http2: fix session memory accounting after pausing

The ability to pause input processing was added in 8a4a193 but
introduced a session memory accounting mismatch leading to potential
NGHTTP2_ENHANCE_YOUR_CALM errors.

After pausing
(https://github.com/nodejs/node/blob/f36331c1bfa4c4c202346b05dc3bd672f653e4df/src/node_http2.cc#L871),
the early return on line 873 skips the
DecrementCurrentSessionMemory(stream_buf_.len) call below (line 878).

When we later finished processing the input chunk
(https://github.com/nodejs/node/blob/f36331c1bfa4c4c202346b05dc3bd672f653e4df/src/node_http2.cc#L1858),
we were calling DecrementCurrentSessionMemory(stream_buf_offset_) [line
1875] which was a no-op since we just set stream_buf_offset_ to 0 [line
1873].

The correct amount to decrement by is still stream_buf_.len, since
that's the amount we skipped previously (line 878).

Fixes: nodejs#29223
Refs: nodejs@164ac5b

src/node_http2.cc

@@ -1872,7 +1872,10 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
     nread = buf.size();
     stream_buf_offset_ = 0;
     stream_buf_ab_.Reset();
-    DecrementCurrentSessionMemory(stream_buf_offset_);
+
+    // We have now fully processed the stream_buf_ input chunk (by moving the
+    // remaining part into buf, which will be accounted for below).
+    DecrementCurrentSessionMemory(stream_buf_.len);
   }
 
   // Shrink to the actual amount of used data.

test/parallel/test-http2-large-writes-session-memory-leak.js

@@ -0,0 +1,55 @@
+'use strict';
+const common = require('../common');
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+const fixtures = require('../common/fixtures');
+const http2 = require('http2');
+
+// Regression test for https://github.com/nodejs/node/issues/29223.
+// There was a "leak" in the accounting of session memory leading
+// to streams eventually failing with NGHTTP2_ENHANCE_YOUR_CALM.
+
+const server = http2.createSecureServer({
+  key: fixtures.readKey('agent2-key.pem'),
+  cert: fixtures.readKey('agent2-cert.pem'),
+});
+
+// Simple server that sends 200k and closes the stream.
+const data200k = 'a'.repeat(200 * 1024);
+server.on('stream', (stream) => {
+  stream.write(data200k);
+  stream.end();
+});
+
+server.listen(0, common.mustCall(() => {
+  const client = http2.connect(`https://localhost:${server.address().port}`, {
+    ca: fixtures.readKey('agent2-cert.pem'),
+    servername: 'agent2',
+
+    // Set maxSessionMemory to 1MB so the leak causes errors faster.
+    maxSessionMemory: 1
+  });
+
+  // Repeatedly create a new stream and read the incoming data. Even though we
+  // only have one stream active at a time, prior to the fix for #29223,
+  // session memory would steadily increase and we'd eventually hit the 1MB
+  // maxSessionMemory limit and get NGHTTP2_ENHANCE_YOUR_CALM errors trying to
+  // create new streams.
+  let streamsLeft = 50;
+  function newStream() {
+    const stream = client.request({ ':path': '/' });
+
+    stream.on('data', () => { });
+
+    stream.on('close', () => {
+      if (streamsLeft-- > 0) {
+        newStream();
+      } else {
+        client.destroy();
+        server.close();
+      }
+    });
+  }
+
+  newStream();
+}));