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Support opening local store with database on read-only filesystem (#8356)

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Previously it was not possible to open a local store when its database is on a read-only filesystem. Obviously a store on a read-only filesystem cannot be modified, but it would still be useful to be able to query it.

This change adds a new read-only setting to LocalStore. When set to true, Nix will skip operations that fail when the database is on a read-only filesystem (acquiring big-lock, schema migration, etc), and the store database will be opened in immutable mode.

Co-authored-by: Ben Radford <benradf@users.noreply.github.com>
Co-authored-by: cidkidnix <cidkidnix@protonmail.com>
Co-authored-by: Dylan Green <67574902+cidkidnix@users.noreply.github.com>
Co-authored-by: John Ericson <git@JohnEricson.me>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
This commit is contained in:
Ben Radford 2023-06-20 10:34:09 +01:00 • committed by GitHub
parent 3910430b9d
commit 6ae35534b7
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GPG key ID: 4AEE18F83AFDEB23
9 changed files with 144 additions and 16 deletions
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45
src/libstore/local-store.cc
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@ -190,7 +190,11 @@ LocalStore::LocalStore(const Params & params)
/* Create missing state directories if they don't already exist. */
createDirs(realStoreDir);
makeStoreWritable();
if (readOnly) {
experimentalFeatureSettings.require(Xp::ReadOnlyLocalStore);
} else {
makeStoreWritable();
}
createDirs(linksDir);
Path profilesDir = stateDir + "/profiles";
createDirs(profilesDir);
@ -204,8 +208,10 @@ LocalStore::LocalStore(const Params & params)
for (auto & perUserDir : {profilesDir + "/per-user", gcRootsDir + "/per-user"}) {
createDirs(perUserDir);
if (chmod(perUserDir.c_str(), 0755) == -1)
throw SysError("could not set permissions on '%s' to 755", perUserDir);
if (!readOnly) {
if (chmod(perUserDir.c_str(), 0755) == -1)
throw SysError("could not set permissions on '%s' to 755", perUserDir);
}
}
/* Optionally, create directories and set permissions for a
@ -269,10 +275,12 @@ LocalStore::LocalStore(const Params & params)
/* Acquire the big fat lock in shared mode to make sure that no
schema upgrade is in progress. */
Path globalLockPath = dbDir + "/big-lock";
globalLock = openLockFile(globalLockPath.c_str(), true);
if (!readOnly) {
Path globalLockPath = dbDir + "/big-lock";
globalLock = openLockFile(globalLockPath.c_str(), true);
}
if (!lockFile(globalLock.get(), ltRead, false)) {
if (!readOnly && !lockFile(globalLock.get(), ltRead, false)) {
printInfo("waiting for the big Nix store lock...");
lockFile(globalLock.get(), ltRead, true);
}
@ -280,6 +288,14 @@ LocalStore::LocalStore(const Params & params)
/* Check the current database schema and if necessary do an
upgrade. */
int curSchema = getSchema();
if (readOnly && curSchema < nixSchemaVersion) {
debug("current schema version: %d", curSchema);
debug("supported schema version: %d", nixSchemaVersion);
throw Error(curSchema == 0 ?
"database does not exist, and cannot be created in read-only mode" :
"database schema needs migrating, but this cannot be done in read-only mode");
}
if (curSchema > nixSchemaVersion)
throw Error("current Nix store schema is version %1%, but I only support %2%",
curSchema, nixSchemaVersion);
@ -344,7 +360,11 @@ LocalStore::LocalStore(const Params & params)
else openDB(*state, false);
if (experimentalFeatureSettings.isEnabled(Xp::CaDerivations)) {
migrateCASchema(state->db, dbDir + "/ca-schema", globalLock);
if (!readOnly) {
migrateCASchema(state->db, dbDir + "/ca-schema", globalLock);
} else {
throw Error("need to migrate to content-addressed schema, but this cannot be done in read-only mode");
}
}
/* Prepare SQL statements. */
@ -475,13 +495,20 @@ int LocalStore::getSchema()
void LocalStore::openDB(State & state, bool create)
{
if (access(dbDir.c_str(), R_OK | W_OK))
if (create && readOnly) {
throw Error("cannot create database while in read-only mode");
}
if (access(dbDir.c_str(), R_OK | (readOnly ? 0 : W_OK)))
throw SysError("Nix database directory '%1%' is not writable", dbDir);
/* Open the Nix database. */
std::string dbPath = dbDir + "/db.sqlite";
auto & db(state.db);
state.db = SQLite(dbPath, create);
auto openMode = readOnly ? SQLiteOpenMode::Immutable
: create ? SQLiteOpenMode::Normal
: SQLiteOpenMode::NoCreate;
state.db = SQLite(dbPath, openMode);
#ifdef __CYGWIN__
/* The cygwin version of sqlite3 has a patch which calls