feat(cosmic-swingset): $SLOGFILE will make the chain write a slogfile

Run e.g. `SLOGFILE=/path/to/chain.slog agoric start local-chain` to cause the
chain's kernel to write "slog" (swingset log) entries to the named file.
Relative pathnames are probably interpreted against the current working
directory. Shortcuts using `~` are unlikely to work unless expanded before
setting the environment variable. The parent directory must exist when the
kernel starts.

We must sadly pass this as an environment variable because the command-line
arguments are consumed by the cosmos-sdk Go code. I hope we can fix that some
day.

packages/cosmic-swingset/lib/chain-main.js

@@ -230,6 +230,7 @@ export default async function main(
       noFakeCurrencies: env.NO_FAKE_CURRENCIES,
     };
     const meterProvider = getMeterProvider(console, env);
+    const slogFile = env.SLOGFILE;
     const s = await launch(
       stateDBDir,
       mailboxStorage,
@@ -238,6 +239,7 @@ export default async function main(
       argv,
       undefined,
       meterProvider,
+      slogFile,
     );
     return s;
   }

packages/cosmic-swingset/lib/launch-chain.js

@@ -34,7 +34,7 @@ async function buildSwingset(
   storage,
   vatsDir,
   argv,
-  { debugName = undefined, slogCallbacks },
+  { debugName = undefined, slogCallbacks, slogFile },
 ) {
   const debugPrefix = debugName === undefined ? '' : `${debugName}:`;
   let config = loadSwingsetConfigFile(`${vatsDir}/chain-config.json`);
@@ -76,6 +76,7 @@ async function buildSwingset(
   await ensureSwingsetInitialized();
   const controller = await makeSwingsetController(storage, deviceEndowments, {
     slogCallbacks,
+    slogFile,
   });
 
   // We DON'T want to run the kernel yet, only when the application decides
@@ -92,6 +93,7 @@ export async function launch(
   argv,
   debugName = undefined,
   meterProvider = DEFAULT_METER_PROVIDER,
+  slogFile = undefined,
 ) {
   console.info('Launching SwingSet kernel');
 
@@ -118,6 +120,7 @@ export async function launch(
     {
       debugName,
       slogCallbacks,
+      slogFile,
     },
   );