FILE_MIGRATE_ errors not handled for file downloads (or: why custom contact photos don't work)

[TKIPisalegacycipher]

FILE_MIGRATE_ errors not handled for file downloads

Problem

Downloading ChatPhotoInfo.Small files for personal photos (photos set by the current user for a contact) permanently fails with FILE_MIGRATE_1 (HTTP 303). The photo is stored on DC 1 but TDLib tries to download from DC 3 and never retries on the correct DC.

Root Cause

NetQueryDispatcher::try_fix_migrate() (td/telegram/net/NetQueryDispatcher.cpp:390-406) handles 303 errors, but only for three prefixes:

static constexpr CSlice prefixes[] = {"PHONE_MIGRATE_", "NETWORK_MIGRATE_", "USER_MIGRATE_"};

FILE_MIGRATE_ is not in this list. So when a file download query returns 303 / FILE_MIGRATE_1:

1. NetQueryDispatcher::dispatch() (line 96-97) sees code 303, calls try_fix_migrate()
2. try_fix_migrate() doesn't match FILE_MIGRATE_, does nothing, returns
3. The error propagates to FileManager::on_download_error_impl() (line 4913)
4. on_download_error_impl() has no handler for it either (lines 4918-4961 handle FILE_ID_INVALID, file reference errors, FILE_DOWNLOAD_RESTART, MTPROTO_CLUSTER_INVALID — but not FILE_MIGRATE)
5. The error is logged at line 4972 and converted to a 400 error at line 4988
6. on_file_load_error() (line 4991) cancels the download permanently

Reproduction

1. Set a personal photo for a contact (via "Set Photo for This Contact")
2. The ChatPhotoInfo.Small file for that contact will have a DC ID that may differ from the DC where the file is actually stored
3. TDLib will attempt to download from the stored DC, receive FILE_MIGRATE_N, and fail permanently
4. The contact's profile picture will only ever show the blurry minithumbnail

Evidence from logs

[Session:3:download_small#0] Receive error [...] [Error : 303 : FILE_MIGRATE_1]
Failed to Download file 4385(0) of type ChatPhoto: [Error : 303 : FILE_MIGRATE_1]
Failed to Download file 4377(0) of type ChatPhoto: [Error : 303 : FILE_MIGRATE_1]

Key files and lines

• td/telegram/net/NetQueryDispatcher.cpp lines 390-406: try_fix_migrate() — missing FILE_MIGRATE_ prefix
• td/telegram/net/NetQueryDispatcher.cpp lines 96-97: 303 error dispatch calls try_fix_migrate()
• td/telegram/files/FileManager.cpp lines 4918-4961: on_download_error_impl() — no FILE_MIGRATE handler
• td/telegram/files/FileManager.cpp lines 4972-4973: Error logged as warning, then treated as fatal

Possible fix

Add FILE_MIGRATE_ handling in try_fix_migrate(), but without changing the main DC (unlike the other migrate errors, this just means resend the query to a different DC):

// In try_fix_migrate(), before the existing prefix loop:
static constexpr CSlice file_migrate_prefix = "FILE_MIGRATE_";
if (begins_with(error_message, file_migrate_prefix)) {
    auto new_dc_id = to_integer<int32>(error_message.substr(file_migrate_prefix.size()));
    if (DcId::is_valid(new_dc_id)) {
        net_query->resend(DcId::internal(new_dc_id));
    }
    return;
}

[levlam]

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