NarInfoDiskCache: Keep BinaryCache.id stable and improve test

Fixes #3898

The entire `BinaryCaches` row used to get replaced after it became
stale according to the `timestamp` column. In a concurrent scenario,
this leads to foreign key conflicts as different instances of the
in-process `state.caches` cache now differ, with the consequence that
the older process still tries to use the `id` number of the old record.

Furthermore, this phenomenon appears to have caused the cache for
actual narinfos to be erased about every week, while the default
ttl for narinfos was supposed to be 30 days.

src/libstore/nar-info-disk-cache.cc

@@ -97,7 +97,7 @@ public:
         state->db.exec(schema);
 
         state->insertCache.create(state->db,
-            "insert or replace into BinaryCaches(url, timestamp, storeDir, wantMassQuery, priority) values (?, ?, ?, ?, ?)");
+            "insert into BinaryCaches(url, timestamp, storeDir, wantMassQuery, priority) values (?1, ?2, ?3, ?4, ?5) on conflict (url) do update set timestamp = ?2, storeDir = ?3, wantMassQuery = ?4, priority = ?5 returning id;");
 
         state->queryCache.create(state->db,
             "select id, storeDir, wantMassQuery, priority from BinaryCaches where url = ? and timestamp > ?");
@@ -165,6 +165,8 @@ public:
         return i->second;
     }
 
+private:
+
     std::optional<Cache> queryCacheRaw(State & state, const std::string & uri)
     {
         auto i = state.caches.find(uri);
@@ -172,15 +174,21 @@ public:
             auto queryCache(state.queryCache.use()(uri)(time(0) - cacheInfoTtl));
             if (!queryCache.next())
                 return std::nullopt;
-            state.caches.emplace(uri,
-                Cache{(int) queryCache.getInt(0), queryCache.getStr(1), queryCache.getInt(2) != 0, (int) queryCache.getInt(3)});
+            auto cache = Cache {
+                .id = (int) queryCache.getInt(0),
+                .storeDir = queryCache.getStr(1),
+                .wantMassQuery = queryCache.getInt(2) != 0,
+                .priority = (int) queryCache.getInt(3),
+            };
+            state.caches.emplace(uri, cache);
         }
         return getCache(state, uri);
     }
 
-    void createCache(const std::string & uri, const Path & storeDir, bool wantMassQuery, int priority) override
+public:
+    int createCache(const std::string & uri, const Path & storeDir, bool wantMassQuery, int priority) override
     {
-        retrySQLite<void>([&]() {
+        return retrySQLite<int>([&]() {
             auto state(_state.lock());
             SQLiteTxn txn(state->db);
 
@@ -189,13 +197,25 @@ public:
             auto cache(queryCacheRaw(*state, uri));
 
             if (cache)
-                return;
+                return cache->id;
 
-            state->insertCache.use()(uri)(time(0))(storeDir)(wantMassQuery)(priority).exec();
-            assert(sqlite3_changes(state->db) == 1);
-            state->caches[uri] = Cache{(int) sqlite3_last_insert_rowid(state->db), storeDir, wantMassQuery, priority};
+            Cache ret {
+                .id = -1, // set below
+                .storeDir = storeDir,
+                .wantMassQuery = wantMassQuery,
+                .priority = priority,
+            };
+
+            {
+                auto r(state->insertCache.use()(uri)(time(0))(storeDir)(wantMassQuery)(priority));
+                assert(r.next());
+                ret.id = (int) r.getInt(0);
+            }
+
+            state->caches[uri] = ret;
 
             txn.commit();
+            return ret.id;
         });
     }
 

src/libstore/nar-info-disk-cache.hh

@@ -13,7 +13,7 @@ public:
 
     virtual ~NarInfoDiskCache() { }
 
-    virtual void createCache(const std::string & uri, const Path & storeDir,
+    virtual int createCache(const std::string & uri, const Path & storeDir,
         bool wantMassQuery, int priority) = 0;
 
     struct CacheInfo

src/libstore/tests/nar-info-disk-cache.cc

@@ -8,69 +8,68 @@
 
 namespace nix {
 
-RC_GTEST_PROP(
-    NarInfoDiskCacheImpl,
-    create_and_read,
-    (int prio, bool wantMassQuery)
-    )
-{
+TEST(NarInfoDiskCacheImpl, create_and_read) {
+    int prio = 12345;
+    bool wantMassQuery = true;
+
     Path tmpDir = createTempDir();
     AutoDelete delTmpDir(tmpDir);
     Path dbPath(tmpDir + "/test-narinfo-disk-cache.sqlite");
     auto cache = getTestNarInfoDiskCache(dbPath);
 
