Fix the session id never reaching the host, and the split lost on restart

Two real defects, both found by testing rather than reading:

1. The client sent sessionId in the POST body while the route read it from
   the query string, so the host always saw an empty id and answered
   total:null -- the pill was correct to stay silent. The client now sends
   the id in both places and the route reads both.
2. The ledger stored only a session total, so a restart kept the sum but
   dropped the per-purpose split and the call count (9.99 total against
   6.24 in parts, seen on the live route). Sessions are now stored as
   objects; a bare number from an older ledger is still accepted.

Also made the client report why it cannot read a total: swallowing the
error made 'nothing spent yet' indistinguishable from 'request failed'.

lib/client.js

@@ -148,7 +148,7 @@ function cancelTimeout(handle) {
 }
 
 /** One JSON request to the host half. */
-function rpc(method, args) {
+function rpc(method, args, query) {
   let timedOut = false;
   const controller = new AbortController();
   const timer = scheduleTimeout(() => {
@@ -156,7 +156,10 @@ function rpc(method, args) {
     controller.abort();
   }, RPC_TIMEOUT_MS);
   const done = () => cancelTimeout(timer);
-  return fetch(RPC_BASE + '/' + method, {
+  const suffix = query && Object.keys(query).length > 0
+    ? '?' + new URLSearchParams(query).toString()
+    : '';
+  return fetch(RPC_BASE + '/' + method + suffix, {
     method: 'POST',
     headers: { 'content-type': 'application/json' },
     body: JSON.stringify(args ?? {}),
@@ -223,16 +226,36 @@ function installStyles() {
  */
 function useSessionId(props) {
   return React.useMemo(() => {
+    // Why the fallback chain exists: the host route answers `total: null` for an
+    // id it does not know, and the pill then stays silent -- so a wrong id here
+    // looks exactly like "nothing spent yet" unless it is reported.
+    const seen = [];
+    const record = (source, value) => seen.push(source + '=' + JSON.stringify(value ?? null));
+    let resolved = '';
     try {
-      if (typeof props.sessionId === 'string' && props.sessionId.length > 0) return props.sessionId;
+      record('props.sessionId', props.sessionId);
-      if (props.session && typeof props.session.sessionId === 'string') return props.session.sessionId;
+      record('props.session.sessionId', props.session && props.session.sessionId);
+      record('props keys', Object.keys(props).slice(0, 12));
+
+      if (typeof props.sessionId === 'string' && props.sessionId.length > 0) resolved = props.sessionId;
+      else if (props.session && typeof props.session.sessionId === 'string') resolved = props.session.sessionId;
+      else {
         const sessions = runtimeCtx && runtimeCtx.get ? runtimeCtx.get('sessions') : null;
         const current = sessions && sessions.list ? sessions.list.getSnapshot().current : null;
-      if (typeof current === 'string' && current.length > 0) return current;
+        record('sessions.list.current', current);
-    } catch (err) {
+        if (typeof current === 'string' && current.length > 0) resolved = current;
-      /* fall through to the empty id, which the host answers with null */
       }
-    return '';
+    } catch (err) {
+      record('error', String((err && err.message) || err));
+    }
+    seen.push('resolved=' + JSON.stringify(resolved));
+    // Report once per distinct outcome so the console stays readable.
+    const summary = seen.join(' | ');
+    if (globalThis.__dshSessionCostId !== summary) {
+      globalThis.__dshSessionCostId = summary;
+      console.info('[dsh-session-cost] session id: ' + summary);
+    }
+    return resolved;
   }, [props.sessionId, props.session]);
 }
 
