Helpers behind sharing pages to another SilverBullet space; see ^Library/Std/Infrastructure/SilverBullet Share and API/share.
A shared page is addressed by a URI of the form <space url>/<page name>, for example https://example.com/notes/Shared/Page. Reads and writes use the other space's .fs API with a token obtained through the device flow, kept under the credential key sb:<space url> (see API/auth).
One net.readURI: / net.writeURI: service pair handles every space. Its match resolves the URI when it is dispatched, so it claims URIs of any space known at that moment (from share.silverbullet.spaces or a stored credential), including spaces configured after this page loaded. Its priority is above the generic https: reader.
Requests go through auth.fetch. Reads never sign in: they send the stored credential if there is one and read anonymously otherwise. A read rejected with 401 drops the credential and is tried once more anonymously; when that is rejected too, it fails with Not connected to <space>: share a page to it in push or sync mode to sign in. A missing page reads as nil. Writes sign in when needed, and when a write is rejected with 401 the credential is dropped and you sign in again, once.
On a new device a space can be known (from share.silverbullet.spaces) without a usable credential on that device (none, or an expired one that can't be refreshed). Before a push or sync share, the share:ensure service then asks Sign in to <space>? and runs the device flow, so a private space is never read anonymously first (with a usable credential it does nothing); declining fails with Not connected to <space> and leaves the page and the remote alone. pull shares and uri: pages never ask: they read anonymously. For a URI of a space that is not known at all, share:ensure first finds the space root with sbShare.findSpaceRoot (for http: and https: URIs), asks Connect to <space>?, signs in and remembers the space.
Only sync writes are conditional (they pass {conditional = true} to net.writeURI): they send the ETag of the last read as If-Match, so a change made in the meantime makes the write return {conflict = true} and sync retries. Push writes never send If-Match: the local page replaces the remote one.
share.silverbullet.spaces is a list of space URLs shared to before, most recent first. It is offered when choosing a space and kept in the space's configuration, so it is the same on every device.
Builds the share URI of a page in a space: the space URL without trailing slashes, then the page name percent-encoded except for /, e.g. sbShare.pageUri("https://h/notes/", "Café Plan") is https://h/notes/Caf%C3%A9%20Plan.
Returns the space URL and page name a share URI points to, choosing the longest matching known space (one in share.silverbullet.spaces or with a stored credential on this device), or nil. The space's own URL (with or without a trailing slash) gives the page name "": it can be a collection root, but isn't a page to read or write, and isn't listed (a collection rooted at a whole space is published member by member, with nothing offered for removal).
Finds the root of the SilverBullet space a URL belongs to by probing .ping at each path prefix, longest first (sending a stored credential, never signing in). Returns nil when it is not a SilverBullet space. Definitive misses (a 404, a 405, or a 200 without an X-Server-Version header) are remembered for the session; failures such as 401, 429, 5xx or network errors are not.