fix(shim): recognize shims at arbitrary paths via PMG_SHIM_PATH (#323)

* fix(shim): recognize shims at arbitrary paths via PMG_SHIM_PATH

The recursion guard in FilterPMGFromPath hardcoded the `/.pmg/bin`
suffix, so shims placed anywhere else (e.g. `/usr/local/lib/pmg/bin`,
`/shims`, or any future system-wide location) would not be stripped
from PATH when PMG resolved the real package manager. The shim would
resolve back to itself and PMG would re-exec it in an infinite loop.

This blocks moving shims out of `~/.pmg/bin` — needed for a future
`pmg setup install --system` (#317) — and also any user attempt to
relocate shims manually.

Have the shim export its own path before exec'ing pmg, and let the
filter use that to strip the exact dir at runtime. Keep the legacy
suffix check as a fallback so already-installed shims keep working
until they are regenerated.

Also drop `PMG_SHIM_PATH` from the env passed to the real package
manager so child processes don't inherit a stale marker.

* docs(shim): clarify PMG_SHIM_PATH is internal and unsupported to set manually

* remove comment

* update comment

internal/shim/path.go

@@ -11,7 +11,24 @@ import (
 	"github.com/safedep/dry/log"
 )
 
-const pmgBinSuffix = "/.pmg/bin"
+const (
+	// pmgBinSuffix matches the legacy per-user shim dir (~/.pmg/bin). Retained
+	// as a fallback so shims installed by older PMG versions keep working after
+	// upgrade, until they are regenerated by a fresh `pmg setup install`.
+	pmgBinSuffix = "/.pmg/bin"
+
+	// pmgShimPathEnv is the env var the shim script exports before exec'ing
+	// pmg. Its value is the absolute path of the shim that was invoked, which
+	// lets FilterPMGFromPath strip exactly the dir the shim lives in — no
+	// matter where it was placed (`~/.pmg/bin`, `/usr/local/lib/pmg/bin`, a
+	// custom location, etc.).
+	//
+	// Internal: set by the shim script, consumed by pmg in the same process
+	// tree. Setting it manually from a user shell is unsupported — it would
+	// cause FilterPMGFromPath to strip the wrong directory from PATH lookup
+	// and could prevent pmg from resolving the real package manager.
+	pmgShimPathEnv = "PMG_SHIM_PATH"
+)
 
 var resolverMu sync.Mutex
 
@@ -20,13 +37,22 @@ func FilterPMGFromPath(pathEnv string) string {
 		return ""
 	}
 
+	var shimDir string
+	if shimPath := os.Getenv(pmgShimPathEnv); shimPath != "" {
+		shimDir = filepath.Clean(filepath.Dir(shimPath))
+	}
+
 	entries := filepath.SplitList(pathEnv)
 	filtered := make([]string, 0, len(entries))
 
 	for _, entry := range entries {
-		if !strings.HasSuffix(entry, pmgBinSuffix) {
-			filtered = append(filtered, entry)
+		if strings.HasSuffix(entry, pmgBinSuffix) {
+			continue
 		}
+		if shimDir != "" && filepath.Clean(entry) == shimDir {
+			continue
+		}
+		filtered = append(filtered, entry)
 	}
 
 	return strings.Join(filtered, string(os.PathListSeparator))
@@ -66,9 +92,14 @@ func FilterPMGFromEnv(env []string) []string {
 		if pathValue, ok := strings.CutPrefix(entry, "PATH="); ok {
 			filtered := FilterPMGFromPath(pathValue)
 			result = append(result, "PATH="+filtered)
-		} else {
-			result = append(result, entry)
+			continue
 		}
+		// Drop PMG_SHIM_PATH so child processes don't inherit a stale marker
+		// from the shim invocation that triggered this exec.
+		if strings.HasPrefix(entry, pmgShimPathEnv+"=") {
+			continue
+		}
+		result = append(result, entry)
 	}
 
