Move the discard warning into ForkSafety

- Make sure it's only emitted once per fork.
- Try to clarify the warning message.

ext/sqlite3/database.c

@@ -27,11 +27,6 @@ close_or_discard_db(sqlite3RubyPtr ctx)
             sqlite3_file *sfile;
             int status;
 
-            rb_warning("An open writable sqlite database connection was inherited from a "
-                       "forked process and is being closed to prevent the risk of corruption. "
-                       "If possible, please close all writable sqlite database connections "
-                       "before forking.");
-
             // release as much heap memory as possible by deallocating non-essential memory
             // allocations held by the database library. Memory used to cache database pages to
             // improve performance is an example of non-essential memory.

lib/sqlite3/fork_safety.rb

@@ -30,10 +30,20 @@ def track(database)
       end
 
       def discard
+        warned = false
         @databases.each do |db|
           next unless db.weakref_alive?
 
           unless db.closed? || db.readonly?
+            unless warned
+              # If you are here, you may want to read
+              # https://github.com/sparklemotion/sqlite3-ruby/pull/558
+              warn("#{__FILE__}:#{__LINE__}: warning: " \
+                   "Writable sqlite database connection(s) were inherited from a forked process. " \
+                   "This is unsafe and the connections are being closed to prevent possible data " \
+                   "corruption. Please close writable sqlite database connections before forking.")
+              warned = true
+            end
             db.close
           end
         end

test/test_database.rb

@@ -752,7 +752,7 @@ def test_fork_discards_an_open_readwrite_connection
         assert_equal("ok", assertion.chomp, "closed? did not return true")
         assert_equal(1, stderr.count, "unexpected output on stderr: #{stderr.inspect}")
         assert_match(
-          /warning: An open writable sqlite database connection was inherited from a forked process/,
+          /warning: Writable sqlite database connection\(s\) were inherited from a forked process/,
           stderr.first,
           "expected warning was not emitted"
         )