Don't set a default PATH for Windows

Commit ecf070a updated DefaultPathEnv to return
platform specific values, but also defined a default PATH for Windows.

However, Windows does not have a default PATH, and the PATH to use must be set
by the container image. Also see moby/moby@3c177dc,
which removed the default PATH on Windows;

> This is deliberately empty on Windows as the default path will be set by
> the container. Docker has no context of what the default path should be.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/build_test.go

@@ -1390,17 +1390,15 @@ func testClientGatewayContainerPlatformPATH(t *testing.T, sb integration.Sandbox
 			Platform *pb.Platform
 			Expected string
 		}{{
-			"default path",
-			nil,
-			utilsystem.DefaultPathEnvUnix,
+			Name:     "default path",
+			Expected: utilsystem.DefaultPathEnvUnix,
 		}, {
-			"linux path",
-			&pb.Platform{OS: "linux"},
-			utilsystem.DefaultPathEnvUnix,
+			Name:     "linux path",
+			Platform: &pb.Platform{OS: "linux"},
+			Expected: utilsystem.DefaultPathEnvUnix,
 		}, {
-			"windows path",
-			&pb.Platform{OS: "windows"},
-			utilsystem.DefaultPathEnvWindows,
+			Name:     "windows path",
+			Platform: &pb.Platform{OS: "windows"},
 		}}
 
 		for _, tt := range tests {

client/llb/exec.go

@@ -160,7 +160,9 @@ func (e *ExecOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 			} else if e.constraints.Platform != nil {
 				os = e.constraints.Platform.OS
 			}
-			env = env.SetDefault("PATH", system.DefaultPathEnv(os))
+			if v, ok := system.DefaultPathEnv(os); ok {
+				env = env.SetDefault("PATH", v)
+			}
 		} else {
 			addCap(&e.constraints, pb.CapExecMetaSetsDefaultPath)
 		}

exporter/containerimage/writer.go

@@ -603,7 +603,9 @@ func defaultImageConfig() ([]byte, error) {
 	img.Variant = pl.Variant
 	img.RootFS.Type = "layers"
 	img.Config.WorkingDir = "/"
-	img.Config.Env = []string{"PATH=" + system.DefaultPathEnv(pl.OS)}
+	if env, ok := system.DefaultPathEnv(pl.OS); ok {
+		img.Config.Env = []string{"PATH=" + env}
+	}
 	dt, err := json.Marshal(img)
 	return dt, errors.Wrap(err, "failed to create empty image config")
 }

frontend/dockerfile/dockerfile2llb/convert.go

@@ -496,7 +496,9 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 
 		// make sure that PATH is always set
 		if _, ok := shell.BuildEnvs(d.image.Config.Env)["PATH"]; !ok {
-			d.image.Config.Env = append(d.image.Config.Env, "PATH="+system.DefaultPathEnv(d.platform.OS))
+			if env, ok := system.DefaultPathEnv(d.platform.OS); ok {
+				d.image.Config.Env = append(d.image.Config.Env, "PATH="+env)
+			}
 		}
 
 		// initialize base metadata from image conf

frontend/dockerfile/dockerfile2llb/image.go

@@ -26,6 +26,8 @@ func emptyImage(platform ocispecs.Platform) image.Image {
 	img.Variant = platform.Variant
 	img.RootFS.Type = "layers"
 	img.Config.WorkingDir = "/"
-	img.Config.Env = []string{"PATH=" + system.DefaultPathEnv(platform.OS)}
+	if env, ok := system.DefaultPathEnv(platform.OS); ok {
+		img.Config.Env = []string{"PATH=" + env}
+	}
 	return img
 }

frontend/gateway/container/container.go

@@ -328,7 +328,9 @@ func (gwCtr *gatewayContainer) Start(ctx context.Context, req client.StartReques
 	if procInfo.Meta.Cwd == "" {
 		procInfo.Meta.Cwd = "/"
 	}
-	procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "PATH", utilsystem.DefaultPathEnv(gwCtr.platform.OS))
+	if env, ok := utilsystem.DefaultPathEnv(gwCtr.platform.OS); ok {
+		procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "PATH", env)
+	}
 	if req.Tty {
 		procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "TERM", "xterm")
 	}

solver/llbsolver/ops/exec.go

@@ -371,7 +371,9 @@ func (e *ExecOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 	if e.platform != nil {
 		currentOS = e.platform.OS
 	}
-	meta.Env = addDefaultEnvvar(meta.Env, "PATH", utilsystem.DefaultPathEnv(currentOS))
+	if env, ok := utilsystem.DefaultPathEnv(currentOS); ok {
+		meta.Env = addDefaultEnvvar(meta.Env, "PATH", env)
+	}
 
 	secretEnv, err := e.loadSecretEnv(ctx, g)
 	if err != nil {

util/system/path.go

@@ -13,16 +13,36 @@ import (
 // ':' character .
 const DefaultPathEnvUnix = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 
-// DefaultPathEnvWindows is windows style list of directories to search for
-// executables. Each directory is separated from the next by a colon
-// ';' character .
-const DefaultPathEnvWindows = "c:\\Windows\\System32;c:\\Windows"
+// DefaultPathEnvWindows is deliberately empty on Windows and the default path
+// must be set by the image. BuildKit has no context of what the default
+// path should be.
+//
+// Deprecated: Windows images must not have a default PATH set.
+const DefaultPathEnvWindows = ""
 
-func DefaultPathEnv(os string) string {
+// DefaultPathEnv returns the default value for the PATH environment-variable
+// if available for the given operating-system. The second return variable
+// indicates whether a default exists.
+//
+// On Linux/Unix, this returns [DefaultPathEnvUnix]. On Windows, no default
+// exists, and PATH must not be set as its image-dependent, and must be
+// configured through other means.
+//
+// More details about default PATH values for Windows images can be found
+// in the following GitHub discussions:
+//
+// - [moby/moby#13833]
+// - [moby/buildkit#3158]
+// - [containerd/containerd#9118]
+//
+// [moby/moby#13833]: https://github.com/moby/moby/pull/13833
+// [moby/buildkit#3158]: https://github.com/moby/buildkit/pull/3158
+// [containerd/containerd#9118]: https://github.com/containerd/containerd/pull/9118
+func DefaultPathEnv(os string) (string, bool) {
 	if os == "windows" {
-		return DefaultPathEnvWindows
+		return "", false
 	}
-	return DefaultPathEnvUnix
+	return DefaultPathEnvUnix, true
 }
 
 // NormalizePath cleans the path based on the operating system the path is meant for.