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

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
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
9 changed files with 144 additions and 16 deletions

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@ -110,6 +110,11 @@ void LocalStore::createTempRootsFile()
void LocalStore::addTempRoot(const StorePath & path)
{
if (readOnly) {
debug("Read-only store doesn't support creating lock files for temp roots, but nothing can be deleted anyways.");
return;
}
createTempRootsFile();
/* Open/create the global GC lock file. */

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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

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@ -46,6 +46,23 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
"require-sigs",
"Whether store paths copied into this store should have a trusted signature."};
Setting<bool> readOnly{(StoreConfig*) this,
false,
"read-only",
R"(
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 for querying (when write access is not needed), causing it to fail if the database is on a read-only filesystem.
Enable read-only mode to disable locking and open the SQLite database with the [`immutable` parameter](https://www.sqlite.org/c3ref/open.html) set.
> **Warning**
> 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.
> While the filesystem the database resides on might appear to be read-only, consider whether another user or system might have write access to it.
)"};
const std::string name() override { return "Local Store"; }
std::string doc() override;
@ -269,6 +286,10 @@ 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);

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@ -1,6 +1,7 @@
#include "sqlite.hh"
#include "globals.hh"
#include "util.hh"
#include "url.hh"
#include <sqlite3.h>
@ -50,15 +51,17 @@ static void traceSQL(void * x, const char * sql)
notice("SQL<[%1%]>", sql);
};
SQLite::SQLite(const Path & path, bool create)
SQLite::SQLite(const Path & path, SQLiteOpenMode mode)
{
// useSQLiteWAL also indicates what virtual file system we need. Using
// `unix-dotfile` is needed on NFS file systems and on Windows' Subsystem
// for Linux (WSL) where useSQLiteWAL should be false by default.
const char *vfs = settings.useSQLiteWAL ? 0 : "unix-dotfile";
int flags = SQLITE_OPEN_READWRITE;
if (create) flags |= SQLITE_OPEN_CREATE;
int ret = sqlite3_open_v2(path.c_str(), &db, flags, vfs);
bool immutable = mode == SQLiteOpenMode::Immutable;
int flags = immutable ? SQLITE_OPEN_READONLY : SQLITE_OPEN_READWRITE;
if (mode == SQLiteOpenMode::Normal) flags |= SQLITE_OPEN_CREATE;
auto uri = "file:" + percentEncode(path) + "?immutable=" + (immutable ? "1" : "0");
int ret = sqlite3_open_v2(uri.c_str(), &db, SQLITE_OPEN_URI | flags, vfs);
if (ret != SQLITE_OK) {
const char * err = sqlite3_errstr(ret);
throw Error("cannot open SQLite database '%s': %s", path, err);

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@ -11,6 +11,27 @@ struct sqlite3_stmt;
namespace nix {
enum class SQLiteOpenMode {
/**
* Open the database in read-write mode.
* If the database does not exist, it will be created.
*/
Normal,
/**
* Open the database in read-write mode.
* Fails with an error if the database does not exist.
*/
NoCreate,
/**
* Open the database in immutable mode.
* In addition to the database being read-only,
* no wal or journal files will be created by sqlite.
* Use this mode if the database is on a read-only filesystem.
* Fails with an error if the database does not exist.
*/
Immutable,
};
/**
* RAII wrapper to close a SQLite database automatically.
*/
@ -18,7 +39,7 @@ struct SQLite
{
sqlite3 * db = 0;
SQLite() { }
SQLite(const Path & path, bool create = true);
SQLite(const Path & path, SQLiteOpenMode mode = SQLiteOpenMode::Normal);
SQLite(const SQLite & from) = delete;
SQLite& operator = (const SQLite & from) = delete;
SQLite& operator = (SQLite && from) { db = from.db; from.db = 0; return *this; }

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@ -12,7 +12,7 @@ struct ExperimentalFeatureDetails
std::string_view description;
};
constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
constexpr std::array<ExperimentalFeatureDetails, 15> xpFeatureDetails = {{
{
.tag = Xp::CaDerivations,
.name = "ca-derivations",
@ -221,6 +221,13 @@ constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
Allow parsing of timestamps in builtins.fromTOML.
)",
},
{
.tag = Xp::ReadOnlyLocalStore,
.name = "read-only-local-store",
.description = R"(
Allow the use of the `read-only` parameter in [local store](@docroot@/command-ref/new-cli/nix3-help-stores.md#local-store) URIs.
)",
},
}};
static_assert(

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@ -31,6 +31,7 @@ enum struct ExperimentalFeature
DaemonTrustOverride,
DynamicDerivations,
ParseTomlTimestamps,
ReadOnlyLocalStore,
};
/**

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@ -136,7 +136,8 @@ nix_tests = \
impure-derivations.sh \
path-from-hash-part.sh \
test-libstoreconsumer.sh \
toString-path.sh
toString-path.sh \
read-only-store.sh
ifeq ($(HAVE_LIBCPUID), 1)
nix_tests += compute-levels.sh

42
tests/read-only-store.sh Normal file
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@ -0,0 +1,42 @@
source common.sh
enableFeatures "read-only-local-store"
needLocalStore "cannot open store read-only when daemon has already opened it writeable"
clearStore
happy () {
# We can do a read-only query just fine with a read-only store
nix --store local?read-only=true path-info $dummyPath
# We can "write" an already-present store-path a read-only store, because no IO is actually required
nix-store --store local?read-only=true --add dummy
}
## Testing read-only mode without forcing the underlying store to actually be read-only
# Make sure the command fails when the store doesn't already have a database
expectStderr 1 nix-store --store local?read-only=true --add dummy | grepQuiet "database does not exist, and cannot be created in read-only mode"
# Make sure the store actually has a current-database, with at least one store object
dummyPath=$(nix-store --add dummy)
# Try again and make sure we fail when adding a item not already in the store
expectStderr 1 nix-store --store local?read-only=true --add eval.nix | grepQuiet "attempt to write a readonly database"
# Test a few operations that should work with the read-only store in its current state
happy
## Testing read-only mode with an underlying store that is actually read-only
# Ensure store is actually read-only
chmod -R -w $TEST_ROOT/store
chmod -R -w $TEST_ROOT/var
# Make sure we fail on add operations on the read-only store
# This is only for adding files that are not *already* in the store
expectStderr 1 nix-store --add eval.nix | grepQuiet "error: opening lock file '$(readlink -e $TEST_ROOT)/var/nix/db/big-lock'"
expectStderr 1 nix-store --store local?read-only=true --add eval.nix | grepQuiet "Permission denied"
# Test the same operations from before should again succeed
happy