image: backends: Implement a cleanup() method

This allows backends to implement their own handling of target umounts.
Notably, the tar backend already umounts the target /dev /proc /sys, so
our hardcoded cleanup in the Creator caused unnecessary and confusing
error output when umount was run a second time.

Closes: #345

5 files changed, 26 insertions, 4 deletions

diff --git a/image/backends/README.rst b/image/backends/README.rst
index e2a91a7..5eb9036 100644
--- a/image/backends/README.rst
+++ b/image/backends/README.rst
@@ -56,6 +56,11 @@ name.  You may also implement ``prepare()``, which is called before create,
 and ``finalise()``, which is called after.  Default no-op implementations
 are provided for you in BasicBackend.
 
+You may also implement ``cleanup()``, which is called after ``finalise()``
+or when an error occurs during one of the other phases.  The default
+implementation in BasicBackend unmounts the target 'dev', 'proc', and 'sys'
+filesystems from ir_dir/target.
+
 
 
 Repository Layout
diff --git a/image/backends/basic.cc b/image/backends/basic.cc
index 0aa1afa..dc4d9ea 100644
--- a/image/backends/basic.cc
+++ b/image/backends/basic.cc
@@ -12,6 +12,9 @@
 
 #include "basic.hh"
 
+#include <sys/mount.h>      /* umount */
+#include "hscript/util.hh"  /* run_command */
+
 namespace Horizon {
 namespace Image {
 
@@ -38,5 +41,12 @@ int BasicBackend::finalise() {
     return 0;
 }
 
+void BasicBackend::cleanup() {
+    /* The default implementation unmounts /dev, /proc, and /sys from the target. */
+    run_command("umount", {"-R", (ir_dir + "/target/sys")});
+    ::umount((ir_dir + "/target/proc").c_str());
+    run_command("umount", {"-R", (ir_dir + "/target/dev")});
+}
+
 }
 }
diff --git a/image/backends/basic.hh b/image/backends/basic.hh
index ff92ec8..0babfd3 100644
--- a/image/backends/basic.hh
+++ b/image/backends/basic.hh
@@ -50,6 +50,10 @@ public:
      */
     virtual int finalise();
 
+    /*! Clean up the target mounts.
+     */
+    virtual void cleanup();
+
     /*! The intermediate directory which contains the sysroot the image
      *  should contain. */
     const string ir_dir;
diff --git a/image/backends/tar.cc b/image/backends/tar.cc
index b05f4a5..a110acb 100644
--- a/image/backends/tar.cc
+++ b/image/backends/tar.cc
@@ -175,6 +175,12 @@ ret:
 
         return 0;
     }
+
+    void cleanup() override {
+        /* We call umount during creation to avoid including the dirs in the tarball,
+         * so we don't need the default cleanup to run. */
+        return;
+    }
 };
 
 __attribute__((constructor(400)))
diff --git a/image/creator.cc b/image/creator.cc
index ce2c537..c45c50f 100644
--- a/image/creator.cc
+++ b/image/creator.cc
@@ -257,10 +257,7 @@ int main(int argc, char *argv[]) {
 
 trouble:        /* delete the Script and exit */
     /* ensure that our target mounts are unmounted */
-    run_command("umount", {"-R", (ir_dir + "/target/sys")});
-    ::umount((ir_dir + "/target/proc").c_str());
-    run_command("umount", {"-R", (ir_dir + "/target/dev")});
-
+    backend->cleanup();
     delete my_script;
 early_trouble:  /* no script yet */
     delete backend;