Open Replica read-only when possible (#3776)

* Open Replica read-only when possible

Specifically, when either
 - the command is read-only; or
 - the command requires GC (including recurrence updates) but GC is disabled by config

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Context.cpp

@@ -598,9 +598,6 @@ int Context::initialize(int argc, const char** argv) {
 
     createDefaultConfig();
 
-    bool create_if_missing = !config.getBoolean("exit.on.missing.db");
-    tdb2.open_replica(data_dir, create_if_missing);
-
     ////////////////////////////////////////////////////////////////////////////
     //
     // [3] Instantiate Command objects and capture command entities.
@@ -674,6 +671,21 @@ int Context::initialize(int argc, const char** argv) {
       if (foundAssumed) header("No command specified - assuming 'information'.");
     }
 
+    ////////////////////////////////////////////////////////////////////////////
+    //
+    // [7.5] Open the Replica.
+    //
+    ////////////////////////////////////////////////////////////////////////////
+
+    bool create_if_missing = !config.getBoolean("exit.on.missing.db");
+    Command* c = commands[cli2.getCommand()];
+
+    // We must allow writes if either 'gc' is enabled and the command performs GC, or the command
+    // itself is read-write.
+    bool read_write =
+        (config.getBoolean("gc") && (c->needs_gc() || c->needs_recur_update())) || !c->read_only();
+    tdb2.open_replica(data_dir, create_if_missing, read_write);
+
     ////////////////////////////////////////////////////////////////////////////
     //
     // [8] Initialize hooks.

src/TDB2.cpp

@@ -50,10 +50,13 @@ bool TDB2::debug_mode = false;
 static void dependency_scan(std::vector<Task>&);
 
 ////////////////////////////////////////////////////////////////////////////////
-void TDB2::open_replica(const std::string& location, bool create_if_missing) {
-  _replica = tc::new_replica_on_disk(location, create_if_missing);
+void TDB2::open_replica(const std::string& location, bool create_if_missing, bool read_write) {
+  _replica = tc::new_replica_on_disk(location, create_if_missing, read_write);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+void TDB2::open_replica_in_memory() { _replica = tc::new_replica_in_memory(); }
+
 ////////////////////////////////////////////////////////////////////////////////
 // Add the new task to the replica.
 void TDB2::add(Task& task) {
@@ -190,11 +193,8 @@ void TDB2::purge(Task& task) {
 
 ////////////////////////////////////////////////////////////////////////////////
 rust::Box<tc::Replica>& TDB2::replica() {
-  // Create a replica in-memory if `open_replica` has not been called. This
-  // occurs in tests.
-  if (!_replica) {
-    _replica = tc::new_replica_in_memory();
-  }
+  // One of the open_replica_ methods must be called before this one.
+  assert(_replica);
   return _replica.value();
 }
 

src/TDB2.h

@@ -46,7 +46,8 @@ class TDB2 {
 
   TDB2() = default;
 
-  void open_replica(const std::string &, bool create_if_missing);
+  void open_replica(const std::string &, bool create_if_missing, bool read_write);
+  void open_replica_in_memory();
   void add(Task &);
   void modify(Task &);
   void purge(Task &);

src/taskchampion-cpp/src/lib.rs

@@ -104,8 +104,11 @@ mod ffi {
         fn new_replica_in_memory() -> Result<Box<Replica>>;
 
         /// Create a new replica stored on-disk.
-        fn new_replica_on_disk(taskdb_dir: String, create_if_missing: bool)
-            -> Result<Box<Replica>>;
+        fn new_replica_on_disk(
+            taskdb_dir: String,
+            create_if_missing: bool,
+            read_write: bool,
+        ) -> Result<Box<Replica>>;
 
         /// Commit the given operations to the replica.
         fn commit_operations(&mut self, ops: Vec<Operation>) -> Result<()>;
@@ -490,11 +493,14 @@ impl From<tc::Replica> for Replica {
 fn new_replica_on_disk(
     taskdb_dir: String,
     create_if_missing: bool,
+    read_write: bool,
 ) -> Result<Box<Replica>, CppError> {
+    use tc::storage::AccessMode::*;
+    let access_mode = if read_write { ReadWrite } else { ReadOnly };
     let storage = tc::StorageConfig::OnDisk {
         taskdb_dir: PathBuf::from(taskdb_dir),
         create_if_missing,
-        access_mode: tc::storage::AccessMode::ReadWrite,
+        access_mode,
     }
     .into_storage()?;
     Ok(Box::new(tc::Replica::new(storage).into()))