Re: [Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

>> +=head2 C<.default_export> >> + >> + const char *default_export (int readonly, int is_tls); > > Oh fun. For some plugins (like ondemand), this is trivial: return a > compile-time constant string. But for others (like sh and eval), > there's a lifetime issue: this callback is used _before_ .open, ergo > there is no handle struct that it can be associated with. What's > more, this is called _after_ .preconnect, which means it is logical > to expect that the default export name might change over time > (consider a plugin that advertises the largest file in a directory > as its default, but where the directory can change _which_ file is > largest between when the first client connects and when the second > client connects). And the string returned by the sh script is in > malloc'd memory (by it's very nature of coming from the user script, > rather than being a compile-time constant). Without a handle to > store this string in, we would have a memory leak: there is no way > to associate this inside the handle's struct so that .close can > reclaim it, but storing it globally is not thread-safe to parallel > client connections. I'm not sure there's an actual problem here, because there is still a connection object inside the server any time we have a TCP connection, so you can just store it there, unless I'm misunderstanding something.

But anyway ... > So I'm thinking I need to add a helper > function: > > const char *nbdkit_string_intern (const char *str); > > Return a pointer to a copy of str, where nbdkit owns the lifetime of > the copy (allowing the caller to not have to worry about persisting > str indefinitely). If called when there is no client connection > (such as during .load), the copy will remain valid through .unload; > if called in the context of a client connection (any callback from > .preconnect through .close), the copy will remain valid through > .close. I'm a bit unclear why the plugin would have to call this (or this function be in the public API at all). Why can't string interning be done inside the server. Have a global struct where strings returned from the plugin are interned, and free it on server exit.