From db2f8fa61cfb613020989eaafccba20fca232a66 Mon Sep 17 00:00:00 2001
From: Nikita Sivukhin <sivukhin@turso.tech>
Date: Wed, 30 Jul 2025 00:42:36 +0400
Subject: [PATCH] add TestContext in order to simplify fault injection

---
 packages/turso-sync/src/test_context.rs | 139 ++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 packages/turso-sync/src/test_context.rs

diff --git a/packages/turso-sync/src/test_context.rs b/packages/turso-sync/src/test_context.rs
new file mode 100644
index 000000000..123b6308d
--- /dev/null
+++ b/packages/turso-sync/src/test_context.rs
@@ -0,0 +1,139 @@
+use std::{
+    collections::{HashMap, HashSet},
+    future::Future,
+    pin::Pin,
+    sync::Arc,
+};
+
+use rand::SeedableRng;
+use rand_chacha::ChaCha8Rng;
+use tokio::sync::Mutex;
+
+use crate::{errors::Error, Result};
+
+pub struct FaultInjectionPlan {
+    pub is_fault:
+        Box<dyn Fn(String, String) -> Pin<Box<dyn Future<Output = bool> + Send>> + Send + Sync>,
+}
+
+pub enum FaultInjectionStrategy {
+    Disabled,
+    Record,
+    Enabled { plan: FaultInjectionPlan },
+}
+
+impl std::fmt::Debug for FaultInjectionStrategy {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Disabled => write!(f, "Disabled"),
+            Self::Record => write!(f, "Record"),
+            Self::Enabled { .. } => write!(f, "Enabled"),
+        }
+    }
+}
+
+pub struct TestContext {
+    fault_injection: Mutex<FaultInjectionStrategy>,
+    faulty_call: Mutex<HashSet<(String, String)>>,
+    rng: Mutex<ChaCha8Rng>,
+}
+
+pub struct FaultSession {
+    ctx: Arc<TestContext>,
+    recording: bool,
+    plans: Option<Vec<FaultInjectionPlan>>,
+}
+
+impl FaultSession {
+    pub async fn next(&mut self) -> Option<FaultInjectionStrategy> {
+        if !self.recording {
+            self.recording = true;
+            return Some(FaultInjectionStrategy::Record);
+        }
+        if self.plans.is_none() {
+            self.plans = Some(self.ctx.enumerate_simple_plans().await);
+        }
+
+        let plans = self.plans.as_mut().unwrap();
+        if plans.len() == 0 {
+            return None;
+        }
+
+        let plan = plans.pop().unwrap();
+        Some(FaultInjectionStrategy::Enabled { plan })
+    }
+}
+
+impl TestContext {
+    pub fn new(seed: u64) -> Self {
+        Self {
+            rng: Mutex::new(ChaCha8Rng::seed_from_u64(seed)),
+            fault_injection: Mutex::new(FaultInjectionStrategy::Disabled),
+            faulty_call: Mutex::new(HashSet::new()),
+        }
+    }
+    pub async fn rng(&self) -> tokio::sync::MutexGuard<ChaCha8Rng> {
+        self.rng.lock().await
+    }
+    pub fn fault_session(self: &Arc<Self>) -> FaultSession {
+        FaultSession {
+            ctx: self.clone(),
+            recording: false,
+            plans: None,
+        }
+    }
+    pub async fn switch_mode(&self, updated: FaultInjectionStrategy) {
+        let mut mode = self.fault_injection.lock().await;
+        tracing::info!("switch fault injection mode: {:?}", updated);
+        *mode = updated;
+    }
+    pub async fn enumerate_simple_plans(&self) -> Vec<FaultInjectionPlan> {
+        let mut plans = vec![];
+        for call in self.faulty_call.lock().await.iter() {
+            let mut fault_counts = HashMap::new();
+            fault_counts.insert(call.clone(), 1);
+
+            let count = Arc::new(Mutex::new(1));
+            let call = call.clone();
+            plans.push(FaultInjectionPlan {
+                is_fault: Box::new(move |name, bt| {
+                    let call = call.clone();
+                    let count = count.clone();
+                    Box::pin(async move {
+                        if &(name, bt) != &call {
+                            return false;
+                        }
+                        let mut count = count.lock().await;
+                        *count -= 1;
+                        *count >= 0
+                    })
+                }),
+            })
+        }
+        plans
+    }
+    pub async fn faulty_call(&self, name: &str) -> Result<()> {
+        tracing::trace!("faulty_call: {}", name);
+        tokio::task::yield_now().await;
+        match &*self.fault_injection.lock().await {
+            FaultInjectionStrategy::Disabled => return Ok(()),
+            _ => {}
+        };
+        let bt = std::backtrace::Backtrace::force_capture().to_string();
+        match &mut *self.fault_injection.lock().await {
+            FaultInjectionStrategy::Record => {
+                let mut call_sites = self.faulty_call.lock().await;
+                call_sites.insert((name.to_string(), bt));
+                Ok(())
+            }
+            FaultInjectionStrategy::Enabled { plan } => {
+                if plan.is_fault.as_ref()(name.to_string(), bt.clone()).await {
+                    Err(Error::DatabaseSyncError(format!("injected fault")))
+                } else {
+                    Ok(())
+                }
+            }
+            _ => unreachable!("Disabled case handled above"),
+        }
+    }
+}