From a313ea6aff0954df09d6f4d822e8d926a0ea9b04 Mon Sep 17 00:00:00 2001
From: Jyrki Gadinger <nilsding@nilsding.org>
Date: Wed, 22 Oct 2025 17:18:22 +0200
Subject: [PATCH] fix(network): fall back to unlimited bandwidth in case the
 legacy global limit is still set

`-2` used to mean "Use global limits", which no longer exist since
3.17.0.  As any negative value results in the auto bandwidth limiter to
engage, this could result in unexpected slower sync speeds after a
client upgrade.

Let's just keep handling that value and assume that we want to use
unlimited bandwidth.

Fixes #8743

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>
---
 src/libsync/account.cpp | 20 +++++++++++++++++---
 src/libsync/account.h   |  1 +
 test/testaccount.cpp    | 31 +++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/src/libsync/account.cpp b/src/libsync/account.cpp
index 08530a00d7..b824df53ab 100644
--- a/src/libsync/account.cpp
+++ b/src/libsync/account.cpp
@@ -1426,7 +1426,14 @@ void Account::setUploadLimitSetting(const AccountNetworkTransferLimitSetting set
         return;
     }
 
-    _uploadLimitSetting = setting;
+    auto targetSetting = setting;
+
+    if (setting == AccountNetworkTransferLimitSetting::LegacyGlobalLimit) {
+        qCInfo(lcAccount) << "Upload limit setting was requested to be set to the legacy global limit, falling back to unlimited";
+        targetSetting = AccountNetworkTransferLimitSetting::NoLimit;
+    }
+
+    _uploadLimitSetting = targetSetting;
     emit uploadLimitSettingChanged();
 }
 
@@ -1440,8 +1447,15 @@ void Account::setDownloadLimitSetting(const AccountNetworkTransferLimitSetting s
     if (setting == _downloadLimitSetting) {
         return;
     }
-    
-    _downloadLimitSetting = setting;
+
+    auto targetSetting = setting;
+
+    if (setting == AccountNetworkTransferLimitSetting::LegacyGlobalLimit) {
+        qCInfo(lcAccount) << "Download limit setting was requested to be set to the legacy global limit, falling back to unlimited";
+        targetSetting = AccountNetworkTransferLimitSetting::NoLimit;
+    }
+
+    _downloadLimitSetting = targetSetting;
     emit downloadLimitSettingChanged();
 }
 
diff --git a/src/libsync/account.h b/src/libsync/account.h
index e61c675f8d..36d17a7b60 100644
--- a/src/libsync/account.h
+++ b/src/libsync/account.h
@@ -99,6 +99,7 @@ class OWNCLOUDSYNC_EXPORT Account : public QObject
 
 public:
     enum class AccountNetworkTransferLimitSetting {
+        LegacyGlobalLimit = -2, // Until 3.17.0 a value of -2 was interpreted as "Use global network settings", it's now used to fall back to "No limit".  See also GH#8743
         AutoLimit = -1, // Value under 0 is interpreted as auto in general
         NoLimit,
         ManualLimit,
diff --git a/test/testaccount.cpp b/test/testaccount.cpp
index 670b6553de..698e2296d1 100644
--- a/test/testaccount.cpp
+++ b/test/testaccount.cpp
@@ -69,6 +69,37 @@ private slots:
         QCOMPARE(account->isPublicShareLink(), expectedResult);
         QCOMPARE(account->davUser(), expectedDavUser);
     }
+
+    void testAccount_setLimitSettings_globalNetworkLimitFallback()
+    {
+        using LimitSetting = Account::AccountNetworkTransferLimitSetting;
+        AccountPtr account = Account::create();
+
+        const auto setLimitSettings = [account](const LimitSetting setting) -> void {
+            account->setDownloadLimitSetting(setting);
+            account->setUploadLimitSetting(setting);
+        };
+
+        const auto verifyLimitSettings = [account](const LimitSetting expectedSetting) -> void {
+            QCOMPARE_EQ(expectedSetting, account->downloadLimitSetting());
+            QCOMPARE_EQ(expectedSetting, account->uploadLimitSetting());
+        };
+
+        // the default setting should be NoLimit
+        verifyLimitSettings(LimitSetting::NoLimit);
+
+        // changing it to ManualLimit should succeed
+        setLimitSettings(LimitSetting::ManualLimit);
+        verifyLimitSettings(LimitSetting::ManualLimit);
+
+        // changing it to AutoLimit should succeed
+        setLimitSettings(LimitSetting::AutoLimit);
+        verifyLimitSettings(LimitSetting::AutoLimit);
+
+        // changing it to LegacyGlobalLimit (-2) should fall back to NoLimit
+        setLimitSettings(LimitSetting::LegacyGlobalLimit);
+        verifyLimitSettings(LimitSetting::NoLimit);
+    }
 };
 
 QTEST_APPLESS_MAIN(TestAccount)