tweak and document version 3->4 upgrade

db/schema.sql

@@ -332,7 +332,7 @@ create table user (
 
   -- Permissions available for newly created tokens or when authenticating via
   -- unix_uid above. A serialized "Permissions" protobuf.
-  permissions blob
+  permissions blob not null default X''
 );
 
 -- A single session, whether for browser or robot use.
@@ -398,7 +398,7 @@ create table user_session (
   use_count not null default 0,
 
   -- Permissions associated with this token; a serialized "Permissions" protobuf.
-  permissions blob
+  permissions blob not null default X''
 ) without rowid;
 
 create index user_session_uid on user_session (user_id);
@@ -481,7 +481,7 @@ create table signal_change (
   -- delta:         1         1         196 (must be non-negative)
   -- states:             1         1              2
   -- varint:        \x01 \x01 \x01 \x01 \xc4 \x01 \x02
-  changes blob
+  changes blob not null
 );
 
 insert into version (id, unix_time,                           notes)

db/upgrade/v3_to_v4.rs

@@ -58,13 +58,18 @@ pub fn run(_args: &super::Args, tx: &rusqlite::Transaction) -> Result<(), Error>
           primary key (signal_id, camera_id)
         ) without rowid;
 
-        create table signal_state (
+        create table signal_change (
           time_90k integer primary key,
-          changes blob
+          changes blob not null
         );
 
-        alter table user add column permissions blob;
-        alter table user_session add column permissions blob;
+        alter table user add column permissions blob not null default X'';
+        alter table user_session add column permissions blob not null default X'';
+
+        -- Set permissions to "view_video" on existing users and sessions to preserve their
+        -- behavior. Newly created users won't have prepopulated permissions like this.
+        update user set permissions = X'0801';
+        update user_session set permissions = X'0801';
     "#)?;
     Ok(())
 }