trace2: remove use of xstrdup()

In the previous change, we removed a use of xsprintf() that caused a
recursive die() loop when failing to allocate memory. The trace2 library is
too low-level to be calling die(), especially because of these recursive
loops that can occur during the die handler.

For full defense in depth, we remove the xstrdup() calls from
trace2/tr2_sysenv.c.

First, in tr2_sysenv_cb(), we need to handle a failed assignment of the
value with a negative return to halt the config parsing loop.

Second, in tr2_sysenv_get(), the method will return NULL when strdup()
returns NULL. This return is indistinguishable from the environment variable
having no value. That means that all callers know how to handle a NULL
response, but no behavior change will occur between the case of no
environment being set and detecting an environment variable exists but we
fail to duplicate it. This seems an appropriate trade-off, as an allocation
failure at this level will likely lead to failure in another system, but at
least the trace2 API will not cause the process to fail early.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

trace2/tr2_sysenv.c

@@ -73,7 +73,9 @@ static int tr2_sysenv_cb(const char *key, const char *value,
 			if (!value)
 				return config_error_nonbool(key);
 			free(tr2_sysenv_settings[k].value);
-			tr2_sysenv_settings[k].value = xstrdup(value);
+			tr2_sysenv_settings[k].value = strdup(value);
+			if (!tr2_sysenv_settings[k].value)
+				return -1;
 			return 0;
 		}
 	}
@@ -109,7 +111,7 @@ const char *tr2_sysenv_get(enum tr2_sysenv_variable var)
 		const char *v = getenv(tr2_sysenv_settings[var].env_var_name);
 		if (v && *v) {
 			free(tr2_sysenv_settings[var].value);
-			tr2_sysenv_settings[var].value = xstrdup(v);
+			tr2_sysenv_settings[var].value = strdup(v);
 		}
 		tr2_sysenv_settings[var].getenv_called = 1;
 	}