feature/taildrop: do not use m.opts.Dir for Android (#16316)

In Android, we are prompting the user to select a Taildrop directory when they first receive a Taildrop: we block writes on Taildrop dir selection. This means that we cannot use Dir inside managerOptions, since the http request would not get the new Taildrop extension. This PR removes, in the Android case, the reliance on m.opts.Dir, and instead has FileOps hold the correct directory.

This expands FileOps to be the Taildrop interface for all file system operations.

Updates tailscale/corp#29211

Signed-off-by: kari-ts <kari@tailscale.com>

restore tstest

feature/taildrop/taildrop_test.go

@@ -4,40 +4,10 @@
 package taildrop
 
 import (
-	"path/filepath"
 	"strings"
 	"testing"
 )
 
-func TestJoinDir(t *testing.T) {
-	dir := t.TempDir()
-	tests := []struct {
-		in     string
-		want   string // just relative to m.Dir
-		wantOk bool
-	}{
-		{"", "", false},
-		{"foo", "foo", true},
-		{"./foo", "", false},
-		{"../foo", "", false},
-		{"foo/bar", "", false},
-		{"😋", "😋", true},
-		{"\xde\xad\xbe\xef", "", false},
-		{"foo.partial", "", false},
-		{"foo.deleted", "", false},
-		{strings.Repeat("a", 1024), "", false},
-		{"foo:bar", "", false},
-	}
-	for _, tt := range tests {
-		got, gotErr := joinDir(dir, tt.in)
-		got, _ = filepath.Rel(dir, got)
-		gotOk := gotErr == nil
-		if got != tt.want || gotOk != tt.wantOk {
-			t.Errorf("joinDir(%q) = (%v, %v), want (%v, %v)", tt.in, got, gotOk, tt.want, tt.wantOk)
-		}
-	}
-}
-
 func TestNextFilename(t *testing.T) {
 	tests := []struct {
 		in    string
@@ -67,3 +37,29 @@ func TestNextFilename(t *testing.T) {
 		}
 	}
 }
+
+func TestValidateBaseName(t *testing.T) {
+	tests := []struct {
+		in     string
+		wantOk bool
+	}{
+		{"", false},
+		{"foo", true},
+		{"./foo", false},
+		{"../foo", false},
+		{"foo/bar", false},
+		{"😋", true},
+		{"\xde\xad\xbe\xef", false},
+		{"foo.partial", false},
+		{"foo.deleted", false},
+		{strings.Repeat("a", 1024), false},
+		{"foo:bar", false},
+	}
+	for _, tt := range tests {
+		err := validateBaseName(tt.in)
+		gotOk := err == nil
+		if gotOk != tt.wantOk {
+			t.Errorf("validateBaseName(%q) = %v, wantOk = %v", tt.in, err, tt.wantOk)
+		}
+	}
+}