PR27863: debuginfod optimization for concurrent requests

Sometimes, due to configuration error, mishap, or DoS misadventure, a
debuginfod server may receive near-concurrent requests for the exact
same data from multiple clients.  In practically all cases, it is
beneficial to the clients, as well as the server, to serialize these
requests.  This way, debuginfod does not waste CPU in repeatedly &
concurrently decompressing large archives or querying upstream
servers.  Second and later requesters can benefit from the fdcache /
client-cache and get their results, probably earlier!

This patch adds an "after-you" queueing phase to servicing
http-buildid requests, whereby thereads serialize themselves on each
query URL being serviced at the moment.  Prometheus metrics are added,
and the http GET trace line is modified to print the queue+service
times separately.

Hand-tested on large kernel-debuginfo's, and shows host CPU refusing
to multiply in the face of concurrent identical queries.  The
automated test tries a hundred concurrent curls, at least some of
which are slow enough to trigger the "after-you" wait here.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>

4 files changed, 100 insertions, 1 deletions

diff --git a/debuginfod/ChangeLog b/debuginfod/ChangeLog
index 21407dc2..286c910a 100644
--- a/debuginfod/ChangeLog
+++ b/debuginfod/ChangeLog
@@ -1,3 +1,9 @@
+2021-06-03  Frank Ch. Eigler <fche@redhat.com>
+
+	PR27863
+	* debuginfod.cxx (unique_set, unique_set_reserver): New classes.
+	(handler_cb): Use them to implement "after-you" queueing.
+
 2021-05-14  Frank Ch. Eigler <fche@redhat.com>
 
 	PR27859
diff --git a/debuginfod/debuginfod.cxx b/debuginfod/debuginfod.cxx
index e0948eab..543044c6 100644
--- a/debuginfod/debuginfod.cxx
+++ b/debuginfod/debuginfod.cxx
@@ -705,6 +705,54 @@ static workq<scan_payload> scanq; // just a single one
 // idler: thread_main_groom()
 
 
+////////////////////////////////////////////////////////////////////////
+
+// Unique set is a thread-safe structure that lends 'ownership' of a value
+// to a thread.  Other threads requesting the same thing are made to wait.
+// It's like a semaphore-on-demand.
+template <typename T>
+class unique_set
+{
+private:
+  set<T> values;
+  mutex mtx;
+  condition_variable cv;
+public:
+  unique_set() {}
+  ~unique_set() {}
+
+  void acquire(const T& value)
+  {
+    unique_lock<mutex> lock(mtx);
+    while (values.find(value) != values.end())
+      cv.wait(lock);
+    values.insert(value);
+  }
+
+  void release(const T& value)
+  {
+    unique_lock<mutex> lock(mtx);
+    // assert (values.find(value) != values.end());
+    values.erase(value);
+    cv.notify_all();
+  }
+};
+
+
+// This is the object that's instantiate to uniquely hold a value in a
+// RAII-pattern way.
+template <typename T>
+class unique_set_reserver
+{
+private:
+  unique_set<T>& please_hold;
+  T mine;
+public:
+  unique_set_reserver(unique_set<T>& t, const T& value):
+    please_hold(t), mine(value)  { please_hold.acquire(mine); }
+  ~unique_set_reserver() { please_hold.release(mine); }
+};
+
 
 ////////////////////////////////////////////////////////////////////////
 
@@ -1961,6 +2009,7 @@ handler_cb (void * /*cls*/,
   off_t http_size = -1;
   struct timespec ts_start, ts_end;
   clock_gettime (CLOCK_MONOTONIC, &ts_start);
+  double afteryou = 0.0;
 
