feature: every write of an instance's variable files is kept as a revision with the schema version of their layout and the application that wrote them.

server/sql/tfvars_store.sql

@@ -1,6 +1,7 @@
 -- The store of Terraform modules, their instances and the instances' variable files: the tables tf_modules,
--- tf_instances and tf_variable_files, and the functions and procedures the store calls. The store applies the file on
--- its first use in the schema its connection's search path names; every statement can run again.
+-- tf_instances, tf_variable_files and tf_variable_revisions, and the functions and procedures the store calls. The
+-- store applies the file on its first use in the schema its connection's search path names; every statement can run
+-- again.
 
 CREATE TABLE IF NOT EXISTS tf_modules (
   name             text PRIMARY KEY CHECK (name ~ '^[a-z][a-z0-9-]*$'),
@@ -28,6 +29,28 @@ CREATE TABLE IF NOT EXISTS tf_variable_files (
   FOREIGN KEY (module, instance) REFERENCES tf_instances (module, name) ON DELETE CASCADE
 );
 
+-- Every write of an instance's variable files: all its files after the write, by name, the schema version of their
+-- layout, null when not known, and the application that wrote them.
+CREATE TABLE IF NOT EXISTS tf_variable_revisions (
+  module   text NOT NULL,
+  instance text NOT NULL,
+  revision integer NOT NULL CHECK (revision > 0),
+  files    jsonb NOT NULL CHECK (jsonb_typeof(files) = 'object'),
+  schema   integer CHECK (schema > 0),
+  producer text NOT NULL CHECK (producer <> ''),
+  saved    timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY (module, instance, revision),
+  FOREIGN KEY (module, instance) REFERENCES tf_instances (module, name) ON DELETE CASCADE
+);
+
+-- The files of an instance that has no revision are its revision 1, of an unknown schema and producer.
+INSERT INTO tf_variable_revisions (module, instance, revision, files, schema, producer, saved)
+SELECT f.module, f.instance, 1, jsonb_object_agg(f.name, f.content), NULL, 'unknown', max(f.updated)
+FROM tf_variable_files f
+WHERE NOT EXISTS (SELECT 1 FROM tf_variable_revisions r WHERE r.module = f.module AND r.instance = f.instance)
+GROUP BY f.module, f.instance
+ON CONFLICT DO NOTHING;
+
 -- The modules, by name.
 CREATE OR REPLACE FUNCTION tf_modules_list()
 RETURNS TABLE (name text, root text, description text, configurator_url text)
@@ -115,3 +138,36 @@ SET search_path FROM CURRENT
 AS $$
   DELETE FROM tf_variable_files WHERE module = module_name AND instance = instance_name AND name = file_name
 $$;
+
+-- Records the variable files of an instance as its next revision, with the schema of their layout and their producer;
+-- called with the lock of tf_variable_files_lock held.
+CREATE OR REPLACE PROCEDURE tf_variable_revision_add(module_name text, instance_name text, revision_schema integer, revision_producer text)
+LANGUAGE sql
+SET search_path FROM CURRENT
+AS $$
+  INSERT INTO tf_variable_revisions (module, instance, revision, files, schema, producer)
+  SELECT module_name, instance_name,
+    (SELECT coalesce(max(r.revision), 0) + 1 FROM tf_variable_revisions r WHERE r.module = module_name AND r.instance = instance_name),
+    (SELECT coalesce(jsonb_object_agg(f.name, f.content), '{}'::jsonb) FROM tf_variable_files f WHERE f.module = module_name AND f.instance = instance_name),
+    revision_schema, revision_producer
+$$;
+
+-- The revisions of an instance, newest first, with the names of their files.
+CREATE OR REPLACE FUNCTION tf_variable_revisions_list(module_name text, instance_name text)
+RETURNS TABLE (revision integer, schema integer, producer text, saved timestamptz, names text[])
+LANGUAGE sql STABLE
+SET search_path FROM CURRENT
+AS $$
+  SELECT r.revision, r.schema, r.producer, r.saved, ARRAY(SELECT jsonb_object_keys(r.files) ORDER BY 1)
+  FROM tf_variable_revisions r WHERE r.module = module_name AND r.instance = instance_name ORDER BY r.revision DESC
+$$;
+
+-- A revision of an instance with its files.
+CREATE OR REPLACE FUNCTION tf_variable_revision_get(module_name text, instance_name text, revision_number integer)
+RETURNS TABLE (revision integer, schema integer, producer text, saved timestamptz, files jsonb)
+LANGUAGE sql STABLE
+SET search_path FROM CURRENT
+AS $$
+  SELECT r.revision, r.schema, r.producer, r.saved, r.files
+  FROM tf_variable_revisions r WHERE r.module = module_name AND r.instance = instance_name AND r.revision = revision_number
+$$;