Only accept installs of usrmerge on unmerged-usr systems

As of bookworm, merged-usr is mandatory, and people got caught
in the crosshairs of the dpkg fsys-unmessusr debacle and inadvertently
reverted back to an unmerged configuration and continue to remain
on an unsupported system unknowingly.

Help them by erroring out when they are installing packages on /,
they are not in a chroot, and a usrmerge package is available.

2 files changed, 33 insertions, 2 deletions

diff --git a/apt-pkg/aptconfiguration.cc b/apt-pkg/aptconfiguration.cc
index 00a97a0e7..cfe38ec83 100644
--- a/apt-pkg/aptconfiguration.cc
+++ b/apt-pkg/aptconfiguration.cc
@@ -28,8 +28,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h>
 #include <unistd.h>
-
 									/*}}}*/
 namespace APT {
 // setDefaultConfigurationForCompressors				/*{{{*/
@@ -538,4 +538,31 @@ bool Configuration::isChroot()
    return once.res;
 }
 									/*}}}*/
+// isUsrMerged - whether usr is merged t			     	/*{{{*/
+// ---------------------------------------------------------------------
+/* */
+bool Configuration::checkUsrMerged()
+{
+   std::string rootDir = _config->FindDir("Dir");
+   for (auto dir : {"bin", "sbin", "lib"})
+   {
+      struct stat root;
+      struct stat usr;
+      std::string dirInRoot = rootDir + dir;
+      std::string dirInUsr = rootDir + "usr/" + dir;
+
+      // Missing directories are a boot strap scenario that needs to work
+      if (stat(dirInRoot.c_str(), &root))
+	 continue;
+      if (stat(dirInUsr.c_str(), &usr))
+	 continue;
+      if (root.st_dev != usr.st_dev)
+	 return _error->Error("%s is on different device than %s", dirInRoot.c_str(), dirInUsr.c_str());
+      if (root.st_ino != usr.st_ino)
+	 return _error->Error("%s resolved to a different inode than %s", dirInRoot.c_str(), dirInUsr.c_str());
+   }
+
+   return true;
+}
+									/*}}}*/
 }
diff --git a/apt-pkg/aptconfiguration.h b/apt-pkg/aptconfiguration.h
index bbeb156b9..3e2636edc 100644
--- a/apt-pkg/aptconfiguration.h
+++ b/apt-pkg/aptconfiguration.h
@@ -125,8 +125,12 @@ namespace Configuration {							/*{{{*/
 
 	std::string const getMachineID();
 
+#ifdef APT_COMPILING_APT
 	/** \return Whether we are running in a chroot */
-	bool isChroot();
+	APT_PUBLIC bool isChroot();
+	/** \return Check usr is merged or produce error. */
+	APT_PUBLIC bool checkUsrMerged();
+#endif
 	/*}}}*/
 }
 									/*}}}*/