   try
     {
@@ -1973,7 +2022,25 @@ handler_cb (void * /*cls*/,
 
       if (slash1 != string::npos && url1 == "/buildid")
         {
+          // PR27863: block this thread awhile if another thread is already busy
+          // fetching the exact same thing.  This is better for Everyone.
+          // The latecomer says "... after you!" and waits.
+          add_metric ("thread_busy", "role", "http-buildid-after-you", 1);
+#ifdef HAVE_PTHREAD_SETNAME_NP
+          (void) pthread_setname_np (pthread_self(), "mhd-buildid-after-you");
+#endif
+          struct timespec tsay_start, tsay_end;
+          clock_gettime (CLOCK_MONOTONIC, &tsay_start);
+          static unique_set<string> busy_urls;
+          unique_set_reserver<string> after_you(busy_urls, url1);
+          clock_gettime (CLOCK_MONOTONIC, &tsay_end);
+          afteryou = (tsay_end.tv_sec - tsay_start.tv_sec) + (tsay_end.tv_nsec - tsay_start.tv_nsec)/1.e9;
+          add_metric ("thread_busy", "role", "http-buildid-after-you", -1);
+          
           tmp_inc_metric m ("thread_busy", "role", "http-buildid");
+#ifdef HAVE_PTHREAD_SETNAME_NP
+          (void) pthread_setname_np (pthread_self(), "mhd-buildid");
+#endif
           size_t slash2 = url_copy.find('/', slash1+1);
           if (slash2 == string::npos)
             throw reportable_exception("/buildid/ webapi error, need buildid");
@@ -2036,10 +2103,12 @@ handler_cb (void * /*cls*/,
 
   clock_gettime (CLOCK_MONOTONIC, &ts_end);
   double deltas = (ts_end.tv_sec - ts_start.tv_sec) + (ts_end.tv_nsec - ts_start.tv_nsec)/1.e9;
+  // afteryou: delay waiting for other client's identical query to complete
+  // deltas: total latency, including afteryou waiting
   obatched(clog) << conninfo(connection)
                  << ' ' << method << ' ' << url
                  << ' ' << http_code << ' ' << http_size
-                 << ' ' << (int)(deltas*1000) << "ms"
+                 << ' ' << (int)(afteryou*1000) << '+' << (int)((deltas-afteryou)*1000) << "ms"
                  << endl;
 
   // related prometheus metrics
@@ -2053,6 +2122,10 @@ handler_cb (void * /*cls*/,
              deltas*1000); // prometheus prefers _seconds and floating point
   inc_metric("http_responses_duration_milliseconds_count","code",http_code_str);
 
+  add_metric("http_responses_after_you_milliseconds_sum","code",http_code_str,
+             afteryou*1000);
+  inc_metric("http_responses_after_you_milliseconds_count","code",http_code_str);
+  
   return rc;
 }
 
diff --git a/tests/ChangeLog b/tests/ChangeLog
index 38e92659..b9e5ed4a 100644
--- a/tests/ChangeLog
+++ b/tests/ChangeLog
@@ -1,3 +1,9 @@
+2021-06-03  Frank Ch. Eigler <fche@redhat.com>
+
+	PR27863
+	* run-debuginfod-find.sh: Test "after-you" queueing via flooding
+	with concurent curls.
+
 2021-05-14  Frank Ch. Eigler <fche@redhat.com>
 
 	PR27859
diff --git a/tests/run-debuginfod-find.sh b/tests/run-debuginfod-find.sh
index 9183cccb..0445bce1 100755
--- a/tests/run-debuginfod-find.sh
+++ b/tests/run-debuginfod-find.sh
@@ -580,6 +580,20 @@ testrun ${abs_top_builddir}/debuginfod/debuginfod-find debuginfo $BUILDID
 # Confirm that some debuginfod client pools are being used
 curl -s http://127.0.0.1:$PORT2/metrics | grep 'dc_pool_op.*reuse'
 
+# Trigger a flood of requests against the same archive content file.
+# Use a file that hasn't been previously extracted in to make it
+# likely that even this test debuginfod will experience concurrency
+# and impose some "after-you" delays.
+(for i in `seq 100`; do
+    curl -s http://127.0.0.1:$PORT1/buildid/87c08d12c78174f1082b7c888b3238219b0eb265/executable >/dev/null &
+ done;
+ wait)
+curl -s http://127.0.0.1:$PORT1/metrics | grep 'http_responses_after_you.*'
+# If we could guarantee some minimum number of seconds of CPU time, we
+# could assert that the after_you metrics show some nonzero amount of
+# waiting.  A few hundred ms is typical on this developer's workstation.
+
+
 ########################################################################
 # Corrupt the sqlite database and get debuginfod to trip across its errors
 curl -s http://127.0.0.1:$PORT1/metrics | grep 'sqlite3.*reset'