fix: no sudo advice for a read-only mount

EROFS was advised like a permission refusal, and sudo fails the same way on a read-only mount. The advice now reads the mount flag and says to check and remount the card or drive.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "1eb1aed96cab57bf3bc4ef2918bf1ef40cafcb1de157af2e5fcd5a23ae69a443"
+    $defaultInstallSha256 = "86c4e8b2a67b5721020dd3629212c741a67a3b3180e8f62fb61bba333303a5b6"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }

install.py

@@ -3186,6 +3186,20 @@ class Invocation:
 _INVOCATION = Invocation()
 
 
+def _on_read_only_mount(path: Path) -> bool:
+    """Whether the file system holding path, or the folder it would go in,
+    is mounted read-only (a card Linux remounts read-only after an error)."""
+    if not hasattr(os, "statvfs"):
+        return False
+    probe = path
+    while not probe.exists() and probe.parent != probe:
+        probe = probe.parent
+    try:
+        return bool(os.statvfs(probe).f_flag & os.ST_RDONLY)
+    except OSError:
+        return False
+
+
 def _write_advice(path: Path, assumed: bool, plat: str = "") -> str:
     """What to do about a folder this run could not write.
 
@@ -3193,8 +3207,16 @@ def _write_advice(path: Path, assumed: bool, plat: str = "") -> str:
     machine it would create that folder on the wrong disk: a platform named
     but not found here is a card or share mounted elsewhere, named by --dest.
     Windows has no sudo, and Termux reaches shared storage once
-    termux-setup-storage has run.
+    termux-setup-storage has run. A read-only mount refuses root as well,
+    so sudo is never offered for it.
     """
+    if _on_read_only_mount(path):
+        again = f"\n    {_INVOCATION.command(plat, path)}" if plat else ""
+        return (f"  {path} is on a file system mounted read-only, which no user can "
+                "write, sudo included. Mount it read-write again (a card or drive "
+                "the system switched to read-only after an error needs a check "
+                "first: fsck, or chkdsk on Windows), then run the same command "
+                f"again.{again}")
     if assumed:
         return (f"  {path} is where the platform keeps its BIOS on its own system, "
                 "and it was not found here. If this machine prepares its card or "

install.sh

@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="1eb1aed96cab57bf3bc4ef2918bf1ef40cafcb1de157af2e5fcd5a23ae69a443"
+DEFAULT_INSTALL_SHA256="86c4e8b2a67b5721020dd3629212c741a67a3b3180e8f62fb61bba333303a5b6"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {

tests/test_install.py

@@ -2425,6 +2425,23 @@ class SudoAdviceNamesTheFolder(unittest.TestCase):
         source = (REPO_ROOT / "install.py").read_text(encoding="utf-8")
         self.assertIn("platform_of={path: plat for plat, path in platforms}", source)
 
+    @unittest.skipUnless(hasattr(os, "statvfs"), "statvfs reads mount flags")
+    def test_a_read_only_mount_is_not_sent_to_sudo(self):
+        """A card Linux mounted read-only after a dirty unplug was answered
+        with 'run it again with sudo', which fails the same way."""
+        real = os.statvfs(".")
+        flags = SimpleNamespace(f_flag=real.f_flag | os.ST_RDONLY)
+        for plat in ("retroarch", ""):
+            with self.subTest(platform=plat), \
+                    unittest.mock.patch.object(install.os, "statvfs", return_value=flags):
+                advice = install._write_advice(Path("/media/card/bios"), False, plat)
+            self.assertIn("mounted read-only", advice)
+            self.assertNotIn("Run it again with sudo", advice)
+            self.assertNotIn("sudo python3", advice)
+        writable = SimpleNamespace(f_flag=real.f_flag & ~os.ST_RDONLY)
+        with unittest.mock.patch.object(install.os, "statvfs", return_value=writable):
+            self.assertIn("sudo", install._write_advice(Path("/media/card/bios"), False, "retroarch"))
+
 
 
 class CaseVariants(unittest.TestCase):

wiki/troubleshooting.md

@@ -272,6 +272,10 @@ The installer needs write access to the target directory.
 - On Linux/macOS: check directory ownership (`ls -la`) and run with appropriate
   permissions. Avoid running as root unless the target directory requires it.
 - On Windows: run PowerShell as Administrator if installing to a protected directory.
+- A file system mounted read-only refuses root too, so the installer does not
+  suggest sudo for it. Linux remounts a card or drive read-only after an error
+  (an unclean removal, for one): check it (`fsck`, or `chkdsk` on Windows) and
+  mount it read-write again.
 
 A file already in the BIOS folder with the read-only attribute is replaced
 like any other: on Windows the installer clears the attribute before putting