The returned string belongs to 'message' and as such, should not be
modified by the caller and will only be valid for as long as the
message is valid, (for example, until the user calls
notmuch_message_destroy on 'message' or until a query from which it
derived is destroyed).
This function will not return NULL since Notmuch ensures that every
message belongs to a single thread.
Get the thread ID of 'message'.
The returned string belongs to 'message' and as such, should not be modified by the caller and will only be valid for as long as the message is valid, (for example, until the user calls notmuch_message_destroy on 'message' or until a query from which it derived is destroyed).
This function will not return NULL since Notmuch ensures that every message belongs to a single thread.