cache DecoderSpec calls

DecoderSpec may be called many times, and deeply recursive calls are
expensive. Since we cannot synchronize the Blocks themselves due to them
being copied in parts of the code, we use a separate cache to store the
generated Specs.

configs/configschema/decoder_spec.go

@@ -1,11 +1,68 @@
 package configschema
 
 import (
+	"runtime"
+	"sync"
+	"unsafe"
+
 	"github.com/hashicorp/hcl/v2/hcldec"
 )
 
 var mapLabelNames = []string{"key"}
 
+// specCache is a global cache of all the generated hcldec.Spec values for
+// Blocks. This cache is used by the Block.DecoderSpec method to memoize calls
+// and prevent unnecessary regeneration of the spec, especially when they are
+// large and deeply nested.
+// Caching these externally rather than within the struct is required because
+// Blocks are used by value and copied when working with NestedBlocks, and the
+// copying of the value prevents any safe synchronisation of the struct itself.
+type specCache struct {
+	sync.Mutex
+	specs map[uintptr]hcldec.Spec
+}
+
+var decoderSpecCache = specCache{
+	specs: map[uintptr]hcldec.Spec{},
+}
+
+// get returns the Spec associated with eth given Block, or nil if non is
+// found.
+func (s *specCache) get(b *Block) hcldec.Spec {
+	s.Lock()
+	defer s.Unlock()
+	k := uintptr(unsafe.Pointer(b))
+	return s.specs[k]
+}
+
+// set stores the given Spec as being the result of b.DecoderSpec().
+func (s *specCache) set(b *Block, spec hcldec.Spec) {
+	s.Lock()
+	defer s.Unlock()
+
+	// the uintptr value gets us a unique identifier for each block, without
+	// tying this to the block value itself.
+	k := uintptr(unsafe.Pointer(b))
+	if _, ok := s.specs[k]; ok {
+		return
+	}
+
+	s.specs[k] = spec
+
+	// This must use a finalizer tied to the Block, otherwise we'll continue to
+	// build up Spec values as the Blocks are recycled.
+	runtime.SetFinalizer(b, s.delete)
+}
+
+// delete removes the spec associated with the given Block.
+func (s *specCache) delete(b *Block) {
+	s.Lock()
+	defer s.Unlock()
+
+	k := uintptr(unsafe.Pointer(b))
+	delete(s.specs, k)
+}
+
 // DecoderSpec returns a hcldec.Spec that can be used to decode a HCL Body
 // using the facilities in the hcldec package.
 //
@@ -18,6 +75,10 @@ func (b *Block) DecoderSpec() hcldec.Spec {
 		return ret
 	}
 
+	if spec := decoderSpecCache.get(b); spec != nil {
+		return spec
+	}
+
 	for name, attrS := range b.Attributes {
 		ret[name] = attrS.decoderSpec(name)
 	}
@@ -111,6 +172,7 @@ func (b *Block) DecoderSpec() hcldec.Spec {
 		}
 	}
 
+	decoderSpecCache.set(b, ret)
 	return ret
 }