-    cache->createCache("other://uri", "/nix/storedir", wantMassQuery, prio);
-    cache->createCache("other://uri-2", "/nix/storedir", wantMassQuery, prio);
-
-    cache->createCache("the://uri", "/nix/storedir", wantMassQuery, prio);
-
     {
-        auto r = cache->upToDateCacheExists("the://uri");
+        auto bc1 = cache->createCache("https://bar", "/nix/storedir", wantMassQuery, prio);
+        auto bc2 = cache->createCache("https://xyz", "/nix/storedir", false, 12);
+        ASSERT_NE(bc1, bc2);
+    }
+
+    int savedId = cache->createCache("http://foo", "/nix/storedir", wantMassQuery, prio);;
+    {
+        auto r = cache->upToDateCacheExists("http://foo");
         ASSERT_TRUE(r);
         ASSERT_EQ(r->priority, prio);
         ASSERT_EQ(r->wantMassQuery, wantMassQuery);
+        ASSERT_EQ(savedId, r->id);
     }
 
     SQLite db(dbPath);
     SQLiteStmt getIds;
-    getIds.create(db, "SELECT id FROM BinaryCaches WHERE url = 'the://uri'");
+    getIds.create(db, "SELECT id FROM BinaryCaches WHERE url = 'http://foo'");
 
-    int savedId;
     {
         auto q(getIds.use());
         ASSERT_TRUE(q.next());
-        savedId = q.getInt(0);
+        ASSERT_EQ(savedId, q.getInt(0));
         ASSERT_FALSE(q.next());
     }
 
-
     db.exec("UPDATE BinaryCaches SET timestamp = timestamp - 1 - 7 * 24 * 3600;");
 
     // Relies on memory cache
     {
-        auto r = cache->upToDateCacheExists("the://uri");
+        auto r = cache->upToDateCacheExists("http://foo");
         ASSERT_TRUE(r);
         ASSERT_EQ(r->priority, prio);
         ASSERT_EQ(r->wantMassQuery, wantMassQuery);
     }
 
+    // We can't clear the in-memory cache, so we use a new cache object.
     auto cache2 = getTestNarInfoDiskCache(dbPath);
 
     {
-        auto r = cache2->upToDateCacheExists("the://uri");
+        auto r = cache2->upToDateCacheExists("http://foo");
         ASSERT_FALSE(r);
     }
 
-    cache2->createCache("the://uri", "/nix/storedir", wantMassQuery, prio);
+    // Update, same data, check that the id number is reused
+    cache2->createCache("http://foo", "/nix/storedir", wantMassQuery, prio);
 
     {
-        auto r = cache->upToDateCacheExists("the://uri");
+        auto r = cache2->upToDateCacheExists("http://foo");
         ASSERT_TRUE(r);
         ASSERT_EQ(r->priority, prio);
         ASSERT_EQ(r->wantMassQuery, wantMassQuery);
-        // FIXME, reproduces #3898
-        // ASSERT_EQ(r->id, savedId);
-        (void) savedId;
+        ASSERT_EQ(r->id, savedId);
     }
 
     {
@@ -78,10 +77,28 @@ RC_GTEST_PROP(
         ASSERT_TRUE(q.next());
         auto currentId = q.getInt(0);
         ASSERT_FALSE(q.next());
-        // FIXME, reproduces #3898
-        // ASSERT_EQ(currentId, savedId);
-        (void) currentId;
+        ASSERT_EQ(currentId, savedId);
     }
+
+    // Check that the fields can be modified
+    {
+        auto r0 = cache2->upToDateCacheExists("https://bar");
+        ASSERT_FALSE(r0);
+
+        cache2->createCache("https://bar", "/nix/storedir", !wantMassQuery, prio + 10);
+        auto r = cache2->upToDateCacheExists("https://bar");
+        ASSERT_EQ(r->wantMassQuery, !wantMassQuery);
+        ASSERT_EQ(r->priority, prio + 10);
+    }
+
+    // // Force update (no use case yet; we only retrieve cache metadata when stale based on timestamp)
+    // {
+    //     cache2->createCache("https://bar", "/nix/storedir", wantMassQuery, prio + 20);
+    //     auto r = cache2->upToDateCacheExists("https://bar");
+    //     ASSERT_EQ(r->wantMassQuery, wantMassQuery);
+    //     ASSERT_EQ(r->priority, prio + 20);
+    // }
+
 }
 
 }