Annotate some errors with context canceled/deadline exceeded info

cache/disk/BUILD.bazel

@@ -17,6 +17,7 @@ go_library(
         "//cache/disk/casblob:go_default_library",
         "//cache/disk/zstdimpl:go_default_library",
         "//genproto/build/bazel/remote/execution/v2:go_default_library",
+        "//utils/annotate:go_default_library",
         "//utils/tempfile:go_default_library",
         "//utils/validate:go_default_library",
         "@com_github_djherbis_atime//:go_default_library",

cache/disk/disk.go

@@ -19,6 +19,7 @@ import (
 	"github.com/buchgr/bazel-remote/v2/cache"
 	"github.com/buchgr/bazel-remote/v2/cache/disk/casblob"
 	"github.com/buchgr/bazel-remote/v2/cache/disk/zstdimpl"
+	"github.com/buchgr/bazel-remote/v2/utils/annotate"
 	"github.com/buchgr/bazel-remote/v2/utils/tempfile"
 	"github.com/buchgr/bazel-remote/v2/utils/validate"
 
@@ -315,7 +316,7 @@ func (c *diskCache) Put(ctx context.Context, kind cache.EntryKind, hash string,
 	removeTempfile = true
 
 	var sizeOnDisk int64
-	sizeOnDisk, err = c.writeAndCloseFile(r, kind, hash, size, tf)
+	sizeOnDisk, err = c.writeAndCloseFile(ctx, r, kind, hash, size, tf)
 	if err != nil {
 		return internalErr(err)
 	}
@@ -340,7 +341,7 @@ func (c *diskCache) Put(ctx context.Context, kind cache.EntryKind, hash string,
 	return nil
 }
 
-func (c *diskCache) writeAndCloseFile(r io.Reader, kind cache.EntryKind, hash string, size int64, f *os.File) (int64, error) {
+func (c *diskCache) writeAndCloseFile(ctx context.Context, r io.Reader, kind cache.EntryKind, hash string, size int64, f *os.File) (int64, error) {
 	closeFile := true
 	defer func() {
 		if closeFile {
@@ -354,14 +355,14 @@ func (c *diskCache) writeAndCloseFile(r io.Reader, kind cache.EntryKind, hash st
 	if kind == cache.CAS && c.storageMode != casblob.Identity {
 		sizeOnDisk, err = casblob.WriteAndClose(c.zstd, r, f, c.storageMode, hash, size)
 		if err != nil {
-			return -1, fmt.Errorf("Failed to write compressed CAS blob to disk: %w", err)
+			return -1, annotate.Err(ctx, "Failed to write compressed CAS blob to disk", err)
 		}
 		closeFile = false
 		return sizeOnDisk, nil
 	}
 
 	if sizeOnDisk, err = io.Copy(f, r); err != nil {
-		return -1, fmt.Errorf("Failed to copy data to disk: %w", err)
+		return -1, annotate.Err(ctx, "Failed to copy data to disk", err)
 	}
 
 	if isSizeMismatch(sizeOnDisk, size) {

utils/annotate/BUILD.bazel

@@ -0,0 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["annotate.go"],
+    importpath = "github.com/buchgr/bazel-remote/v2/utils/annotate",
+    visibility = ["//visibility:public"],
+)

utils/annotate/annotate.go

@@ -0,0 +1,18 @@
+package annotate
+
+import (
+	"context"
+	"fmt"
+)
+
+func Err(ctx context.Context, prefix string, err error) error {
+	ctxErr := ctx.Err()
+
+	if ctxErr == nil {
+		// The context is still valid/not done, we don't have extra info to add.
+		return fmt.Errorf("%s: %w", prefix, err)
+	}
+
+	// Add info that the context was cancelled or deadline exceeded.
+	return fmt.Errorf("%s: %w (%w)", prefix, err, ctxErr)
+}