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>

2 files changed, 44 insertions, 1 deletions

diff --git a/tests/local.mk b/tests/local.mk
index 8e387fe45..88848926b 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -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
diff --git a/tests/read-only-store.sh b/tests/read-only-store.sh
new file mode 100644
index 000000000..d63920c19
--- /dev/null
+++ b/tests/read-only-store.sh
@@ -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