config: Allow specifying helper dirs with $BINDIR as base directory

This should make it easier to locate helper binaries relative to where the main
binary was installed, which should be useful in installations such as Homebrew
which install packages under a versioned directory.

Use a `$BINDIR` magic token as a prefix in the helper path to indicate it should
be relative to the directory where the binary is located. This is somewhat familiar
to the syntax used in the shell and Makefile and is still quite explicit about the
behavior (as opposed to, say, making all relative paths be relative to the directory
of the binary.)

Update `podman` config on Darwin to look for helpers such as `gvproxy` under
`$BINDIR/../libexec/podman`, which is the ultimate objective of this code change.

Tested: Updated vendored package in podman, built it with `podman-remote`,
copied `gvproxy` to a `libexec/podman` at the same level as `bin/podman` and
confirmed that `podman machine start` worked as expected.

Fixes: containers/podman#12161

Related: PR containers#946

Signed-off-by: Filipe Brandenburger <[email protected]>

pkg/config/config.go

@@ -27,6 +27,8 @@ const (
 	_configPath = "containers/containers.conf"
 	// UserOverrideContainersConfig holds the containers config path overridden by the rootless user
 	UserOverrideContainersConfig = ".config/" + _configPath
+	// Token prefix for looking for helper binary under $BINDIR
+	bindirPrefix = "$BINDIR"
 )
 
 // RuntimeStateStore is a constant indicating which state store implementation
@@ -1245,13 +1247,35 @@ func (c *Config) ActiveDestination() (uri, identity string, err error) {
 // If searchPATH is set to true it will also search in $PATH.
 func (c *Config) FindHelperBinary(name string, searchPATH bool) (string, error) {
 	dirList := c.Engine.HelperBinariesDir
+	var bindirPath string
+	var bindirErr error
 
 	// If set, search this directory first. This is used in testing.
 	if dir, found := os.LookupEnv("CONTAINERS_HELPER_BINARY_DIR"); found {
 		dirList = append([]string{dir}, dirList...)
 	}
 
 	for _, path := range dirList {
+		if path == bindirPrefix || strings.HasPrefix(path, bindirPrefix+string(filepath.Separator)) {
+			// Calculate the path to the executable first time we encounter a $BINDIR prefix.
+			if bindirPath == "" && bindirErr == nil {
+				podman, bindirErr := os.Executable()
+				if bindirErr == nil {
+					bindirPath = filepath.Dir(podman)
+				}
+			}
+			// If there's an error, don't stop the search for the helper binary, but store the
+			// error message  to include in the final error result if one is returned.
+			if bindirPath == "" {
+				continue
+			}
+			// Replace the $BINDIR prefix with the path to the directory of the current binary.
+			if path == bindirPrefix {
+				path = bindirPath
+			} else {
+				path = filepath.Join(bindirPath, strings.TrimPrefix(path, bindirPrefix+string(filepath.Separator)))
+			}
+		}
 		fullpath := filepath.Join(path, name)
 		if fi, err := os.Stat(fullpath); err == nil && fi.Mode().IsRegular() {
 			return fullpath, nil
@@ -1261,6 +1285,9 @@ func (c *Config) FindHelperBinary(name string, searchPATH bool) (string, error)
 		return exec.LookPath(name)
 	}
 	configHint := "To resolve this error, set the helper_binaries_dir key in the `[engine]` section of containers.conf to the directory containing your helper binaries."
+	if bindirErr != nil {
+		configHint = fmt.Sprintf("Error looking up path relative to %s: [%s]  %s", bindirPrefix, bindirErr, configHint)
+	}
 	if len(c.Engine.HelperBinariesDir) == 0 {
 		return "", fmt.Errorf("could not find %q because there are no helper binary directories configured.  %s", name, configHint)
 	}

pkg/config/config_darwin.go

@@ -35,4 +35,6 @@ var defaultHelperBinariesDir = []string{
 	"/usr/local/lib/podman",
 	"/usr/libexec/podman",
 	"/usr/lib/podman",
+	// Relative to the binary directory
+	"$BINDIR/../libexec/podman",
 }