[pacman-dev] Extending download callback interface

Erich Eckner arch at eckner.net
Tue Feb 11 09:49:18 UTC 2020


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On Tue, 11 Feb 2020, Anatol Pomozov wrote:

> Hello folks

Hi Anatol,

>
> While working on multiplexed download API I hit one issue that requires
> some alpm API changes.
>
> Current ALPM download api handles one file at a time. And interaction
> between pacman and ALPM looks like:
>
> - pacman iterates over list of files to download
> - pacman calls alpm API to download one file
> - alpm initializes curl with progress callback implemented by pacman
> - during the download curl calls the pacman callback
> - once download is done alpm returns a result code of the download
> transaction
>
> In this single-download scenario pacman knows when the download starts,
> progresses and finished.
>
> With multiplexed download feature alpm_download() will change its API. It
> will get a list of files as a parameter and handle downloads for all of
> them with a single function call.
>
> This unfortunately makes impossible to intercept "start file download" and
> "complete file download" events. These events are needed because we want to
> render UI correctly and print download information like "file up to date",
> "failed to download"... at the exact moment when the event happens.
>
> To mitigate this problem I propose to extend the callback API to make it
> possible for ALPM to provide other types of download events.
>
> The signature will change from:
> typedef void (*alpm_cb_download)(const char *filename, off_t xfered, off_t
> total);
> that implies only download progress events, to:
> typedef void (*alpm_cb_download)(const char *filename,
> alpm_download_event_type_t event, void *data);
>
> where alpm_download_event_type_t is enum
>
> typedef enum {
>       ALPM_DOWNLOAD_INIT,
>       ALPM_DOWNLOAD_PROGRESS,
>       ALPM_DOWNLOAD_ERROR,
>       ALPM_DOWNLOAD_COMPLETED
> } alpm_download_event_type_t;
>
> and *data is a pointer to event specific structure (different for each type
> of event), e.g.:
> typedef struct {
>       int result; /* 1 - file is uptodate, 0 - download completed
> successfully, -1 failed */
> } alpm_download_event_completed_t;
>
> Note this is an ALPM breaking API change. It means it can be done only with
> major version bump.
>
> What do you think about it?
>

I'm sure, you have considered this, but: Why not only(?) add a 
"this_event_refers_to_that_file" pointer of some kind to the callback (the 
index of the file in the parameter array, for example)? This might be 
nice, even if you change the interface in the direction you proposed. Then 
the frontend could update the progress/status of the respective file 
separately (xfered==0 would be "start", xfered==total would be 
"finished").

Or is this, what "const char *filename" gets used for in the callback?

regards,
Erich

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