73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
<!--
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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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-->
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# Multi Identifiers (mid)
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All transfers (easy handles) added to a multi handle are assigned
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a unique identifier until they are removed again. The multi handle
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keeps a table `multi->xfers` that allow O(1) access to the easy
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handle by its `mid`.
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References to other easy handles *should* keep their `mid`s instead
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of a pointer (not all code has been converted as of now). This solves
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problems in easy and multi handle life cycle management as well as
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iterating over handles where operations may add/remove other handles.
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### Values and Lifetime
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An `mid` is an `unsigned int`. There are two reserved values:
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* `0`: is the `mid` of an internal "admin" handle. Multi and share handles
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each have their own admin handle for maintenance operations, like
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shutting down connections.
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* `UINT_MAX`: the "invalid" `mid`. Easy handles are initialized with
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this value. They get it assigned again when removed from
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a multi handle.
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This makes potential range of `mid`s from `1` to `UINT_MAX - 1` *inside
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the same multi handle at the same time*. However, the `multi->xfers` table
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reuses `mid` values from previous transfers that have been removed.
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`multi->xfers` is created with an initial capacity. At the time of this
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writing that is `16` for "multi_easy" handles (used in `curl_easy_perform()`
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and `512` for multi handles created with `curl_multi_init()`.
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The first added easy handle gets `mid == 1` assigned. The second one receives `2`,
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even when the fist one has been removed already. Every added handle gets an
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`mid` one larger than the previously assigned one. Until the capacity of
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the table is reached and it starts looking for a free id at `1` again (`0`
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is always in the table).
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When adding a new handle, the multi checks the amount of free entries
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in the `multi->xfers` table. If that drops below a threshold (currently 25%),
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the table is resized. This serves two purposes: one, a previous `mid` is not
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reused immediately and second, table resizes are not needed that often.
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The table is implemented in `uint-table.[ch]`. More details in [`UINT_SETS`](UINT_SETS.md).
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### Tracking `mid`s
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There are several places where transfers need to be tracked:
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* the multi tracks `process`, `pending` and `msgsent` transfers. A transfer
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is in at most one of these at a time.
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* connections track the transfers that are *attached* to them.
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* multi event handling tracks transfers interested in a specific socket.
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* DoH handles track the handle they perform lookups for (and vice versa).
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There are two bitset implemented for storing `mid`s: `uint_bset` and `uint_spbset`.
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The first is a bitset optimal for storing a large number of unsigned int values.
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The second one is a "sparse" variant good for storing a small set of numbers.
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More details about these in [`UINT_SETS`](UINT_SETS.md).
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A multi uses `uint_bset`s for `process`, `pending` and `msgsent`. Connections
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and sockets use the sparse variant as both often track only a single transfer
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and at most 100 on an HTTP/2 or HTTP/3 connection/socket.
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These sets allow safe iteration while being modified. This allows a multi
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to iterate over its "process" set while existing transfers are removed
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or new ones added.
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