osdc/Journaler: avoid executing on_safe contexts prematurely

src/osdc/Journaler.cc

@@ -568,8 +568,12 @@ uint64_t Journaler::append_entry(bufferlist& bl)
     ldout(cct, 10) << " flushing completed object(s) (su " << su << " wro "
 		   << write_obj << " flo " << flush_obj << ")" << dendl;
     _do_flush(write_buf.length() - write_off);
-    if (write_off) {
-      // current entry isn't being flushed, set next_safe_pos to the end of previous entry
+
+    // if _do_flush() skips flushing some data, it does do a best effort to
+    // update next_safe_pos.
+    if (write_buf.length() > 0 &&
+	write_buf.length() <= wrote) { // the unflushed data are within this entry
+      // set next_safe_pos to end of previous entry
       next_safe_pos = write_pos - wrote;
     }
   }
@@ -634,6 +638,17 @@ void Journaler::_do_flush(unsigned amount)
     next_safe_pos = write_pos;
   } else {
     write_buf.splice(0, len, &write_bl);
+    // Keys of waitfor_safe map are journal entry boundaries.
+    // Try finding a journal entry that we are actually flushing
+    // and set next_safe_pos to end of it. This is best effort.
+    // The one we found may not be the lastest flushing entry.
+    auto p = waitfor_safe.lower_bound(flush_pos + len);
+    if (p != waitfor_safe.end()) {
+      if (p->first > flush_pos + len && p != waitfor_safe.begin())
+       --p;
+      if (p->first <= flush_pos + len && p->first > next_safe_pos)
+       next_safe_pos = p->first;
+    }
   }
 
   filer.write(ino, &layout, snapc,
@@ -952,7 +967,16 @@ void Journaler::_issue_read(uint64_t len)
     if (pending_safe.empty()) {
       _flush(NULL);
     }
-    waitfor_safe[flush_pos].push_back(new C_RetryRead(this));
+
+    // Make sure keys of waitfor_safe map are journal entry boundaries.
+    // The key we used here is either next_safe_pos or old value of
+    // next_safe_pos. next_safe_pos is always set to journal entry
+    // boundary.
+    auto p = pending_safe.rbegin();
+    if (p != pending_safe.rend())
+      waitfor_safe[p->second].push_back(new C_RetryRead(this));
+    else
+      waitfor_safe[next_safe_pos].push_back(new C_RetryRead(this));
     return;
   }
 
@@ -1062,6 +1086,7 @@ bool Journaler::_is_readable()
     // adjust write_pos
     prezeroing_pos = prezero_pos = write_pos = flush_pos = safe_pos = next_safe_pos = read_pos;
     assert(write_buf.length() == 0);
+    assert(waitfor_safe.empty());
 
     // reset read state
     requested_pos = received_pos = read_pos;