handle lookup misses without exceptions

By using `Resolv#each_name` and `Resolv#each_address`, and returning the first
match from the block or returning nil if no matches were found, we can mirror
the behavior of `Resolv#getname` and `Resolv#getaddress` without raising
exceptions on lookup misses.

Adjusts flow-control to handle a nil return value as a lookup miss instead of
relying on exception handlers.

Resolves: logstash-plugins#60

lib/logstash/filters/dns.rb

@@ -195,11 +195,12 @@ def resolve(event)
         else
           address = retriable_getaddress(raw)
         end
-      rescue Resolv::ResolvError => e
-        @failed_cache[raw] = true if @failed_cache
-        @logger.debug("DNS: couldn't resolve the hostname.",
-                      :field => field, :value => raw, :message => e.message)
-        return
+        if address.nil?
+          @failed_cache[raw] = true if @failed_cache
+          @logger.debug("DNS: couldn't resolve the hostname.",
+                        :field => field, :value => raw)
+          return
+        end
       rescue Resolv::ResolvTimeout, Timeout::Error
         @failed_cache[raw] = true if @failed_cache
         @logger.warn("DNS: timeout on resolving the hostname.",
@@ -275,11 +276,12 @@ def reverse(event)
         else
           hostname = retriable_getname(raw)
         end
-      rescue Resolv::ResolvError => e
-        @failed_cache[raw] = true if @failed_cache
-        @logger.debug("DNS: couldn't resolve the address.",
-                      :field => field, :value => raw, :message => e.message)
-        return
+        if hostname.nil?
+          @failed_cache[raw] = true if @failed_cache
+          @logger.debug("DNS: couldn't resolve the address.",
+                        :field => field, :value => raw)
+          return
+        end
       rescue Resolv::ResolvTimeout, Timeout::Error
         @failed_cache[raw] = true if @failed_cache
         @logger.warn("DNS: timeout on resolving address.",
@@ -350,13 +352,43 @@ def retriable_getaddress(name)
 
   private
   def getname(address)
-    name = @resolv.getname(address).force_encoding(Encoding::UTF_8)
-    IDN.toUnicode(name)
+    name = resolv_getname_or_nil(@resolv, address)
+    name && name.force_encoding(Encoding::UTF_8)
+    name && IDN.toUnicode(name)
   end
 
   private
   def getaddress(name)
     idn = IDN.toASCII(name)
-    @resolv.getaddress(idn).force_encoding(Encoding::UTF_8)
+    address = resolv_getaddress_or_nil(@resolv, idn)
+    address && address.force_encoding(Encoding::UTF_8)
+  end
+
+  private
+  def resolv_getname_or_nil(resolver, address)
+    # `Resolv#each_name` yields to the provided block zero or more times;
+    # to prevent it from yielding multiple times when more than one match
+    # is found, we return directly in the block.
+    # See also `Resolv#getname`
+    resolver.each_name(address) do |name|
+      return name
+    end
+
+    # If no match was found, we return nil.
+    return nil
+  end
+
+  private
+  def resolv_getaddress_or_nil(resolver, name)
+    # `Resolv#each_address` yields to the provided block zero or more times;
+    # to prevent it from yielding multiple times when more than one match
+    # is found, we return directly in the block.
+    # See also `Resolv#getaddress`
+    resolver.each_address(name) do |address|
+      return address
+    end
+
+    # If no match was found, we return nil.
+    return nil
   end
 end # class LogStash::Filters::DNS

spec/filters/dns_spec.rb

@@ -6,13 +6,16 @@
 describe LogStash::Filters::DNS do
   describe "with stubbed Resolv" do
     before(:each) do
-      allow_any_instance_of(Resolv).to receive(:getaddress).with("carrera.databits.net").and_return("199.192.228.250")
-      allow_any_instance_of(Resolv).to receive(:getaddress).with("does.not.exist").and_raise(Resolv::ResolvError)
-      allow_any_instance_of(Resolv).to receive(:getaddress).with("nonexistanthostname###.net").and_raise(Resolv::ResolvError)
-      allow_any_instance_of(Resolv).to receive(:getname).with("199.192.228.250").and_return("carrera.databits.net")
-      allow_any_instance_of(Resolv).to receive(:getname).with("127.0.0.1").and_return("localhost")
-      allow_any_instance_of(Resolv).to receive(:getname).with("128.0.0.1").and_raise(Resolv::ResolvError)
-      allow_any_instance_of(Resolv).to receive(:getname).with("199.192.228.250").and_return("carrera.databits.net")
+      # We use `Resolv#each_address` and `Resolv#each_name`, which have
+      # undefined return values but _yield_ once per result, so our stubs
+      # need to either yield a result or not yield at all if there is no result.
+      allow_any_instance_of(Resolv).to receive(:each_address).with("carrera.databits.net").and_yield("199.192.228.250")
+      allow_any_instance_of(Resolv).to receive(:each_address).with("does.not.exist") # no yield
+      allow_any_instance_of(Resolv).to receive(:each_address).with("nonexistanthostname###.net") # no yield
+      allow_any_instance_of(Resolv).to receive(:each_name).with("199.192.228.250").and_yield("carrera.databits.net")
+      allow_any_instance_of(Resolv).to receive(:each_name).with("127.0.0.1").and_yield("localhost")
+      allow_any_instance_of(Resolv).to receive(:each_name).with("128.0.0.1") # no yield
+      allow_any_instance_of(Resolv).to receive(:each_name).with("199.192.228.250").and_yield("carrera.databits.net")
     end
 
     describe "dns reverse lookup, replace (on a field)" do
@@ -305,7 +308,7 @@
       let(:event2) { LogStash::Event.new("message" => "unkownhost") }
 
       before(:each) do
-        allow(subject).to receive(:getaddress).and_raise Resolv::ResolvError
+        allow(subject).to receive(:getaddress).and_return(nil)
         subject.register
       end