http.c: Fix infinite loop in GTK apps #1439
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GTK has a specific IPP processing which stopped working after CVE-2025-58436 fix. GTK depends on internal behavior of `_httpUpdate()` which read a line from connection at the start of function, which was one of culprits behind CVE-2025-58436. To mitigate CVE-2025-58436 `_httpUpdate()` started to read from connection only if there was data in internal HTTP buffer and there was at least one newline buffered - otherwise the function returns HTTP_ERROR/HTTP_CONTINUE, which caused the loop in GTK. The change which fixes GTK behavior in the PR is to read data from connection at the start of `_httpUpdate()` for non-blocking connections immediately with no timeout if internal HTTP buffer is not full. The change mitigates the CVE as well as the previous implementation. Fixes OpenPrinting#1429
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GTK has a specific IPP processing which stopped working after fix for CVE-2025-58436. GTK depended on internal behavior of
_httpUpdate()which read a line from connection regardless of already buffered data.To mitigate CVE-2025-58436
_httpUpdate()started to read another data from connection only if there is already a line buffered in internal HTTP structure in order to prevent being stuck on slow clients - the function now returns HTTP_STATUS_CONTINUE in such cases.The fix which fixes GTK behavior here, which caused the GTK print dialogs not being opened at all, is to read from the connection if we have a signal there are data to read, and only if there is no newline after this data read, return error/continue to client.
Fixes #1429