toStringz lifetime

[Ali Çehreli]

toStringz documentation is clear on why, when, and how to extend the 
lifetime of a D string:

   https://dlang.org/phobos/std_string.html#.toStringz

Assume foo is a D library function that passes a "string" result to e.g. C:

extern(C)
void foo(ref const(char) * name) {
   name = format!"file%s.txt"(42).toStringz;  // Allocates from GC memory
}

This may be fine for "immediate use" on the C side because at first 
glance no garbage collection can take place between our returning the 
result and their using it:

// C caller:
   const char * name = NULL;
   foo(&name);                 // Calls us
   printf("%s", name);         // Uses 'name' immediately

Is it really safe? Imagine a multi-threaded environment where another D 
function is executed that triggers a GC collection right before the printf.

Does the GC see that local variable 'name' that is on the C side? What I 
don't know is whether the GC is aware only of the stack frames of D 
functions or the entire thread, which would include the C caller's 'name'.

Ali