 	return result

internal/shim/path_test.go

@@ -14,6 +14,7 @@ func TestFilterPMGFromPath(t *testing.T) {
 	tests := []struct {
 		name     string
 		path     string
+		shimEnv  string
 		expected string
 	}{
 		{
@@ -51,10 +52,45 @@ func TestFilterPMGFromPath(t *testing.T) {
 			path:     "/usr/local/bin:/home/user/.pmg/binaries:/usr/bin",
 			expected: "/usr/local/bin:/home/user/.pmg/binaries:/usr/bin",
 		},
+		{
+			name:     "env var strips non-legacy shim dir",
+			path:     "/usr/local/lib/pmg/bin:/usr/local/bin:/usr/bin",
+			shimEnv:  "/usr/local/lib/pmg/bin/npm",
+			expected: "/usr/local/bin:/usr/bin",
+		},
+		{
+			name:     "env var strips arbitrary shim dir",
+			path:     "/shims:/usr/local/bin:/usr/bin",
+			shimEnv:  "/shims/npm",
+			expected: "/usr/local/bin:/usr/bin",
+		},
+		{
+			name:     "env var and legacy suffix both strip",
+			path:     "/shims:/home/user/.pmg/bin:/usr/bin",
+			shimEnv:  "/shims/npm",
+			expected: "/usr/bin",
+		},
+		{
+			name:     "env var matches even with trailing slash in PATH entry",
+			path:     "/shims/:/usr/bin",
+			shimEnv:  "/shims/npm",
+			expected: "/usr/bin",
+		},
+		{
+			name:     "env var unset falls back to legacy suffix only",
+			path:     "/shims:/home/user/.pmg/bin:/usr/bin",
+			shimEnv:  "",
+			expected: "/shims:/usr/bin",
+		},
 	}
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
+			if tc.shimEnv != "" {
+				t.Setenv(pmgShimPathEnv, tc.shimEnv)
+			} else {
+				t.Setenv(pmgShimPathEnv, "")
+			}
 			result := FilterPMGFromPath(tc.path)
 			assert.Equal(t, tc.expected, result)
 		})
@@ -245,6 +281,18 @@ func TestFilterPMGFromEnv(t *testing.T) {
 			env:      []string{},
 			expected: []string{},
 		},
+		{
+			name: "drops PMG_SHIM_PATH from child env",
+			env: []string{
+				"HOME=/home/user",
+				"PMG_SHIM_PATH=/home/user/.pmg/bin/npm",
+				"PATH=/home/user/.pmg/bin:/usr/bin",
+			},
+			expected: []string{
+				"HOME=/home/user",
+				"PATH=/usr/bin",
+			},
+		},
 	}
 
 	for _, tc := range tests {

internal/shim/shim.go

@@ -124,6 +124,8 @@ if [ ! -x "$PMG_BIN" ]; then
   echo "[pmg] error: run 'pmg setup install' again or remove shims with 'pmg setup remove'" >&2
   exit 127
 fi
+PMG_SHIM_PATH=$(cd -- "$(dirname -- "$0")" && pwd)/$(basename -- "$0")
+export PMG_SHIM_PATH
 exec "$PMG_BIN" %s "$@"
 `, pmgBin, pm)
 

internal/shim/shim_test.go

@@ -52,6 +52,8 @@ func TestShimManagerInstall(t *testing.T) {
 		assert.Contains(t, string(content), "#!/bin/sh")
 		assert.Contains(t, string(content), "PMG_BIN='"+pmgBin+"'")
 		assert.Contains(t, string(content), `exec "$PMG_BIN" `+pm+` "$@"`)
+		assert.Contains(t, string(content), `PMG_SHIM_PATH=$(cd -- "$(dirname -- "$0")" && pwd)/$(basename -- "$0")`)
+		assert.Contains(t, string(content), "export PMG_SHIM_PATH")
 		assert.NotContains(t, string(content), "command -v pmg")
 		assert.NotContains(t, string(content), "exec pmg")
 		assert.NotContains(t, string(content), "falling back to native")

main.go

@@ -90,7 +90,7 @@ func main() {
 				ui.ErrorExit(err)
 			}
 
-			// Initialize event logging (silently fail if it can't be initialized)
+			// Initialize event logging
 			var eventlogErr error
 			if logFile != "" {
 				// If a custom log file is specified, use it for event logging too