refactor the cache.Cache interface

Since all the proxy backends use the same copy+pasted logic to use
the disk cache, refactor to put that functionality in the disk cache.
This simplifies the proxy backends.

Contains is now also proxied to the backend, instead of only
returning whether or not the local disk cache contains the item.

Fixes buchgr#161.

cache/BUILD.bazel

@@ -5,8 +5,4 @@ go_library(
     srcs = ["cache.go"],
     importpath = "github.com/buchgr/bazel-remote/cache",
     visibility = ["//visibility:public"],
-    deps = [
-        "@com_github_bazelbuild_remote_apis//build/bazel/remote/execution/v2:go_default_library",
-        "@com_github_golang_protobuf//proto:go_default_library",
-    ],
 )

cache/cache.go

@@ -2,10 +2,6 @@ package cache
 
 import (
 	"io"
-	"io/ioutil"
-
-	pb "github.com/bazelbuild/remote-apis/build/bazel/remote/execution/v2"
-	"github.com/golang/protobuf/proto"
 )
 
 // EntryKind describes the kind of cache entry
@@ -49,81 +45,20 @@ func (e *Error) Error() string {
 	return e.Text
 }
 
-// Cache is the interface for a generic blob storage backend. Implementers should handle
-// locking internally.
-type Cache interface {
-
-	// Put stores a stream of `size` bytes from `rdr` into the cache. If `hash` is
-	// not the empty string, and the contents don't match it, a non-nil error is
-	// returned.
-	Put(kind EntryKind, hash string, size int64, rdr io.Reader) error
-
-	// Get returns an io.ReadCloser with the content of the cache item stored under `hash`
-	// and the number of bytes that can be read from it. If the item is not found, `rdr` is
-	// nil. If some error occurred when processing the request, then it is returned.
-	Get(kind EntryKind, hash string) (rdr io.ReadCloser, sizeBytes int64, err error)
-
-	// Contains returns true if the `hash` key exists in the cache.
-	Contains(kind EntryKind, hash string) (ok bool)
-
-	// MaxSize returns the maximum cache size in bytes.
-	MaxSize() int64
-
-	// Return the current size of the cache in bytes, and the number of
-	// items stored in the cache.
-	Stats() (int64, int)
-}
-
-// If `hash` refers to a valid ActionResult with all the dependencies
-// available in the CAS, return it and its serialized value.
-// If not, return nil values.
-// If something unexpected went wrong, return an error.
-func GetValidatedActionResult(c Cache, hash string) (*pb.ActionResult, []byte, error) {
-	rdr, sizeBytes, err := c.Get(AC, hash)
-	if err != nil {
-		return nil, nil, err
-	}
-
-	if rdr == nil || sizeBytes <= 0 {
-		return nil, nil, nil // aka "not found"
-	}
-
-	data, err := ioutil.ReadAll(rdr)
-	if err != nil {
-		return nil, nil, err
-	}
-
-	result := &pb.ActionResult{}
-	err = proto.Unmarshal(data, result)
-	if err != nil {
-		return nil, nil, err
-	}
-
-	for _, f := range result.OutputFiles {
-		if len(f.Contents) == 0 && f.Digest.SizeBytes > 0 {
-			if !c.Contains(CAS, f.Digest.Hash) {
-				return nil, nil, nil // aka "not found"
-			}
-		}
-	}
-
-	for _, d := range result.OutputDirectories {
-		if !c.Contains(CAS, d.TreeDigest.Hash) {
-			return nil, nil, nil // aka "not found"
-		}
-	}
-
-	if result.StdoutDigest != nil && result.StdoutDigest.SizeBytes > 0 {
-		if !c.Contains(CAS, result.StdoutDigest.Hash) {
-			return nil, nil, nil // aka "not found"
-		}
-	}
-
-	if result.StderrDigest != nil && result.StderrDigest.SizeBytes > 0 {
-		if !c.Contains(CAS, result.StderrDigest.Hash) {
-			return nil, nil, nil // aka "not found"
-		}
-	}
-
-	return result, data, nil
+// Cache backends implement this interface, and are optionally used
+// by DiskCache. CacheProxy implementations are expected to be safe
+// for concurrent use.
+type CacheProxy interface {
+	// Put should make a reasonable effort to proxy this data to the backend.
+	// This is allowed to fail silently (eg when under heavy load).
+	Put(kind EntryKind, hash string, size int64, rdr io.Reader)
+
+	// Get should return the cache item identified by `hash`, or an error
+	// if something went wrong. If the item was not found, the io.ReadCloser
+	// will be nil.
+	Get(kind EntryKind, hash string) (io.ReadCloser, int64, error)
+
+	// Contains returns whether or not the cache item exists on the
+	// remote end.
+	Contains(kind EntryKind, hash string) bool
 }

cache/disk/BUILD.bazel

@@ -10,7 +10,9 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//cache:go_default_library",
+        "@com_github_bazelbuild_remote_apis//build/bazel/remote/execution/v2:go_default_library",
         "@com_github_djherbis_atime//:go_default_library",
+        "@com_github_golang_protobuf//proto:go_default_library",
         "@com_github_prometheus_client_golang//prometheus:go_default_library",
         "@com_github_prometheus_client_golang//prometheus/promauto:go_default_library",
     ],
@@ -25,6 +27,7 @@ go_test(
     embed = [":go_default_library"],
     deps = [
         "//cache:go_default_library",
+        "//cache/http:go_default_library",
         "//utils:go_default_library",
     ],
 )