Support leaking additional file descriptors to the container

This mirrors the --preserve-fds option of Podman.

Converting an unsigned 'uint', which is what Podman uses for its
--preserve-fds option, to a string is surprisingly annoying.
strconv.Itoa [1] takes a signed 'int', which would require a cast, and
there's no unsigned counterpart.  There's strconv.FormatUint [2] which
takes an unsigned 'uint64', which is better, but would still require a
cast.

So, fmt.Sprint [3] it is, if the cast is to be avoided.  It's more
expensive than the other two functions, but there's no need to worry
unless it's proven to be a performance bottle neck.

Some changes by Debarshi Ray.

[1] https://pkg.go.dev/strconv#Itoa

[2] https://pkg.go.dev/strconv#FormatUint

[3] https://pkg.go.dev/fmt#Sprint

https://github.com/containers/toolbox/issues/1066

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>

doc/toolbox-run.1.md

@@ -6,6 +6,7 @@ toolbox\-run - Run a command in an existing toolbox container
 ## SYNOPSIS
 **toolbox run** [*--container NAME* | *-c NAME*]
             [*--distro DISTRO* | *-d DISTRO*]
+            [*--preserve-fds N*]
             [*--release RELEASE* | *-r RELEASE*]
             [*COMMAND*]
 
@@ -37,6 +38,11 @@ Run command inside a toolbox container for a different operating system DISTRO
 than the host. Has to be coupled with `--release` unless the selected DISTRO
 matches the host system.
 
+**--preserve-fds** N
+
+Pass down to command N additional file descriptors (in addition to 0, 1,
+2). The total number of file descriptors will be 3+N.
+
 **--release** RELEASE, **-r** RELEASE
 
 Run command inside a toolbox container for a different operating system

src/cmd/enter.go

@@ -144,6 +144,7 @@ func enter(cmd *cobra.Command, args []string) error {
 		defaultContainer,
 		image,
 		release,
+		0,
 		command,
 		emitEscapeSequence,
 		true,

src/cmd/rootMigrationPath.go

@@ -92,6 +92,7 @@ func rootRunImpl(cmd *cobra.Command, args []string) error {
 		true,
 		image,
 		release,
+		0,
 		command,
 		emitEscapeSequence,
 		true,

src/cmd/run.go

@@ -34,9 +34,10 @@ import (
 
 var (
 	runFlags struct {
-		container string
-		distro    string
-		release   string
+		container   string
+		distro      string
+		preserveFDs uint
+		release     string
 	}
 
 	runFallbackCommands = [][]string{{"/bin/bash", "-l"}}
@@ -66,6 +67,11 @@ func init() {
 		"",
 		"Run command inside a toolbox container for a different operating system distribution than the host")
 
+	flags.UintVar(&runFlags.preserveFDs,
+		"preserve-fds",
+		0,
+		"Pass down to command N additional file descriptors (in addition to 0, 1, 2)")
+
 	flags.StringVarP(&runFlags.release,
 		"release",
 		"r",
@@ -134,6 +140,7 @@ func run(cmd *cobra.Command, args []string) error {
 		defaultContainer,
 		image,
 		release,
+		runFlags.preserveFDs,
 		command,
 		false,
 		false,
@@ -156,6 +163,7 @@ func run(cmd *cobra.Command, args []string) error {
 func runCommand(container string,
 	defaultContainer bool,
 	image, release string,
+	preserveFDs uint,
 	command []string,
 	emitEscapeSequence, fallbackToBash, pedantic bool) error {
 	if !pedantic {
@@ -277,14 +285,21 @@ func runCommand(container string,
 
 	logrus.Debugf("Container %s is initialized", container)
 
-	if err := runCommandWithFallbacks(container, command, emitEscapeSequence, fallbackToBash); err != nil {
+	if err := runCommandWithFallbacks(container,
+		preserveFDs,
+		command,
+		emitEscapeSequence,
+		fallbackToBash); err != nil {
 		return err
 	}
 
 	return nil
 }
 
-func runCommandWithFallbacks(container string, command []string, emitEscapeSequence, fallbackToBash bool) error {
+func runCommandWithFallbacks(container string,
+	preserveFDs uint,
+	command []string,
+	emitEscapeSequence, fallbackToBash bool) error {
 	logrus.Debug("Checking if 'podman exec' supports disabling the detach keys")
 
 	var detachKeysSupported bool
@@ -295,6 +310,7 @@ func runCommandWithFallbacks(container string, command []string, emitEscapeSeque
 	}
 
 	envOptions := utils.GetEnvOptionsForPreservedVariables()
+	preserveFDsString := fmt.Sprint(preserveFDs)
 
 	var stderr io.Writer
 	var ttyNeeded bool
@@ -320,6 +336,7 @@ func runCommandWithFallbacks(container string, command []string, emitEscapeSeque
 
 	for {
 		execArgs := constructExecArgs(container,
+			preserveFDsString,
 			command,
 			detachKeysSupported,
 			envOptions,
@@ -459,7 +476,7 @@ func constructCapShArgs(command []string, useLoginShell bool) []string {
 	return capShArgs
 }
 
-func constructExecArgs(container string,
+func constructExecArgs(container, preserveFDs string,
 	command []string,
 	detachKeysSupported bool,
 	envOptions []string,
@@ -483,6 +500,7 @@ func constructExecArgs(container string,
 
 	execArgs = append(execArgs, []string{
 		"--interactive",
+		"--preserve-fds", preserveFDs,
 	}...)
 
 	if ttyNeeded {