fix: gc policy for windows to use 10% 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 10% as it is
on Linux. For instance, after the change, on a
dev machine, the configured space was
74694367232 / (1 << 30) = 69.5 GiB.

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

@@ -104,3 +104,19 @@ 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 := getDiskSize(root)
+	return diskSize * d.Percentage / 100
+}

cmd/buildkitd/config/gcpolicy_unix.go

@@ -5,25 +5,18 @@ package config
 
 import (
 	"syscall"
-)
 
-func DetectDefaultGCCap() DiskSpace {
-	return DiskSpace{Percentage: 10}
-}
+	"github.com/moby/buildkit/util/bklog"
+)
 
-func (d DiskSpace) AsBytes(root string) int64 {
-	if d.Bytes != 0 {
-		return d.Bytes
-	}
-	if d.Percentage == 0 {
-		return 0
-	}
+var DiskSpacePercentage int64 = 20
 
+func getDiskSize(root string) int64 {
 	var st syscall.Statfs_t
 	if err := syscall.Statfs(root, &st); err != nil {
+		bklog.L.Warnf("failed to get disk size: %v", err)
 		return defaultCap
 	}
 	diskSize := int64(st.Bsize) * int64(st.Blocks)
-	avail := diskSize * d.Percentage / 100
-	return (avail/(1<<30) + 1) * 1e9 // round up
+	return diskSize
 }

cmd/buildkitd/config/gcpolicy_windows.go

@@ -3,10 +3,32 @@
 
 package config
 
-func DetectDefaultGCCap() DiskSpace {
-	return DiskSpace{Bytes: defaultCap}
-}
+import (
+	"github.com/moby/buildkit/util/bklog"
+	"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 {
+	rootUTF16, err := windows.UTF16FromString(root)
+	if err != nil {
+		bklog.L.Warnf("failed to convert root dir to UTF16: %v", err)
+		return defaultCap
+	}
+	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 {
+		bklog.L.Warnf("failed to get disk size: %v", err)
+		return defaultCap
+	}
+	return int64(totalBytes)
 }