Merge branch 'read-only-local-store' into overlayfs-store

src/libstore/local-store.cc

@@ -363,7 +363,7 @@ LocalStore::LocalStore(const Params & params)
         if (!readOnly) {
             migrateCASchema(state->db, dbDir + "/ca-schema", globalLock);
         } else {
-            throw Error("need to migrate to CA schema, but this cannot be done in read-only mode");
+            throw Error("need to migrate to content-addressed schema, but this cannot be done in read-only mode");
         }
     }
 
@@ -502,7 +502,7 @@ int LocalStore::getSchema()
 void LocalStore::openDB(State & state, bool create)
 {
     if (create && readOnly) {
-        throw Error("unable to create database while in read-only mode");
+        throw Error("cannot create database while in read-only mode");
     }
 
     if (access(dbDir.c_str(), R_OK | (readOnly ? 0 : W_OK)))

src/libstore/local-store.hh

@@ -50,16 +50,16 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
         false,
         "read-only",
         R"(
-          Allow this store to be opened when its database is on a read-only filesystem.
+          Allow this store to be opened when its [database](@docroot@/glossary.md#gloss-nix-database) is on a read-only filesystem.
 
-          Normally Nix will attempt to open the store database in read-write mode, even
+          Normally Nix will attempt to open the store database in read-write mode, even for querying (when write access is not needed).
-          for querying (when write access is not needed). This causes it to fail if the
+          This causes it to fail if the database is on a read-only filesystem.
-          database is on a read-only filesystem.
 
-          Enable read-only mode to disable locking and open the SQLite database with the
+          Enable read-only mode to disable locking and open the SQLite database with the [`immutable` parameter](https://www.sqlite.org/c3ref/open.html) set.
-          **immutable** parameter set. Do not use this unless the filesystem is read-only.
+
-          Using it when the filesystem is writable can cause incorrect query results or
+          **Warning**
-          corruption errors if the database is changed by another process.
+          Do not use this unless the filesystem is read-only.
+          Using it when the filesystem is writable can cause incorrect query results or corruption errors if the database is changed by another process.
         )"};
 
     const std::string name() override { return "Local Store"; }
@@ -292,6 +292,10 @@ public:
 
 private:
 
+    /**
+     * Retrieve the current version of the database schema.
+     * If the database does not exist yet, the version returned will be 0.
+     */
     int getSchema();
 
     void openDB(State & state, bool create);

src/libutil/experimental-features.cc

@@ -227,14 +227,13 @@ constexpr std::array<ExperimentalFeatureDetails, 15> xpFeatureDetails = {{
         .description = R"(
             Allow the use of the `read-only` parameter in local store URIs.
 
-            Set this parameter to `true` to allow stores with databases on read-only
+            Set this parameter to `true` to allow stores with databases on read-only filesystems to be opened for querying; ordinarily Nix will refuse to do this.
-            filesystems to be opened for querying; ordinarily Nix will refuse to do this.
 
-            Enabling this setting disables the locking required for safe concurrent
+            This is because SQLite requires write access to the database file to perform the file locking operations necessary for safe concurrent access.
-            access, so you should be certain that the database will not be changed.
+            When `read-only` is set to `true`, the database will be opened in immutable mode.
-            While the filesystem the database resides on might be read-only to this
+
-            process, consider whether another user, process, or system, might have
+            Under this mode, SQLite will not do any locking at all, so you should be certain that the database will not be changed.
-            write access to it.
+            While the filesystem the database resides on might be read-only to this process, consider whether another user, process, or system, might have write access to it.
         )",
     },
 }};

tests/read-only-store.sh

@@ -2,6 +2,8 @@ source common.sh
 
 enableFeatures "read-only-local-store"
 
+needLocalStore "cannot open store read-only when daemon has already opened it writeable"
+
 clearStore
 
 happy () {