fix: gc policy for windows to use percentage of disk space

Initially we had the GC Policy for Windows use only
2 GB (2e9 bytes) of disk space and this was limiting
for some build scenarios that need more than that,
especially ServerCore images.

This commit makes the policy to use percentages
as it is on Linux. Also going for 20%, double that
of Linux, since Windows images tend to be larger.

fixes #4858 docker/buildx#2411

Also, refactors the diskSize logic to simplify
it by bringing the `d.AsByte` function back from
platform specific files to `gcpolicy.go`.

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

cmd/buildkitd/config/gcpolicy.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/docker/go-units"
+	"github.com/moby/buildkit/util/bklog"
 	"github.com/pkg/errors"
 )
 
@@ -104,3 +105,25 @@ func stripQuotes(s string) string {
 	}
 	return s
 }
+
+func DetectDefaultGCCap() DiskSpace {
+	return DiskSpace{Percentage: DiskSpacePercentage}
+}
+
+func (d DiskSpace) AsBytes(root string) int64 {
+	if d.Bytes != 0 {
+		return d.Bytes
+	}
+	if d.Percentage == 0 {
+		return 0
+	}
+
+	diskSize, err := getDiskSize(root)
+	if err != nil {
+		bklog.L.Warnf("failed to get disk size: %v", err)
+		return defaultCap
+	}
+	avail := diskSize * d.Percentage / 100
+	rounded := (avail/(1<<30) + 1) * 1e9 // round up
+	return rounded
+}

cmd/buildkitd/config/gcpolicy_unix.go

@@ -7,23 +7,13 @@ import (
 	"syscall"
 )
 
-func DetectDefaultGCCap() DiskSpace {
-	return DiskSpace{Percentage: 10}
-}
-
-func (d DiskSpace) AsBytes(root string) int64 {
-	if d.Bytes != 0 {
-		return d.Bytes
-	}
-	if d.Percentage == 0 {
-		return 0
-	}
+var DiskSpacePercentage int64 = 10
 
+func getDiskSize(root string) (int64, error) {
 	var st syscall.Statfs_t
 	if err := syscall.Statfs(root, &st); err != nil {
-		return defaultCap
+		return 0, err
 	}
 	diskSize := int64(st.Bsize) * int64(st.Blocks)
-	avail := diskSize * d.Percentage / 100
-	return (avail/(1<<30) + 1) * 1e9 // round up
+	return diskSize, nil
 }

cmd/buildkitd/config/gcpolicy_windows.go

@@ -3,10 +3,29 @@
 
 package config
 
-func DetectDefaultGCCap() DiskSpace {
-	return DiskSpace{Bytes: defaultCap}
-}
+import (
+	"golang.org/x/sys/windows"
+)
+
+// set as double that for Linux since
+// Windows images are generally larger.
+var DiskSpacePercentage int64 = 20
+
+func getDiskSize(root string) (int64, error) {
+	rootUTF16, err := windows.UTF16FromString(root)
+	if err != nil {
+		return 0, err
+	}
+	var freeAvailableBytes uint64
+	var totalBytes uint64
+	var totalFreeBytes uint64
 
-func (d DiskSpace) AsBytes(root string) int64 {
-	return d.Bytes
+	if err := windows.GetDiskFreeSpaceEx(
+		&rootUTF16[0],
+		&freeAvailableBytes,
+		&totalBytes,
+		&totalFreeBytes); err != nil {
+		return 0, err
+	}
+	return int64(totalBytes), nil
 }