, too.

[mgorny]

In D134905#3826111, @MaskRay wrote:

during testing

This part of the subject is probably not needed. I think exposing the idea to user can probably be useful, too.

Sure but the larger part of the commit is actually about disabling them in the test suite ;-).

Thanks for all the reviews.

[sepavloff]

A more flexible way is to implement support for environment variable like CL and _CL (https://learn.microsoft.com/en-us/cpp/build/reference/cl-environment-variables?view=msvc-170 ). They could be used to deliver --no-default-config and would have wide usage than just this one.

[MaskRay]

Wait. The effect can be achieved today with CCC_OVERRIDE_OPTIONS=+--no-default-config

[MaskRay]

But CCC_OVERRIDE_OPTIONS=+--no-default-config leaves an annoying notice, so I guess introducing a new environment variable is still fine.

clang/lib/Driver/Driver.cpp

 #include "llvm/Support/MD5.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
+#include <cstdlib> // ::getenv
 #include <map>
 #include <memory>
 #include <utility>
 #if LLVM_ON_UNIX
 #include <unistd.h> // getpid
 #endif
 
 using namespace clang::driver;
if (CLOptions) {
     }
   }
 
   // No error occurred.
   return false;
 }
 
 bool Driver::loadDefaultConfigFiles(ArrayRef<StringRef> CfgFileSearchDirs) {
+  // Disable default config if CLANG_NO_DEFAULT_CONFIG is set to a non-empty
+  // value.
+  if (const char *NoConfigEnv = ::getenv("CLANG_NO_DEFAULT_CONFIG")) {
+    if (*NoConfigEnv)
+      return false;
+  }
   if (CLOptions && CLOptions->hasArg(options::OPT_no_default_config))
     return false;
 
   std::string RealMode = getExecutableForDriverMode(Mode);
   std::string Triple;
 
   // If name prefix is present, no --target= override was passed via CLOptions
   // and the name prefix is not a valid triple, force it for backwards

clang/test/Driver/config-file3.c

 // REQUIRES: shell
 // REQUIRES: x86-registered-target
 
+// RUN: unset CLANG_NO_DEFAULT_CONFIG
 // RUN: rm -rf %t && mkdir %t
 
 //--- If config file is specified by relative path (workdir/cfg-s2), it is searched for by that path.
 
 // RUN: mkdir -p %t/workdir/subdir
 // RUN: echo "@subdir/cfg-s2" > %t/workdir/cfg-1
 // RUN: echo "-Wundefined-var-template" > %t/workdir/subdir/cfg-s2
 //

clang/test/Driver/env.c

 // These tests try to ensure that the driver operates reasonably when run with
 // a strange environment. Unfortunately, it requires a normal shell and the
 // 'env' command that understands arguments, unlike the LIT built-in env.
 //
 // REQUIRES: shell
 // The PATH variable is heavily used when trying to find a linker.
-// RUN: env -i LC_ALL=C LD_LIBRARY_PATH="$LD_LIBRARY_PATH" \
+// RUN: env -i LC_ALL=C LD_LIBRARY_PATH="$LD_LIBRARY_PATH" CLANG_NO_DEFAULT_CONFIG=1 \
 // RUN:   %clang %s -### -o %t.o --target=i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --rtlib=platform -no-pie \
 // RUN:     --gcc-toolchain="" 2>&1 | FileCheck --check-prefix=CHECK-LD-32 %s
 //
-// RUN: env -i LC_ALL=C PATH="" LD_LIBRARY_PATH="$LD_LIBRARY_PATH" \
+// RUN: env -i LC_ALL=C PATH="" LD_LIBRARY_PATH="$LD_LIBRARY_PATH" CLANG_NO_DEFAULT_CONFIG=1 \
 // RUN:   %clang %s -### -o %t.o --target=i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --rtlib=platform -no-pie \
 // RUN:     --gcc-toolchain="" 2>&1 | FileCheck --check-prefix=CHECK-LD-32 %s
 //
 // CHECK-LD-32-NOT: warning:
 // CHECK-LD-32: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"
 // CHECK-LD-32: "{{.*}}/usr/lib/gcc/i386-unknown-linux/10.2.0{{/|\\\\}}crtbegin.o"
 // CHECK-LD-32: "-L[[SYSROOT]]/usr/lib/gcc/i386-unknown-linux/10.2.0"
 // CHECK-LD-32: "-L[[SYSROOT]]/usr/lib/gcc/i386-unknown-linux/10.2.0/../../../../i386-unknown-linux/lib"
 // CHECK-LD-32: "-L[[SYSROOT]]/lib"
 // CHECK-LD-32: "-L[[SYSROOT]]/usr/lib"

clang/test/Unit/lit.cfg.py

shlibpath = os.path.pathsep.join(
         (config.shlibdir,
          config.llvm_libs_dir,
          config.environment.get(shlibpath_var, '')))
     config.environment[shlibpath_var] = shlibpath
     break
 else:
     lit_config.warning("unable to inject shared library path on '{}'"
                        .format(platform.system()))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"

clang/test/lit.cfg.py

 # controls the kind of objects they will support. If there is no "OBJECT_MODE"
 # environment variable specified, the default behaviour is to support 32-bit
 # objects only. In order to not affect most test cases, which expect to support
 # 32-bit and 64-bit objects by default, set the environment variable
 # "OBJECT_MODE" to 'any' for llvm-nm on AIX OS.
 
 if 'system-aix' in config.available_features:
         config.substitutions.append(('llvm-nm', 'env OBJECT_MODE=any llvm-nm'))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"