feat: allow forcing in-memory SQLite dbs (#29026)

This commit adds two env vars:
- "DENO_CACHE_DB_MODE"
- "DENO_KV_DB_MODE"

Both of these env vars accept either "disk" or "memory" values and
control the modes of backing databases for Web Cache API and
"Deno.openKv()" API.

By default both APIs use disk backed DBs, but they can be changed to use
in-memory
DB, making them effectively ephemeral.

ext/cache/sqlite.rs

@@ -36,15 +36,37 @@ pub struct SqliteBackedCache {
   pub cache_storage_dir: PathBuf,
 }
 
+#[derive(Debug)]
+enum Mode {
+  Disk,
+  InMemory,
+}
+
 impl SqliteBackedCache {
   pub fn new(cache_storage_dir: PathBuf) -> Result<Self, CacheError> {
+    let mode = match std::env::var("DENO_CACHE_DB_MODE")
+      .unwrap_or_default()
+      .as_str()
     {
+      "disk" | "" => Mode::Disk,
+      "memory" => Mode::InMemory,
+      _ => {
+        log::warn!("Unknown DENO_CACHE_DB_MODE value, defaulting to disk");
+        Mode::Disk
+      }
+    };
+
+    let connection = if matches!(mode, Mode::InMemory) {
+      rusqlite::Connection::open_in_memory()
+        .unwrap_or_else(|_| panic!("failed to open in-memory cache db"))
+    } else {
       std::fs::create_dir_all(&cache_storage_dir).map_err(|source| {
         CacheError::CacheStorageDirectory {
           dir: cache_storage_dir.clone(),
           source,
         }
       })?;
+
       let path = cache_storage_dir.join("cache_metadata.db");
       let connection = rusqlite::Connection::open(&path).unwrap_or_else(|_| {
         panic!("failed to open cache db at {}", path.display())
@@ -57,14 +79,17 @@ impl SqliteBackedCache {
         PRAGMA optimize;
       ";
       connection.execute_batch(initial_pragmas)?;
-      connection.execute(
-        "CREATE TABLE IF NOT EXISTS cache_storage (
+      connection
+    };
+
+    connection.execute(
+      "CREATE TABLE IF NOT EXISTS cache_storage (
                     id              INTEGER PRIMARY KEY,
                     cache_name      TEXT NOT NULL UNIQUE
                 )",
-        (),
-      )?;
-      connection
+      (),
+    )?;
+    connection
         .execute(
           "CREATE TABLE IF NOT EXISTS request_response_list (
                     id                     INTEGER PRIMARY KEY,
@@ -82,11 +107,10 @@ impl SqliteBackedCache {
                 )",
           (),
         )?;
-      Ok(SqliteBackedCache {
-        connection: Arc::new(Mutex::new(connection)),
-        cache_storage_dir,
-      })
-    }
+    Ok(SqliteBackedCache {
+      connection: Arc::new(Mutex::new(connection)),
+      cache_storage_dir,
+    })
   }
 }