tests: Only change the behaviour of getpwuid() inside each test function

The `gutils-user-database` test uses `LD_PRELOAD` to load
`getpwuid-preload.so`, which contains overrides for `getpwuid()`,
`getpwnam_r()` and `getpwuid_r()`. These overrides clear the `pw_name`
field to `NULL` to test handling of a weird error case from an old
systemd implementation of those functions (see !2244).

When running the `G_DISABLE_ASSERT` CI job, this test was crashing
without any debugging output. The `G_DISABLE_ASSERT` CI job runs its
tests under `--setup thorough`, which was the culprit here: the function
overrides in `getpwuid-preload.so` were being loaded (via `LD_PRELOAD`)
into the `bash` process which runs `thorough-test-wrapper.sh` and
causing it to crash.

Interestingly, this couldn’t be reproduced locally with a comparable
version of bash. Perhaps it depends on the CI machine’s environment,
triggering bash to make a `getpwuid()` (or similar) call which it
wouldn’t normally do locally.

So, slightly change `getpwuid-preload.so` to additionally link to
`libglib-2.0.so` and query `g_test_get_path()` to work out whether it’s
being called inside a unit test. If not, don’t modify the return value.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3272

glib/tests/getpwuid-preload.c

@@ -21,6 +21,7 @@
  */
 
 #include <dlfcn.h>
+#include <glib.h>
 #include <pwd.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -58,13 +59,24 @@
 
 #undef getpwuid
 
+/* Only modify the result if running inside a GLib test. Otherwise, we risk
+ * breaking test harness code, or test wrapper processes (as this binary is
+ * loaded via LD_PRELOAD, it will affect all wrapper processes). */
+static inline int
+should_modify_result (void)
+{
+  const char *path = g_test_get_path ();
+  return (path != NULL && *path != '\0');
+}
+
 static struct passwd my_pw;
 
 DEFINE_WRAPPER (struct passwd *, getpwuid, (uid_t uid))
 {
   struct passwd *pw = GET_REAL (getpwuid) (uid);
   my_pw = *pw;
-  my_pw.pw_name = NULL;
+  if (should_modify_result ())
+    my_pw.pw_name = NULL;
   return &my_pw;
 }
 
@@ -75,7 +87,8 @@ DEFINE_WRAPPER (int, getpwnam_r, (const char     *name,
                                   struct passwd **result))
 {
   int code = GET_REAL (getpwnam_r) (name, pwd, buf, buflen, result);
-  pwd->pw_name = NULL;
+  if (should_modify_result ())
+    pwd->pw_name = NULL;
   return code;
 }
 
@@ -86,6 +99,7 @@ DEFINE_WRAPPER (int, getpwuid_r, (uid_t           uid,
                                   struct passwd **result))
 {
   int code = GET_REAL (getpwuid_r) (uid, pwd, buf, buflen, result);
-  pwd->pw_name = NULL;
+  if (should_modify_result ())
+    pwd->pw_name = NULL;
   return code;
 }

glib/tests/meson.build

@@ -261,7 +261,7 @@ else
     getpwuid_preload = shared_library('getpwuid-preload',
                                       'getpwuid-preload.c',
                                       name_prefix : '',
-                                      dependencies: libdl_dep,
+                                      dependencies: [libdl_dep, libglib_dep],
                                       install_dir : installed_tests_execdir,
                                       install_tag : 'tests',
                                       install: installed_tests_enabled)