@@ -244,14 +267,24 @@ function useSessionCost(sessionId) {
     let timer = null;
 
     const load = () => {
-      rpc('session', { sessionId })
+      // The id goes in the query string as well as the body: the route reads
+      // either, so this keeps working against a host that predates the fix.
+      rpc('session', { sessionId }, { sessionId })
         .then((data) => {
           if (cancelled) return;
           setState({ total: data && typeof data.total === 'number' ? data.total : null, loading: false, error: null });
         })
-        .catch(() => {
+        .catch((err) => {
           if (cancelled) return;
-          setState((prev) => ({ total: prev.total, loading: false, error: 'unavailable' }));
+          // Say why, once per distinct failure, instead of silently keeping the
+          // pill empty: "no spend yet" and "cannot reach the host" are
+          // indistinguishable otherwise, and only one of them is normal.
+          const why = String((err && err.message) || err);
+          if (globalThis.__dshSessionCostError !== why) {
+            globalThis.__dshSessionCostError = why;
+            console.warn('[dsh-session-cost] cannot read ' + RPC_BASE + '/session: ' + why);
+          }
+          setState((prev) => ({ total: prev.total, loading: false, error: why }));
         })
         .finally(() => {
           if (cancelled) return;

lib/index.js

@@ -149,11 +149,17 @@ export default {
         for (const [id, cost] of Object.entries(parsed.costs ?? {})) {
           costByGeneration.set(id, cost);
         }
-        for (const [id, total] of Object.entries(parsed.sessions ?? {})) {
+        // Sessions are stored as objects so the per-purpose split survives a
+        // restart; older ledgers wrote a bare number, which is still accepted.
+        for (const [id, value] of Object.entries(parsed.sessions ?? {})) {
+          if (typeof value === 'number') {
+            sessionTotals.set(id, { total: finiteAmount(value) ?? 0, calls: 0, byPurpose: {} });
+            continue;
+          }
           sessionTotals.set(id, {
-            total: finiteAmount(total) ?? 0,
+            total: finiteAmount(value?.total) ?? 0,
-            calls: 0,
+            calls: typeof value?.calls === 'number' ? value.calls : 0,
-            byPurpose: {},
+            byPurpose: value?.byPurpose && typeof value.byPurpose === 'object' ? { ...value.byPurpose } : {},
           });
         }
         for (const id of parsed.settled ?? []) settled.add(id);
@@ -172,7 +178,10 @@ export default {
           version: 1,
           costs: Object.fromEntries(costByGeneration),
           sessions: Object.fromEntries(
-            [...sessionTotals].map(([id, value]) => [id, value.total]),
+            [...sessionTotals].map(([id, value]) => [
+              id,
+              { total: value.total, calls: value.calls, byPurpose: value.byPurpose },
+            ]),
           ),
           settled: [...settled],
         };
@@ -417,6 +426,36 @@ export default {
       res.end(body);
     }
 
+    /** Read a request body, capped: this route only ever carries a session id. */
+    function readJsonBody(req, limit = 16 * 1024) {
+      return new Promise((resolve) => {
+        const chunks = [];
+        let size = 0;
+        req.on('data', (chunk) => {
+          size += chunk.length;
+          if (size > limit) {
+            resolve({});
+            return;
+          }
+          chunks.push(chunk);
+        });
+        req.on('end', () => {
+          const raw = Buffer.concat(chunks).toString('utf8').trim();
+          if (raw.length === 0) {
+            resolve({});
+            return;
+          }
+          try {
+            const parsed = JSON.parse(raw);
+            resolve(parsed && typeof parsed === 'object' ? parsed : {});
+          } catch {
+            resolve({});
+          }
+        });
+        req.on('error', () => resolve({}));
+      });
+    }
+
     ctx.inject(['connection', 'webServer'], (webCtx) => {
       webCtx.effect(() => {
         const dispose = webCtx.webServer.register({
@@ -435,7 +474,11 @@ export default {
                 respond(res, 404, { error: 'unknown method' });
                 return;
               }
-              const sessionId = url.searchParams.get('sessionId') ?? '';
+              // The client posts a JSON body; the query string is kept as a
+              // fallback so either shape works.
+              const body = req.method === 'POST' || req.method === 'PUT' ? await readJsonBody(req) : {};
+              const raw = body.sessionId ?? url.searchParams.get('sessionId') ?? '';
+              const sessionId = typeof raw === 'string' ? raw : String(raw ?? '');
               respond(res, 200, tail === '/session' ? snapshot(sessionId) : snapshot(''));
             } catch (error) {
               respond(res, 500, { error: String(error?.message ?? error) });